/ ’

/'•

nr de Historie v/d Radi

i

HR9

t

.; ;X..

: W ■ '..........'

11 ' > '1 ®T '

L ■ '.sc..:.;-'.;?"'

■ - • •

<< . .

r ■■: » rt: ■■

-■r-^._____ _ ;

UXi■

■

■

/'. .. < v -- J/ -

I ■-‘-'.I

' ''

j

PHILIPS TRANSMITTING NEWSMARCH 1941 VOLUME VIII No. 1

CONTENTS

H. B. R. Boosman and R. P. Wirix

Transmitter and receiver tuning components

5 kW tropicproof shortwave broadcast transmitter

• PHILIPS TRANSMITTING NEWS”

N.V. Philips9 Gloeilampenfabrieken,

;■

Tj. Douma

Resonance of circuits and lines

..

All rights reserved. Articles acknowledgement of this source:

All inquiries regarding subscription should Transmitting Division, Eindhoven, Holland. Subscription rate: 3’50 florins per annum (4-6 numbers). Single numbers: 1.— florin.

or in part must be accompanied by fullor illustrations reproduced in whole

be addressed to

TRANSMITTER AND RECEIVER TUNING COMPONENTS

by H. B. R. BOOSMAN and R. P. WIRIX

2

I

3

9 99

1.2.3.

a.b.

This article deals with the electrical and mechanical requirements with which transmitter tuning components should comply. Various constructions, such as those relating to ap­pliances for a quick and accurate adjustment of several predetermined wavelengths, are treated at some length.

§ 1. Introduction

The majority of wireless telegraphy- and teleph­ony transmitters require repeated changes of frequency. These changes necessarily occur during the traffic between coastal stations and marine transmitters for naval communication, as well as between ground stations and aeroplane transmitters for air communication, both classes of traffic having the disposal of so called calling- and operating frequencies, and also of distinct frequencies for definite areas. This also applies to the military stations, which nowadays are very numerous, and that repeatedly change frequency with a view to enhancing secrecy and reducing the possibility of d.f. bearings being taken. In all these cases sim­plicity, speed and accuracy of tuning arc of primary importance; this article deals with the mechanical auxiliary components designed to meet these requirements.

It may be emphasized, that for simplicity and speed of tuning the electrical design is all-impor­tant. For each object a circuit diagram will have to be developed, containing the very smallest possible number of variable elements consistent with the proper and economical performance of the equip­ment; moreover it should offer the possibility of combining several tuning condensers or variom­eters.

As regards accuracy the mechanical and elec­trical design band together; there is no sense, for instance, in fitting a tuning knob permitting of an adjustment correct to fifty cycles or so per second, to an oscillator which has a frequency drift of several kilocycles per second caused by the gener­ation of heat during operation or by variation of the ambient temperature.

The ideal expedient for obtaining a high degree of accuracy of adjustment — from the electrical as well as from the mechanical point of view — is the quartz crystal. For transmitters of some power, however, its use calls for rather a high number of amplifying stages which impairs the simplicity of the svslem; besides its flexibility is inadequate for many of the applications mentioned. Trans­mitters that are required to operate within a fre-

Fig. 1. Drive circuit of 2 kW. coastal transmitter with 5 pre­determined operating frequencies at medium- and long wave­lengths.Switches 1, 2 and 3 serve the following purposes respectively:

tapping switch of inductance coil (4). switch for fixed condensers.switch, connecting one of the five variometers (5) to the drive circuit.

Motor (6) drives these switches.Inset: transmitter front panel with 5x3 spindles, for:

fine adjustment of drive circuit (8), ditto of intermediate circuit (9), ditto of aerial circuit (10),

to 5 fixed operating frequencies. The adjustment is effected by means of a slotted handle (11).

quency band should be able to “deviate” with a view to the elimination of jamming; in case of military equipments the adversary will soon be familiar with the particulars of the collection of crystals available. It may be pointed out, though, that crystal control may prove advantageous when

§ 2. Tuning to fixed frequencies

continuously variable

I

I

4

L

Fig. 3. Medium wave drive circuit with finely subdivided fre­quency range and 8 predetermined frequencies.Choice of one of 5 main-ranges with switcli (1) controlling tappings on coil (2) and of 10 sub-ranges with switch (3) and condensers (4). Knob (5) drives both switches simultaneously. The tuning condenser is fitted with a catch knob type Illa: catch knob with rings (7): catch selector (8). Adjustment of catch selector with knob (9); catch knob is turned by means of fine adjustment (10). Total frequency range: 250-750 kc/s.

Fig. 2. Short- and medium wave aircraft transmitter with 9 pre­determined operating frequencies.Power in aerial circuit: 40—100 and 200 W. Fixed frequency is selected by means of handle (1); aerial fine adjustment with knob (2) for short waves; with knobs (3) (call) or (4) (traffic) for medium waves. The inset shows both aerial variometers. In case of jamming, deviation from the operating frequency is effected with knobs (5) after releasing catches (6).

As to the methods of tuning it is fundamentally necessary to discriminate between equipments that are required to operate at one of a few pre-set fre­

quencies and those having a frequency range.

In the former case it will often be possible to find a suitable solution by effecting a rough ad­justment to the desired frequency, by means of switches controlling fixed inductance coils and/or condensers; further to this manipulation a variable coil or condenser of a low (pre-set) value is added for any of the particular frequencies required. Figs 1 and 2 show examples of this sort of design.

For the coastal transmitter of fig. 1 use is also made of separate aerial variometers for each spot frequency; for the aeroplane transmitter of fig. 2 the fine adjustment of the aerial tuning is effected by hand. In the latter case, however, two variom­eters are fitted for the medium wavelength, viz. one for the calling- and the other for the operating frequency; after the call has been answered this system thus permits of changing-over to the oper­ating frequency by the turn of a band (in the true sense of the word!). The transmitter of fig. 1 uses a tuning switch driven by a remotely controlled servomotor; that of the transmitter of fig. 2, how­ever, is worked by hand.

n>ed lor hundreds and even thousands of low

powered equipments if only it is possible to cir­culate the multitude of crystals, required for such a scheme, over the various units.

Thus, in the majoritv of cases, the oscillator will have to comprise an oscillatory circuit with a variable inductance coil and/or a variable condenser; the design should be sufficiently robust to prevent vibration, and a high degree of rigidity of each individual component, as well as of the various components mutually, is necessary. With a view to the specific use — for portable- or aircraft equip­ments. for instance — the permissible weight must often be kept within very narrow bounds; besides, the construction should not be unnecessarily mas­sive. as this would soon give rise to difficulties caused by deformation of the driving mechanism.

5

the results of thiswave­

fig. 4 is

Fig. 6. Long linear scale used on medium wave d.f. receiver (220—560 kc/s). Upper pari of scale: frequency calibration; on lower part: linear scale with vernier. Total number of scale divisions: 500. Maximum number of kc/s per scale division: 1-5. Tuning by means of fine adjustment knob (1).

Fig. -I . Shortwave drive circuit with finely subdivided frequency range. Choice of one of 5 main-ranges with coil drum (1) and of one of 10 sub-ranges with condenser drum (2). Fine adjust­ment by means of variable condenser (3). Total frequency range 3—15 Mc/s.

uniform maximum-to-minimum-wavelength ratio; consequently the number of kc/s per scale division will be much larger for the higher frequencies than for the lower. This is accentuated by the higher self-capacity of the inductance coils for the longer waves and the smaller influence of the variable capacity resulting therefrom.

For this reason, the effect of the tuning condenser on the 4 upper frequency ranges of the receiver of fig. 5 is diminished by a fixed series condenser.

to be preferred. The inductance coils, as well as the condensers — for the main- and sub-ranges respectively — are mounted on drums and the required units are switched on by rotating same Thus the length of the connecting leads in the oscil­latory circuit is reduced to a minimum.

The final tuning of receivers will often be effected by hand; for these, therefore, only a main-range division will be needed. If, however, the total fre­quency range is large and the same tuning conden­ser is used for all sub-ranges, the latter will have a

Fig. 5. Use of ''micrometer scale" on receiver with large fre­quency range (15—20,000 kc/s).Main division in 10 ranges by coils on drum (1). Main scale (2) with 10 sections; concentric fine adjustment scale (3) with 3x100 subdivisions; covers */3 of each main section. Total number of scale divisions: 1000. Maximum number of kc/s per scale division: 10. Tuning is effected by means of fine adjustment knob (4).

§ 3. Circuit diagram and design of tuning elements

The most convenient means to obtain an accurate adjustment of frequency in the case of continuously variable transmitters and receivers is, no doubt, the use of a great many subdivisions of the whole frequency range. This method is advantageous from an electrical standpoint as well, if the induc­tance is varied for the rough adjustment, fixed condensers with the desired temperature coefficient being added for the fine adjustment. Compensation at its middle value of the small variable conden­ser, used for the precise adjustment of each sub­range thus can only give rise to the possibility of but small differences at the beginning and the end of each of these ranges.

Fig. 3 shows a design with two frequency range switches, viz. an inductance coil tapping switch for the main ranges and a condenser selector switch for the subranges. A common knob controls both; therefore this system does not call for additional tuning operations.

For medium wavelengths system are very satisfactory; for the shorter lengths, however, the design shown in

9

§ 4. Tuning dial design

6

1

larger frequency range per called frequency-linear condensers

view to obtaining ap-

are moremore

I

A high degree of accuracy of reading is achieved by: a.

such condensers is not so to vibration, however.

a fine adjustment scale.

ad. a

I

Fig. 9. Application of long drum scale in s. w. duplex receiver. Total frequency range 11—21-8 Mcs/s, in 6 sub-ranges, choice of sub-range by handwheel (1); scale (2) with frequency cali­bration rotates at the same time. Below this: stationary scale (3) with linear division and vernier. Total number of scale divi­sions: 500; maximum number of kc/s per scale division: 14. Tuning by means of fine adjustment knob (4); when this knob is pressed, an electric motor provides for quick adjustment of pointer: clockwise or anti-clockwise, corresponding with di­rection of rotation of motor.

a scale of large diameter or great

If either a long- or large diameter scale is con­sidered, the dimensions of the particular installation

whilst for the 4 lower frequency ranges two tuning condensers are connected in parallel. These series­and parallel condensers are automatically controlled by the contacts of the coil in use.

The result is a maximum of 10 kc/s per scale division and a minimum of ’2 kc/s; without the measures mentioned these figures would be of the order of 30 and -006 kc/s respectively.

Moreover, for a scale division, so are to be preferred, with a proximately equal values of kc/s per scale division for the upper and lower parts of the range. The oblong shape of the condenser plates required for

easily made insusceptible as is the normal semi­

circular form: besides milled packs of condenser plates of the frequency linear type are more dif­ficult to make and consequently more costly,

given maximum capacity, the considerably larger; specially for

aircraft- and portable equipment this fact weighs heavily.

Finally, for a dimensions are

In addition to the foregoing an accurate adjust­ment is only possible if the tuning dial permits of precise readings. There is no sense, however, in raising the accuracy of reading beyond the amount of play in the driving mechanism between the tuning element and the pointer of the dial; in other words: the distance between two scale divisions should exceed the total amount of play in this transmission.

Fig. 7. Short wave aircraft receiver with ''spiral scale" for one frequency range (3-8—10 Mc/s). Scale (1) with frequency calibration makes 3J/< revs, to 1 rev. of the mask (2) and is rotated by means of fine adjustment knob (3). The mask shows but a small part of the scale. Total length of scale: 900 mm.

the use of length.

b. the addition of

Fig. 8. Short- and medium wave aircraft receiver with main- and fine adjustment scale.The main scale (1) is provided with 5 concentric frequency­calibrations in addition to a main division. Fine adjustment scale (2) has 2x 100 divisions and a 1 : 8 gearing; total number of scale divisions: 1600. Maximum number of kc/s per scale division: range 3-10 Mc/s: 2-5 kc/s; range 326—725 kc/s: •2 kc/s. Tuning by means of fine adjustment knob (3): choice of frequency ranges with handle (4). The latter also turns the mask that shows a part of the frequency range chosen.

ad. b

§ 5. Tuning scale calibration

a.

7

iteven with

Fig. 11. Application of catch knob type I in medium wave marine transmitter. Large scale (1) with 360 scale divisions and cams; one frequency range: 375—500 kc/s. Maximum number of kc/s per scale division: -6; accuracy of readjustment of pre­determined frequency: 40 c/s approximately. On turning handwheel (2), cam (3) is locked by catch (4) after readjust­ment of the eccentric lever (5).

©

This means is being put into practice to a great extent; it easily furnishes a proper scale length,

a smallsized front plate.

should lend themselves to this; a properly divided front plate, however, is very important. In this connection attention is drawn to fig. 6.

If little space is available, “the spiral scale” of fig. / offers a solution: a diameter of 150 mm yields a direct scale length of 900 mm.

The magnifying glass is a simple, yet effective expedient for the purpose of interpolation with the eye between two strokes of the scale. An ap­plication, producing a magnification of rather more than two-fold, is shown in fig. 11.

Finally, for any type of dial, it is true that it should be able to obviate reading errors due to parallax, e.g. either by using an edgewise mounted piece of strip metal as a reference stroke (figs 8 and 9), or by using two reference strokes, engraved respectively on the under- and the upper surface of a small plate of transparent material (fig. 6).

■JT7J39

Fig. 10. Principle of catch knob with one cam disk and one catch (type 1). Disk (2) is permanently fixed to spindle (1); on the latter adjustable cams (3) arc filled. The spindle is locked, when catch (4) holds one of the cams. For hand tuning the catch is lifted by an eccentric lever (5).

For the accurate frequency adjustment of any set, a highly precise calibration of same is obviously necessary. This calibration is achieved with the aid of a series of frequencies supplied by a standard signal generator. In the frequency ranges 20—3 Mc/s, 3000—60 kc/s and 60—15 kc/s calibration points at 50, 10 and 1 kc/s arc customary, the tuning scale being provided with a linear division. Unless calibration curves of an inconveniently large size are used, same are to be rejected with a view to the unavoidable drawing- and reading errors; calibration tables, in which 10 intermediate fre­quencies are interpolated within each interval are to be preferred instead. Such tables consequently take a form comparable to that of logarithmic tables, etc.

No doubt, one of the pioneer designs is the “mi­crometer scale” for which some of the broadcasting receivers of 1932 were noted; the receiver shown in fig. 5 is fitted with a precision design of this type of scale. The main scale and its subdivisions arc indicated by characters; the fine adjustment scale has a 0—100 division and the two scales are mounted concentrically.

A high degree of accuracy is achieved in a simpler way by fitting the fine adjustment scale, next to the main scale, directly on the fine adjustment knob; fig. 8 shows an example of this design. (Here the main scale is subdivided into numbered sections).

Fig. 21 shows an entirely different design: the fine reading is done with the aid of the micrometer screws that are used for the adjustment of the stop pins. A scale diameter of only 57 mm thus affords 1800 scale divisions of 2 mm.

A vernier or a magnifying glass is used with a view to enhancing the accuracy of reading the scale. For rotating dials the vernier is fitted near the marker stroke; the latter is the right hand part of it. An interval of 9 scale divisions is divided into 10 equal parts, so that the reading will be correct to x/10 of a scale division.

Scales with a stationary division are fitted with a vernier, consisting of an array of straight lines intersected by a serrated line (vide fig. 6).

desired transmitter.

(?) (5) used, several frequency

§ 6. Simplicity and speed of tuning

(D®

d

8

©

I uning curves do suffice for receivers, if the tuning is accurately readjusted by hand to the

i

by the use of a adjustment, as After pressing the tuning knob

obtain approximately equal numbers of kc/s pcr- scale division (vide fig. 8; in this case a mask, con­trolled by the frequency range switch, shows the

calibration to be used).When straight scales are

calibrations can be made above each other; by the application of a drum type dial, as shown in fig. 9. it is again possible to show only the scale to be used. At the foot a fixed linear scale for precise calibration is provided; it is fitted with a vernier by means oi which particular settings can easily be recorded with a high degree of accuracy.

Fig. 13. Application of catch knob type II for 4 predetermined frequencies in three-^lage short wave aircraft transmitter (70 W). Waverange switch with knob (8); aerial circuit hand tuning (rough) with knob (9): fine adjustment with knob (10). Reversal of the eccentric lever (5) puts catch (3) into action. Fine adjustment of predetermined frequency after loosening screw (6) by turning knob (7). Frequency range: 2-6—6-8 Mc/s; maximum number of kc/s per scale division: 11; ac­curacy of readjustment of predetermined frequencies: better than 2 kc/s.

Several expedients may be employed to add to the speed of tuning. Thus for short wave receivers, generally fitted with a high ratio fine adjustment, a

Fig. 12. Principle of catch knob with one disk and several catches (type II).Spindle (1) with cog wheel (2) is locked by catch (3). Fine adjustment of predetermined frequency by shifting catch (3) after loosening screws (6). For choice of one of the predetermined frequencies, mark on cog wheel is set opposite to corresponding mark (4); next knob of eccentric lever (5) is turned.

Fig. 14. Principle of catch knob with several disks and several catches (type III). Drum (2) is fixed on spindle (1). On the former a catch ring (3) with notch (6) is clamped. In a certain position of the catch selector (5) catch (I) will be pressed against the ring and drop into the notch, when the spindle is rotated. On drum (2) several rings with their corresponding catches are mounted behind each other, one set of each being required for each predetermined frequency.

In that case, however, it is much more desirable to use a frequency calibrated scale; the bigger the direct scale length, the more accurate this cali­bration can be made. From this point of view, therefore, the “spiral scale” mentioned in par. 4 sub a is to be preferred to the fine adjustment scale sub b, as in the latter case only the main scale can be calibrated. When several frequency ranges are engraved on a circular dial, the length of the scale will decrease with the frequency; if, however, the outermost scale is used for the highest fre­quency band, it will nevertheless be possible to

particular type of knob for which this ratio is auto­matically decreased to 1/i after one revolution, is customary. Speed of tuning may also be improved

small electric motor for the rough applied to the receiver of fig. 9.

a small clockwise or anti-clockwise turn of same causes the reading stroke to move quickly to the right or to the left.

In order to tune an apparatus in a quick an simple manner to definite predetermined frequen­cies, however, knobs with cams or catches are the obvious means.

In comparison with the usual method of hand adjustment, catch knobs, as a matter of fact, spe­cially add to speed of operation, when they are used to control the continuously variable tuning ele­ments. Compared with a rational indication of the various settings — by means of colour marks, for instance — the advantage of using these knobs for

3.

9

i1

Fig. 17. Application of catch knob Illa in preliminary stages of a short wave transmitter. Each catch knob has its individual catch selector (1) and knob(2) for subsequent rotation.

Fig. 16. Catch knob, with which the catch rings (3) arc fixed together with 4 screws (7). “Tail” of chosen catch (4) drops into milled space of ring (5). For accurate adjustment of pre­determined frequency a fine adjustment scale (8) and fine adjustment knob (9) (type Illb) are provided.

the frequency range switches is small. Unless, however, each switch is given its own driving me­chanism, the use of catch knobs is also imperative in case of remote control; this applies likewise if the catch knobs are being worked together. For, in the end. the optimum conditions for speed and simplicity of tuning arc obtained by the provision of means for simultaneous adjustment of all tuning knobs of the equipment.

§ 8. Requirements of catch knobs

These can be worded as follows:1. The catch knob should suit the particular type

of set and be instrumental in making the latter answer its purpose.

2. Apart from the adjustment to one of the preset frequencies, simple means for normal hand adjustment to any other frequency should be available.“Deviation” — consisting in a small change of the oscillator frequency — should specially be practicable.

4. With military transmitting-receiving equip­ments, operating in a “network”, it is necessary that the transmitter can be adjusted very care­fully to the local receiver, and conversely (zero beat adjustment).

§ 7. Advantages of catch knobs

As compared with the normal method of hand tuning the use of catch knobs offers the following advantages:1. The accurate adjustment of the various fre­

quencies may entirely be carried out beforehand whenever time and accommodation permit.

2. This preadj ustment, therefore, may be performed by skilled personnel; the subsequent tuning may, without objection, be left to unskilled operators.

3. Irrespective of adverse conditions or of the speed of operation the same degree of accuracy of adjustment is always obtained.

4. The adjustment of the knobs takes so little time that the possibility presents itself of making some adjustments variable that, otherwise, would be invariable for the sake of simplicity and speed of tuning. In this way an optimum performance at all frequencies results.

I'ig. 15. Catch knob, with which each catch ring (1) is fixed by an eccentric (2). Spring catches arc fitted in a common holder (3). (type Illa).

Obviously, the degree of accuracy that can be attained by means of a catch knob is also governed by the quality of the scale used with it, the accuracy of calibration and the carefulness of adjustment. The quality of the knob itself is thus determined by the degree of “accuracy of re-adjustment”, i.e. the accuracy with which a predetermined setting can be reiterated. It is found by taking the average of 10 successive adjustments. If the system permits of doing so, 5 of these adjustments should be clock­wise and 5 anti-clockwise.

The limit of the degree of accuracy of readjust­ment is determined by the inevitable play in and the deformation of the mechanism.

For a rotating knob of first class workmanship the degree of accuracy of rc-adjustment thus amounts to 2'—4'. dependent on the load. The variation in c/s occurring in this case naturally depends on the variation of frequency per 180° or 360°; this is another advantage of a finely sub­divided total frequency range, as mentioned in paragraph 3.

5.

■I

4

f

number of fixed

catch

10

§ 11. Catch knobs with several disks and several catches (type III)

Catch knob type II, though attractive because of its simplicity and small dimensions, is unser­viceable in many cases, especially if the combined adjustment of several knobs is desired. This is due to the fact that the knob is to be adjusted to

one disk and several catches

Fig. 19. Application of catch knob type IV.The arm can be moved between the two stops (3), adjustable along ring (4). The position of the spindle (1) is shown by a spiral scale, fitted behind the window (5). lor the fine adjust­ment knob (2) is used.

For all equipments manufactured the only tool required is a screwdriver and the adjustments are effected at the front of the apparatus. In the fol­lowing some of the designs will be considered.

§ 9. Catch knob with one cam disk and one(type I)

The principle of this knob is shown and explained in fig. 10; it may be said that this knob is of the simplest and oldest type. If it is properly made, however, it complies with the latest tolerance re­quirements. Fig. 11, for instance, shows it being used in the drive circuit of a modern medium wave

cams; thus, in some degree, it resembles a cog wheel; the cams can be shifted over a distance slightly exceeding the breadth of the cogs.

As shown in the photograph, up to 4 catches can be fitted in a square of which the scale constitutes the inscribed circle; thus practically no extra space on the frontplate is needed.

’I hereforc, it should be possible to effect small deviations from the “catch-frequency” by hand. If a motor-driven device is used facilities are to be provided for working the tuning controls by hand, in case this device should develop trouble.

6. Finally the adjustment of the catch knobs should be simple and require no special tools.

Fig. 20. Principle of catch knob with two cams and two stops (type V).A disk (2) is fitted on spindle (1). Position of spindle (1) for both predetermined frequencies is fixed by the adjustable cams (3), limited by the stationary stops (4).

§ 10. Catch knob with(type II)

Both objections mentioned in § 9 may be met by fitting individual catches for each predetermined frequency.

The principle of such a knob is shown and ex­plained in fig. 12, a design with 4 catches is shown in fig. 13.

The cam disk is fitted with a

marine transmitter w ith a waverange of 600- 800 m (500—375 kc/s). The maximum frequency range per scale division amounts to *6 kc/s approximately and the accuracy of readjustment is of the order of 40 c/s.

Evidently hand adjustment is easy; a drawback, however, is the fact that the cams require at least 10° of the scale — even when the latter is large — therefore the predetermined frequencies will have to be rather far apart from each other.

Moreover, if the apparatus has several frequency ranges, the cam corresponding with one frequency in a particular range, will occupy a part of another frequency range, unless provision is made to prevent this.

jrzz*/

Fig. 18. Principle of catch knob with one cam and two slops (type IV). A cam (2) fitted on spindle (1). Preset frequencies are determined by cam (2) being limited between two stops (3) adjustable along ring (4).

■ • ]■'

7) @®

11

<D

Fig. 21. Short leave aircraft transmitter-receiver with catch knob type V.For each of the three independently adjustable groups of tuning elements the spindle (1) is fitted with a disk (2) with holes for both cams (3). Stops (4) arc adjustable by means of micrometer screws (5). Owing to their elasticity the cams press against cither left- or right-hand stop. The mechanical device (6) driven by spindle (7) changes direction of action of spring; spindle (7) is driven by motor (9). (Sparc for motor: handle (8)). Hand adjustment by means of knob (10) in recess (11).Frequency range: 2-9—5-1 kc/s. Maximum number of kc/s per scale division: 2-5. Accuracy of repeated adjustment of predetermined frequencies: -4 kc/s. r

5 WI

The diameter will not now be dependent upon the number of catches; besides, a specific catch can be selected before the knob is roughly set.

The procedure of adjustment now includes:1. The selection of the desired catch by means of

a catch selector with clearly defined positions.2. The knob is turned until the selected catch

grips and thus the former is locked in the required position.

The cam disks have now become notched rings that can be fitted in any position round about a drum on the tuning spindle, as shown in fig. 14.

With the design shown in fig. 15 the catches are spring hooks; the desired hook is pushed in. The specific feature of this design, however, is the fitting on the drum of each notched ring individ­

ually; this is done by turning a screw over 180° at the front side, each ring having its corresponding screw.

In this case, however, the rings have to be re­silient, their radial dimensions should therefore be small, the thickness in the direction of the spindle being 2 mm. Together with a distance ring of 1 mm thickness, each predetermined frequency conse­quently requires a depth of 3 mm. so that, for a large number of frequencies, the height of the knob will be considerable.

(Besides, because of the room required by the locking devices, the diameter would have to be correspondingly larger, if more than 8 — as in the photograph — predetermined frequencies should be available.)

the approximately correct position before the catch can be made to hold; this calls for closer attention of the operating personnel. Besides, for more than 4 fixed frequencies the diameter becomes unduly large.

Therefore, the next step is the provision of a catch and a disk for each fixed frequency, these catches and disks being fitted behind one another.

12

Fig. 22. Principle of special catch knob for two predetermined frequencies.Via a slip coupling catch-rings (1) and (2) arc connected to spindle (3). Disk (4) constitutes a slip-coupling between this spindle and a rod (5) with a cross piece (6) with groove, into which moves a pin (7), riveted to a disk (8). On turning handle (9) rod (5) is first moved to the right a little by pin (7), then to the left. This results in the righthand catch (10) being lifted from notch in ring (1) by pin (11); leftband catch (13) is pressed against ring (2) by spring (12). On further movement to the left of rod (5), spindle (3) is rotated, until catch (13) holds in notch (14).

Fig. 23. Transportable short wave transceiver with special catch knob.Adjustment of predetermined frequencies and hand tuning arc cfi’ectcd by means of knobs (1). Change of predetermined frequencies: handle (2). Aerial fine adjustment: knob (3). Fre­quency range: 75—85-8 Mc/s. 36 kc/s. per scale division. Accuracy of readjustment of predetermined frequency: approximately 8 kc/s.

§ 12. Catch knobs for two predetermined frequencies

12

beat of the

Fig. 24. Mechanism of special catch knob of Jig. 22.1. catch disk.2. spindle to be adjusted.9. handle for change of predetermined frequency.

10. catch.14. notch.

§ 13. Special catch knob for two predetermined frequencies

Although the knob shown in fig. 21 apparently complies with all requirements touching upon military applications, a still simpler solution seemed to be desirable for a portable u.s.w. telephony transmitter-receiver with two preset frequencies; the adjustment and operation of the catch knobs

diameter. As described in § 4 the micrometer screw of the stop adjustment is also used for the fine reading.

The coupling between the tuning spindle and the cam disk can be disengaged by pushing away a catch; normal hand adjustment is then possible by putting a key (10) into the hole (11).

The accurate adjustment of frequency (zero adjustment) is easily performed by using one micrometer screws of the oscillator tuning knob for the fine adjustment; as the cam is pressed against the screw by a spring, each movement of the screw is closely followed.

Fig. 25. Central catch selector for three catch knobs.The central catch selector (1) selects a catch of each of the three catch knobs (2) simultaneously. Next these arc turned together by means of handle (3). By putting the handle in recess (4), each individual tuning element may be adjusted to any position required if the catch selector is in the “hand­position”. Fine adjustment of aerial: knob (5). When the catches are being changed, the anode voltage is automatically reduced; after this the full energy is put on again by pressing knob (6).

Ina later design — shown in fig. 16 — therefore, the individual fitting of the notched rings was abandoned, so that their thickness could be reduced to 1 mm. the whole pile of rings being clamped on to the drum by means of 4 screws at the front side. Each catch is pressed down by a separate spring; the catch selector releases the desired catch.

Applications of the first and second design are shown in figs. 17 and 27 respectively.

If only two predetermined frequencies arc called for, the drawbacks mentioned in § 9 with respect to the knob of fig. 11 can also be removed by fitting one fixed cam to the disk and two adjustable stop pins instead of catches. The principle is shown in fig. 18, the practical design in fig. 19.

Sometimes it may’ be of advantage to use fixed stops, which is possible if two adjustable cams are fitted, as shown in fig. 20. The angle between the stops equals 180° -}- twice the breadth of a cam, so that rotation over clear 180° remains possible.

A triple application is shown in fig. 21; here the cams are only used for the rough adjustment, the stops being fitted with a fine adjustment device with a view to providing an easy way of adjustment that is also very accurate in spite of the small

1

a

§ 14. Combined drive of catch knobs

13

A consideration of fig. 17 will make it obvious, that the desirability of applications of a more uni­versal character than that previously described was felt. For all knobs the method of operation is identical and performed almost mechanically, the

by the line adjustment of the aerial with knob (5). Lastly, by pushing knob (6), full power is again applied, because the power is automatically de­creased, when the catch selector is worked in order to prevent the switches from burning and to guard against the occurrence of an overload brought about by the temporarily detuned aerial circuit.

For larger types of transmitters with more than 3 tuning knobs the combined subsequent rotation calls for rather a considerable quantity of power; primarily a motor is used, timed to function until all knobs have rotated over slightly more than 360°, so that all catches are sure to clutch. Each knob drive, therefore, requires a slip coupling so that the tuning spindle remains stationnary whilst the drive keeps turning.

Fig. 26. 800 IF short wave transmitter with 8 predetermined frequencies. Choice of each frequency by means of catch se­lectors (1) and (2); subsequent rotation is automatically ef­fected by 2 built-in motors. Total frequency range: 3—12 kc/s; accuracy of readjustment of predetermined frequencies: better than -8 kc/s.

catch selector on the left and the knob for the sub­sequent rotation on the right. The process is rather a quick one but its speed may still be improved by a combination of movements.

A combination of 3 knobs that provides for the whole tuning of a three stage short- and medium wave aircraft transmitter — the fine tuning of the aerial circuit excepted is shown in fig. 25. The knobs are placed in the angular points of an equi­lateral triangle, without detriment to the proper internal arrangement.

The tuning process now merely requires operation of the catch selector and the subsequent rotation of the knobs by means of the handle (3), followed

had to be such that they could be effected under service conditions by any soldier without tools and without risk of errors.

For a simple adjustment of the predetermined frequencies the principle of using one disk and one catch for each frequency — as outlined in fig. 11 — was applied; for convenience of operation, however, the selection of the catch and the subsequent ro­tation of the knob were made to constitute one com­bined movement. This principle is shown in fig. 22, the transmitter-receiver in question in fig. 23.

Both transmitter and receiver have single knob control; for this reason the adjustment of the Kol- ster circuit — used with a view to the frequency stability of the oscillators in transmitter and re­ceiver (superheterodyne) — is ganged with that of normal rotary condensers. Only the aerial cor­rection has an extra control (3).

When it is desired to change over transmitter and receiver from one predetermined frequency to the other, it is only necessary to push the handle (2) slightly down and turn it until the mark, denoting the other fixed frequency, appears in the aperture above the handle. On turning further, the handle slips; it is provided with a freewheel as the sense of rotation is unidirectional for each change of frequency.

All other adjustments of the tuning knobs are solely effected by means of both fine adjustment knobs (1) next to the scale-windows. When these knobs are pushed in the following operations can be performed:

) For the adjustment of the predetermined fre­quencies the scales are set to the required value in accordance with a calibration chart, after the mark corresponding with the desired fre­quency has been made visible.

b) For the fine adjustment of frequency the trans­mitter is tuned to the receiver, or the receiver to the transmitter, by means of the transmitter* or receiver tuning knobs respectively.

c) If transmitter and receiver are tuned by hand, the corresponding knobs are turned; no additional operations are required.

Fig. 24 gives an idea of the mechanism. The mechanical accuracy is considerable: for a fre­quency range of 75—85-8 Mc/s the accuracy of a reiterated adjustment is 5 kc/s approximately.

Fig. 27b. Exterior vieiv of the transmitter of fig. 27a.

14

i

' a large number conductor saving of which the num-

A combined movement of each selector and catch knob, as described in § 13, is naturally confined to two predetermined frequencies only. A closer con­sideration of fig. 27a shows that — in consequence of the separated movements of selector and knob — some forethought is necessary with regard to pro­viding catch knobs in accordance with figs. 15 and 16 with a combined drive.

Besides, the catch selectors placed next to the knobs require a rather considerable space.

For this reason a knob was finally developed, for which the catch selector is fitted round the knob; although the diameter is smaller, an improved method of design yields the same accuracy of re­adjustment; besides knob and selector arc now being driven by a common spindle.

nothing against fitting the a distance; in this way the

Fig. 27a. Combined drive of 6 catch knobs in a three stage short leave transmitter (200 IF)Catch selectors (1) and catch drums (2) are interconnected by a system of spindles (3—4). This system is connected to motor (5) via freewheels (6) and (7) for anti-clockwise and clockwise rotation respectively. Catch selector switch (8) closes motor circuit until desired catches have been selected. Pilot disk (9) shows that motor has chosen the correct catch. Deviation: knob (10). Frequency range: 3—15 Mc/s; overall accuracy of readjustment of drive circuit catch knobs: 1 kc/s. § 15. Catch knob with one spindle

ber of leads can be cut down. The whole control of such transmitters, however, is remotely operated and a twin wire distant control device has been developed to serve this purpose. For this reason the catch selector itself will, in fact, be mounted near the transmitter in the majority of cases.

Fig. 26 shows an example of this type, fitted to a short wave transmitter, with catch knobs for the continuously variable tuning elements. After the catch selectors (1) and (2) — corresponding with the left- and right hand knobs respectively — have been given identical positions by hand, the groups of knobs arc subsequently rotated by 2 motors.

Besides, a motor that is being stopped in its correct position with the aid of a selector switch, may also be considered for the adjustment of the catch selectors. A short wave transmitter with one motor, common to all catch devices, is shown in fig. 27a.

Naturally, there is catch selector switch at

electrical remote tuning control of a transmitter with a continuously variable frequency range is realized.

For an equipment employing of predetermined frequencies a circuit may be utilized by means <

360°,

Ezfc] Htj

(57)-® IB

5 ts ITu

□ qq P

2

1

J 13

-

§ 16. Catch knob for linear movement

15

ii

T TI -1— TT—-

L

system of signalling lamps, that at a

h r________

All catch knobs so far mentioned, serve the pur­pose of fixing tuning elements, controlled by a rotating motion. Lately, however, so called “sliding

ds^ill I i 11

In.---- JTtr

that all knobs commence

Fig. 28. Single spindle catch knob for 16 positions, central driving unit and catch selector switch.Catch knob is driven by worm on spindle (1). Firstly, catch selector (3) choses the desired catch (2): next, by turning spindle (1) in the opposite direction, spindle (4) is adjusted to the required position. Direction of rotation of driving motor is controlled by switches (5) and (6) in central driving unit. The catch selector switch (7) controls the electric switch (6).

condensers” are also used, each set of plates con­sisting of a structure of concentric rings telescoping into each other. Owing to the higher rigidity of the rings the distance between the “plates” may be made very small indeed; on account of their small dimensions these condensers, therefore, are very suitable for short wave sets.

Fig. 29. Principle of catch knob for linear movement (**Caroussel”- knob).Linear displacement of annular packets (1) of disk condenser brings about change of capacity. Spring (2) always tries to bring condenser (13) to its maximum value. A choice of a predetermined frequency will firstly rotate spindle (4) with attached cam disk (5) over 180°. and bring condenser to its minimum position by means of the pressure plate (3). Next the knob (7) with its adjusting screws is rotated, until the selected screw arrives at point (8). Finally the cam disk (5) is made to return to its initial position and spring (2) forces the condenser towards its maximum position until the pressure plate (3) is stemmed by the chosen adjusting screw (8). For this purpose the cog wheel (9) on spindle of motor (10) is successively coupled with cog wheels (11), (12) and again with (11).

--------- '5=3—'gj-------®---------- -s';

HrS 7----®

In fig. 28 this is shown on the left; at the top the driving spindle, with its two free ends, can be seen. Rotation in one direction operates the catch selec­tor, whilst rotation in the opposite direction operates the knob. The slip couplings for both directions ol rotation are built in. Eighteen catches can be fixed on the periphery; one catch, however, is to be omitted for the position “Hand-setting”, another being reserved for the adjustment of the knobs, so that 16 predetermined frequencies are available. All switches required for the electric driving me­chanism are mounted together in a central control box, shown in the middle of fig 28, and connected in the driving spindle transmission. Its dimensions arc but 85x65x65 mm and without further aid from any relays, it performs the following func­tions:a It stops the motor in the position determined

by the catch selector, so that the correct catches are chosen for all knobs.

b. It reverses the direction of rotation of the motor and restarts it, so turning.

c. It stops the motor after all knobs have turned slightly over 360°, so that all driven spindles arc locked.

d. It again reverses the direction of rotation of the motor; the latter thus being ready for a sub­sequent choice among the catches.

e. It controls apermits checking the correct operation distant point.

For setting the tuning to other than the pre­determined frequencies it is only necessary to turn the catch selector switch (on the right in fig. 28) to the “hand-position”; the catch selector then lifts all catches and by turning the knobs they are all released from the driving mechanism; they arc now freely adjustable by hand.

The adjustment of the knobs to the predeter­mined frequencies can be effected in a simple way by setting the catch selector switch in the “adjust­position”. In this position three screws become free at the front of all knobs; these screws are then disengaged. All 16 frequencies are next succes­sively adjusted by hand; each time the catch selec­tor switch is brought into the position that corre­sponds with the desired catch. Finally this switch is again put back into its “adjust-position” and then

the screws are lightened.

1

§ 17. Conclusion

EMISSIONPHILIPS

16

il

■

i

I

■

Fig. 30. Application of "Caroussel"-knob in aircraft beacon receiver. Adjusting screw (8) determines position of sliding condenser (1). Motor (10) rotates adjusting disk (5) and knob (7) when another predetermined frequency is chosen. Fre­quency range: 37—39 Me s; maximum number of kc/s per scale division: 4. Accuracy of repeated adjustment of pre­determined frequency: better than -8 kc/s.Inset: adjustment of predetermined frequency of receiver by means of screwdriver (recess 14).

from a distance; the receiver may thus be mounted near the lead-in of the special aerial used.

It is worth mentioning that the remote tuning is effected electrically, which also applies to the equipment for small fighter aircraft shown in fig. 21. The type of electric cable used’ here is far easier to install than a flexible shaft or a Bowden cable for mechanical transmission; besides the latter arc likely to develop troublesome play if they are of some length and moreover they may make certain demands upon the position of the apparatus.

Fig. 30 shows an example of such condensers in an u.s.w. aircraft beacon receiver; the principle of the catch knob designed for this particular pur­pose is shown in fig. 29. The knob is arranged for 12 predetermined frequencies within the range of 32—^0 MHz (decided on internationally for beacon transmitters in 1938). As the adjustment of such receivers is performed by means of a calibrating apparatus and an indicator, the scale is of but secondary importance and has consequently been made simple. The desired frequency may be selected

From the foregoing it will have become apparent that, for almost any sort of application, a suitable design of scale or knob is available. However dif­ferent the catch knobs may be, they have one thing in common: they are always fitted to the front of the equipment and never constitute an integral part of the transmitter or receiver proper. Moreover, the operation of the catch knobs is always kept separated from the tuning proper, so that, for in­stance with a transmitter having several frequency ranges, all predetermined frequencies may, without any restriction, be spread at will over the whole frequency scale of the transmitter.

Thus, without appreciable modifications and cer­tainly without changes of a fundamental nature, each type of equipment may, at wish or as required by conditions, be fitted either with one or more catch knobs or with the “ordinary” hand tuning compo­nents. Therefore, fundamentally identical types of equipment will be available for different appli­cations; this is a matter of importance to the user of the equipment, with a view to the economy, of maintenance (spare parts) and of instruction of the operating personnel.

On the other hand these facts have permitted the development of a limited number of standard types of which the design has been further improved in an economical manner until the present high degree of accuracy has been attained.

I

5 kW TROPICPROOF SHORTWAVE BROADCAST TRANSMITTER

Output, modulation depth

Waverange

2) See Transmitting News Vol. V No. 2 p. 12.

The aerial power, measured at the beginning of the transmission line, amounts to 5 kW.The maximum modulation depth is 100%.

as the general con- calculated to meet

The transmitter is designed for operation on 2 fixed, crystal-controlled wavelengths between 40— 131 in (7500—2290 kc/s). For quickest changeover from one wavelength to the other the tuning con­densers of all the stages, excepting the oscillator stage, are provided with precision spring-catch

17

The construction of the transmitter is based on the same principles which we first applied to design of the 10 kW broadcast transmitter KVFH 10/12 with a view to achieving the most practical and surveyable grouping of the transmitter stages and their elements, easy accessibility of all parts and a stylish outward appearance. The 10 kW transmitter in question T) is now being manufac­tured in series as a standard type and is in operation in numerous broadcast stations, mostly in the tropics.

The ground plan of the 5 kW equipment KV2 FL5/91 is likewise horseshoe-shaped, which allows of complete surveyability from the control desk set up in the middle (top illustration). The trans­mitter stages, and also the mains switching and rectifier part, are separate units mounted in metal frames.

The lefthand part contains the devices for switch­ing the installation on and oil' and also the power supply elements for the valve filaments. The central part comprises, from left to right, the 5 H.F. stages and the modulator. The righthand part contains the elements for feeding the anodes and the screen grids of the transmitting valves, as well as the smoothing condensers and the choke coil.

The individual parts as well struction of the equipment are the heaviest demands of tropical climates.

For instance, the construction is specially damp resistant.

The PB 3/1000 is, for normal use, a so-called “radiation-cooled” valve. Yet we have introduced here forccd-aircooling, because otherwise there is a risk of the ambient temperature becoming too high in the whole transmitter cabinet. It is for this reason as well that an additional fan is built into the rear of the transmitter for drawing out the hot air.

Great constancy of the wavelength

IHigh fidelity

A

LLow non-linear distortionE Low background-noise and hum

Fig. 1. Final stage with pentodes PAL 12/15.

I Required LF input voltage

Modulation 1

>: I

Fig. 2. Rear view of the transmitter with covers removed.

18

The modulation depth due to background noise and hum does not exceed 0'25%.

!

i

Crystal-control and self-oscillation

The oscillator stage can also function as a self-excited stage; changeover from crystal-control to self-excitation is ef­fected bv means of a switch.

devices ensuring greatest precision. The oscillator stage is provided in duplicate; consequently there is a special oscillator for each of the two fixed wave­lengths. The time required for changing over from one wavelength to the other is not more than 2 minutes!

Modulation takes place in the sup­pressor grid circuits of the pentodes PB 3 1000 of the 4th HF stage (see diagram fig. 5). The modulator com­prises 2 pentodes PE 06/40. In the control desk is the modulation pre­amplifier (line amplifier): this is connected to the modulator via a transformer. For greatest possible reduction of the non­linear distortion inverse feedback is provided. This rectifier contains 4 diodes EZ 4.

For frequencies between 30-5000 c/s and a max­imum modulation depth of 90% the non-linear distortion does not exceed 4%.

Provided the voltage at the input of the line am­plifier is maintained constant, the LF component of the rectified aerial current shows a maximum de­viation from the zero level of dB within the frequency range of 30—10,000 c/s at a modulation depth of 90%.

For a modulation depth of 100% a voltage of 6 V is required at the input terminals of the line amplifier (input impedance 600 ohms) built into the control desk.

For the crystal-controlled wavelengths the toler­ance of the carrier wave frequency is maximum

0 005% when the mains voltage fluctuates not more than i^% and the ambient temperature not more than ^IS0 C.

Ima-

power

Fig. 3. One of (he forced-aircooled PAL 12/15 valves icithradiator.

Explanation of the circuit diagram

L

Power supply

Fig. 4. Anode circuit of the power stage with aerial coupling coil.

19

With the exception of the modulation and output stage, all the filaments are AC fed. The high ten­sions arc delivered by valve rectifiers. The primary

Cooling

All the valves are air-cooled. The valves in the last two HF stages function with forced air-cooling by means of fans.

power consumption amounts to 33 kW; the factor is 1 when the condensers for improving the power factor are in circuit.

The transmitter comprises the following stages:1. A twin, crystal-controlled oscillator stage, each

stage with 2 valves CC2. For each oscillator a thermostat with 2 crystals is provided, one of which is kept in reserve. Tn order to prevent possible frequency variations when switching on, both oscillators remain switched on perma­nently.

2. Separator stage with two pentodes PE 04/10.This stage functions entirely without grid cur­rent, so that no reaction on the oscillator stage can occur.

3. First HF amplifier stage with 2 pentodes PE 1/80. This stage operates in class C adjust­ment.

4. Second HF amplifier stage with 2 pentodes PB 3/1000. Modulation is effected in this stage, namely in the suppressor grid of the two valves.

5. Power stage with 2 pentodes PAL 12/15. This stage operates as class B amplifier.

6. Modulator with 2 parallel-connected pentodes PE 06/40. In the control desk is the modulation amplifier (line amplifier) with valves EB4, EBC 3 and EL 5).

Valve complement

The transmitter KV 2 FL 5/91 is equipped with the following valves:Oscillator stage: 2 X CC 2 2 x CC 2.Separator stage: 2xPE 04/10.1st HF amplifier stage: 2xPE 1/80.2nd HF amplifier stage: 2xPB 3/1000.3rd HF amplifier stage: 2xPAL 12/15.Modulator: 2 X PE 06/40.Modulator-preamplifier: EB 4, EBC 3, EL 5 (rectifier valves 2 X EZ 4, 1561)Monitor amplifier: 2 X EBC 3, EL 3(rectifier valves 2 X EZ 4, 1561).Inverse feedback: 4 X EZ 4.HT rectifier: 2 X 506; 4 X DCG 4/1000; 2 X DCG 4/1000; 6 X DCG 5/2500; 2 x 1561; 2 X DCG 4/1000; 3 X DCG 4/1000.

IK1 01

SMjrj»

Fig. 5. Schematical diagram of the transmitter.

L

PHILIPS EMISSION

»

20

HFU HFM• 2PB*ooo\-*\ 2 PAL 12/i5>

| 2PE °tyo

T HFI2PE04to\—\2PE1/80^K2

Q2 \y-{2 CC2\' ^K4 __________________ I

instrument for the average modulation depth and a modulation limiter, with indicator, which limits the modulation to slightly under 100%. The desk also contains the two-stage modulation preamplifier (line amplifier) with amplifier valves EBC 3 and EL 5. In the desk are further housed a monitor amplifier which can be connected to different parts of the transmitter by means of press buttons. Reg­ulation of the control grid voltages of the gasfilled rectifier valves for the anode feeding of the power stage, as well as adjustment of the anode voltage of the power stage, are likewise carried out at the control desk.

A press-button switch on the control desk enables to cut out the whole installation in case of emer­gency; by means of a further press button the 12 kV rectifier can be switched off instantly.

Blocking system and safety devices

The equipment can only be operated when the necessary switching manipulations are carried out in a certain order of sequence. If this is not done the blocking system comes into operation. Conse­quently no harm to the equipment can occur when switching is effected in a careless manner. With a dew to protecting the attendants against the danger of high tension all doors behind which there are live parts carrying a high tension are provided with automatic door contacts. Should one of these con­tacts be interrupted the high tension is cut out in the whole equipment.

Control desk

The control desk contains all the devices for supervising the modulation, such as a measuring

RESONANCE OF CIRCUITS AND LINES

by TJ. DOUMA

Introduction

If

<1 and (rx 4- r2) wC<l ... (2)

• ■ (lb)

when

• • (3a)Zl in which case

Z = Zo = 74 -I- r2t

a)

a>-U-

. . (la)

21

ri + r2 (oL

In the case of the parallel circuit of fig. lb the impedance between points A and B is

The most important equations of weakly damped circuits and lines, and also of a number of simple configurations of lines with weakly damped reactances arc deduced.Furthermore the methods of measurement in connection herewith arc discussed. Particular attention is paid to the relationship between circuits and lines.

I

: (3b)

LC

>0

Tc r B

are small with respect to 1. It will

1 H w2C2(r14-r2)/

In the accepted assumption the imaginary part of the impedance in resonance can always be neglected with respect to the real part.

For the purpose of measurement it is, moreover, important to know when the real and the imaginary part of the impedance are equal.

a weak

§ 2. With the series circuits in fig. la the imped­ance between points A and B is

/ 1 \(ri+jwL) (r2+

=------------------------------ =1

r 1 + r2 + J4- T—p,jcdC

f1 + 44) (1 + ^2>C) \ jcdL;

ri + r2 +j^L + — - jwC

§ 1. In ultra short wave technics the quasi- stationary LC circuits, due to their ever decreasing dimensions, are often replaced by lengths of trans­mission line having similar characteristics. It is therefore necessary to know the equations by which their most important characteristics, as for in­stance resonance impedance, sharpness of resonance and resonance wavelength, are expressed as func­tion of given magnitudes or of those to be measured. It is also essentional to have this information for simple configurations of lines and reactances, which often occur in practice.

In the following article we shall confine ourselves to the so-called “low-loss lines”, i.e. the lines where

r j # ------ and-------- a)Lo coCq

also be assumed that the length is not greater than a few wavelengths at the most. With most of the lines used for radio g can be taken as zero. In order to make the relationship between lines and circuits stand out clearly the most important equations for ordinary LC circuits will be given first.

In resonance the imaginary part of the impedance = 0.

If in equation (lb) to, L and C are successively considered as variable, it will be found that Z passes a maximum (parallel resonance) when

1= LC

In this case

z = zo = L--C(ri+r2)

as will be assumed (being concerned with damped circuit) the equation

L 1Z =----------------------------- .c 1

rl + r2 4- JwL 4- ;jcoC

applies by approximation for fig. lb.If in equation (la) w, L and C are successively con­sidered as variable, it will be found that Z passes a minimum (series resonance)

1

LC

1Z = 74 4- r2 4- ja)L 4~ -—7;

j(i)L

In both(6a)

(4)

(5a)

co-variation :|Z| = |ZJ = . . (5b)

1 1rcsp. Z

L-variation :

• • P)1

a case

C-variation :

ZL

=t= CL<($> V

r1 r2

37360

Fig. 2

thus when u)L = C the

of the value in resonance. The circuit

Q (6)

) applies

Zlft)4

/12

Zo

12

4f

B

b)

dL

Lo

as high. In and Jco, Lo and are known. The

check on each

dLZ — rr r2 ~r ju>oLo - - ■>

Lq

1o)oCo(rY-\-r2)

Q signifying the circuit quality.In principle it is naturally also possible to satisfy

(4) by varying the magnitudes o>, L and C two at a time or even all three together. These cases will, however, not be considered. It goes without saying that

22

1 dCOJqCq Cq

(OqLq

I1§ 3. The condition (4), at a given rT 4- r2, can be

', L and C. If the values indicated in resonance

points can

1

ctjo 2 Lo 2 Co/ho AL AC

also applies.In the neigbourhood of resonance it is possible to

write for (la) and (lb) in good approximation with

resp. Z = —

ri + r2 + j^oCo

for the parallel circuit.Vi ith the parallel circuit, (4) is also the condition

for the imaginary part of the impedance reaching an extreme. This extreme is thus half the resonance impedance.

Up to now the requirement of a low damping for a series circuit was unnecessary, in other words the equations (la), (3a), (4), and (5a) also apply when condition (2) is not complied with. Later, however, it will be assumed that if it is a case of a series circuit, (2) has been satisfied.

zi-o

cases the condition is

r 1 foL — =

(r)CIf this is satisfied, then

|Z| = |Z,| - Z„ ) 2

for the series circuit and

da)Z — ri + r2 2jcooLo

O)o

Lo

ri + r2 + ZjtooLo -O)o

1 dC ri + r2 + J -----T- ’

12 0fcB

a}

,, L° resp. Z = -Co . -

rl + r2 + J '

satisfied by variation of a). having these magnitudes are by the index 0, and if furthermore Af, AL and AC are the absolute value of respectively the frequency difference, self-induction difference and capacity difference between the points, whereby the real and the imaginary part of the impedance are equal (in the case of a series circuit these are therefore the points where the impedance is ZO12, and for a parallel circuit the points where the impedance is Zo 1— ), it then follows from (4) and u)o2 = that

J 2 Lo Co

§ 4. The most usual determination of circuit quality, series resistance and parallel impedance, is based on the equations (6). It therefore pos­sible to apply the frequency variation (= wave­length variation), self-induction variation and ca­pacity variation methods, for a series circuit as indicated for instance in fig. 2a. A constant e.m.f. E with as low an internal resistance as possible sends a current I through the circuit. The current is maximum when the circuit is in resonance and

1—. By varying co, L or

a)Cbe determined at which the current has

1dropped to

impedance has then become | 2 times this way Q is found, provided u)0 AL or Co and AC respectively methods can of course be used as a other. In the following way for instance for a pa­rallel circuit: (fig. 2b) A constant e.m.f. E, via an impedance Z, which is large with respect to the Zo of the parallel circuit (i.e. large with respect to

------- —------- ) applies a voltage V to this circuit, C (rj + r2)which is measured with a voltmeter (T.V.M. for

Q =Q =

of the valueri + r2 = (8) (9)

> too, has become Zo = z0 —

J

(10)

Zo =

With the aid of cd02 they can also be red-

(11)Q2 =

and C-variation methods

be

=±=cZ1C= 2(rl4-r2)oJoCo2 =

r2

J736!

. If, as is so often thebecomes proportional toFig. 3

while if rt 4- r2 is proportional

23

CDq

zlco

(rl ~i~ r2)

wo3Lo2

maximum when the 1

<oC determined

long easily

1

LOCO

1AcdCo

on frequency, Z1C

Q=2^v AC

AC

2coCo22

cdoAC

can now be

tant circuit magnitudes: quality, scries resistance and parallel impedance, are respectively:

2LO AL

(»OAL '(

2cdoLo2 | AL /

This amounts to the same principle as fig- 2a, yet a voltmeter (T.V.M.) instead of a current meter is used.

§ 5. If the co-, L- or C-variation method is em­ployed the simplest equation for the most impor-

uced to other forms.Besides the measured values for Acd, AL and AC

respectively, it is necessary to be acquainted with two of the magnitudes (do- Lo and Co in order to know the three above-mentioned circuit values.

Between Q, Zo and rY 4- r2 there is furthermore the relationship:

instance). This voltage is

circuit is in resonance and thus when cdL — — -

C the points

where the voltage has dropped

in resonance.

to cd, AC is proportional to—In each case AC CD2

By varying co, L or

The cd- and C-variation methods are the most generally employed of the three above-mentioned ones. Yet it appears that the L-variation method is also quite serviceable. As small variable self­induction of known magnitude a section of trans­mission line provided with a movable shunt can serve very well. For wavelengths, which are with respect to the line this represents an calculable self-induction. In most cases the resistance of this section of transmission line, which comes into series with the circuit coil, may be neglected.

Sometimes the L method even offers advantages over the C method, for instance when one wishes to determine rt -J- r2 over a fairly large frequency region. As a matter of fact it follows from (10) that

arc1

to —J 2.In this case the circuit impedance,

I y— times as high. Q

found in the same manner as above.In the case of fig. 2a it is also possible to measure

the voltage across the coil of the condenser, instead of the current I, as in the region under consideration this voltage is practically proportional to I. Thus in the case of fig. 2b the current in one of the bran­ches of the circuit can be measured instead of the voltage V. The constant e.m.f. E in fig. 2a can gen­erally be induced by means of a coil loosely coupled with L, through which an II.F. current passes. Strictly speaking this produces a voltage proportional to the frequency, but this is of little significance for the small frequency region round about the resonance, within which measurement is carried out by the frequency variation method. The methods indicated in fig. 2a and 2b do not offer the sole possibility of measuring Q.

As a rule a method can be based on each of the following considerations. A constant e.m.f. produces on a series circuit in resonance a current maximum (fig. 2a), and on a parallel circuit a minimum for the current to points A or B. A constant e.m.f. E in series with an impedance which is large with respect to the circuit impedance, produces in the case of a scries circuit a voltage minimum in res­onance, and a voltage maximum in resonance in the case of a parallel circuit (fig. 2b).

In resonance a source of constant current pro­duces a voltage minimum in a series circuit and a voltage maximum in a parallel circuit.

Usually the circuit to be measured can be con­nected in series and also in parallel. In practice we generally use the circuit of fig. 3:

Thus if rl 4" r» *s independent1

CD3

case, rx -f- r2 is proportional to | cd, AC becomes1

proportional to —. , cd

- ), there

1

Q

■ ■ (12)ri + r>

• ■ (16)rl + r2 = rs

must therefore also be

• • (13)rl r2 ~ Lozl<o

Zo = • • (11)Zo =

kmay be considered as a series resistance• • (14)

Transmission lines (general)

• • (13)

Zk (18)

24

1

dLdC

(Oq zJ L

2

4 ■

I

Z0

zo

I2I2-I2

2fOo /I C

l*/(zla>)2 (ZL)3 44C”

as per (9), AL in the three becomes respectively pro-

_ 1/r H- jcoLo

I g+ja>Co

now be known.all determined.

with the circuit can for with weakly dam

«p

Instead of the current in the circuit the voltage across coil or condenser can also be measured (fig. 3), whereby the resistance rs may be inserted in scries with either the coil or the condenser. If, moreover, in the circuit of fig. 2b the voltage over the circuit in resonance is and, if after a known resistance Rp has been placed across the circuit the voltage is K2» then

Here it is also possible to measure the current in the circuit instead of the voltage over the circuit.

In the circuit of fig. 3, too, a resistance in parallel be inserted and (17) applied,

ped circuits a parallel resistance Lo

CORP ’If the series resistance and parallel impedance

of a circuit are to be determined according to (16) and (17). Q can then be determined with the aid of (11). The objection to the substitution methods, expressed by (16) and (17), is that at the frequency employed rs and Rp respectively must be known.

2 Q=-

4Z1L(ZJ)2 (AC)* ■

Besides Aco and AL. Co known.b. When Aco and AC

With a

2

L0AC(Ao)2

1Zo —-----

AcoCo

If the two parts of the resonance curve at which the real and the imaginary parts are equal are not determined, but a few other points, it is of course also possible to calculate from them the various circuit constants, but this makes the equations more complicated.

§ 7. Besides the above three indirect methods of

determining rT -f- r2 and Zo (= —------- °-------Y thereTo (^1 + rz)'

is also a more direct substitution method where a resistance of known magnitude is placed in the circuit or in parallel with it.

From the current or voltage ratio in both cases the series resistance and the parallel impedance respectively can then be calculated.

For instance, if in the case of fig. 2a the HF current in resonance is IY and if after insertion of a known series resistance rs it is J2, then

portional to

§ 8. Taking r, Lo, g and Co as respectively the resistance, self-inductance, leakance and capacity per cm of a transmission line then, as is probably known, the characteristic impedance is

are determined, then

1 CoAL(Aco)- 1

r /M/JL)2’ -4CT~ [

4LO

Aco (AC)2

Besides Aco and AC, Lo must also be known, c. When AL and AC arc determined:

are determined:3/"

<>=]/

therefore varies very considerably with the fre­quency. On the high side of the frequency region to be examined IC max become so small that a condenser with a very fine calibration will be required.

With the AL method, above mentioned cases

1 1. —, and frequency independent.

small variable self-induction rx -J- r2 can already be measured over a large frequency region.

§ 6. If two methods instead of one are applied at the same time:a. when Aco and AL

Besides AL and AC, coo mustIn the event Aco, AL and AC are

all the circuit magnitudes can be calculated from the same, in the following way:

]/ 4 Q I /TlAC(Z^)2 j

constant:

m =

z3

[Us

therefore

s)* |Z|2 = (y2 d2) . (26a)

I

Z, = Zk th= R' 4- jX' . . (25b) (20)

therefore

(21) |Z|2= (y*+d*) . (26b)

R =qi^l and 7c«l • (22). . (27a)

X =

with :

II (1 + q?) (1 + </c=) - (1

• -(23)and

(</zI (1 + qf) (1 + </?) +

Zk y—j6with

1 + </c2■ • (24)and

1(1 + 7f-) (1 + 7C2)_

1 + gc2

R' =. . (27b)

X' =Ao- A >

fio B^>J7J66

b)a)Fig- 5

25

\VS/77\ {WM\

ill

Closed line1

Open line1

ch 2al — ch 2al +

?Lzilc2

ch 2al + cos 2 pl ch 2al — cos 2frl

z4-1—

12

12

12

12

/I' 2

and the propagation

I (r + jcoLo) (g + j(oCo). ■ ■ (19)

For the open line (fig. 5b) the following applies:

Z —- Zk cth ml —sh 2al — j sin 2 plch 2al — cos 2 pi

§ 10. For the closed line (fig. 5a) the impedance between points A and B amounts to:

Z = Zk th ml =sh 2al + j sin 2 plch 2al + cos 2 pi

('ll + </c)2 — 4<?c2)8 \

. 'll + <1C" 2

Co

2

equations (25a) and

LoCo

)'roc,

LoCo

tr

coC-o

cos 2 pi cos 2 pi

7 -= [

For R and X. R' and X' (25b) give:

;r.

a=i

r

(oL0

(ll(lc) I LOC(

n J(1 (ll(Jc)^

cl — - co | LOCO

- 7c)28

] (1 + ?(2) (1 + <Zc2) +I Lo \ ( /.' Co (

y sh 2al — d sin 2 pl ch 2al — cos 2 pi /

y sin 2 pl + d sh 2al\

ch 2al — cos 2 pi

y sh 2al + <5 sin 2pl ch 2al + cos 2 pi

y sin 2 pi — d sh 2al ch 2al + cos 2 pi

mx .v /\

§ 9. If m and Z/f arc expressed in a real and an imaginary part, we obtain, after some deduction:

m = a + jp

(1 r We)—

Up to here all the equations are exact.The eqations for the open line follow' from the

closed line by substituting 2.pl + n for 2pi

zi zi

b)Fig. 4

whilst a line section of length x (fig. 4a) is equivalent to fig. 4b and fig. 4c, whereby

Henceforth it is assumed that we arc concerned with weakly damped lines, in other words:

cj

P (0 | LOCO

Z2=.sh tux

Z^ sh jnxmx >Zj — Z/c dll (

R + jX . . (25a)

(1 + gi<?c)

. . (28a)A’

with al^l and cos

. . (28b)R'

rule, except

in the small region wheretoo closely;

when pl

be found from (26a) and (26b)

Z^ytgPl with

for<5 sin 2 pl^2yal or

sin 2pl«2pl (29) Z?&yctfll with

a

becomes. (30a)X^y tg pl with

and

. (30b)X' — y cot pl with t

• • (32)fo =

if plFor

immediate

26

ti

2

whole multiple

2/?Z»a2/2

2yal — <5 sin 2 pl ~~2~sirT2pi

71

1/1 L«CO

aZ«lcos 2pi^a2l2 . . (31a) sin 2fil^>a2l2

same order as

we find

~7c2 4

aZ«l, \ sin 2£Z>«2/2

- <[c* 4

§11. Now al is small with respect to 1 for short,

, so that sh 2al = 2 al and

an open line:

a closed line, (30a) therefore applies

does not approach (2n —1) — too closely (?i = 0,

1, 2,...) and furthermore pl must also not be in the

neighbourhood of 2n- (n — 1, 2,...) ex- 2

aZ«l, cos 2pl^a2l2

i sin 2pl qi2F ’ 2 pi

( aZ«l] cos 2pi^a2l2 . . (31b)

( sin 2pl^a2l2

In both cases pl may not approach too closely

of —. The imaginary part of the 2

wave resistance may be neglected.

7/2-7c24

For the absolute value of the impedance the approximations can for a closed line:

However, it is assumed that r and g are indep­endent of frequency. Assuming that r and g are proportional to co‘\ the following is found for both cases as first approximation for the closed line:

cept when ql = qc- However, as can be seen from (30a) the latter region is very small, namely of the second order, which is the result of neglecting <5.

For an open line, (30b) applies if pi docs not

approach 2n- too closely (n = 0, 1, 2...) and fur­

thermore pi may not be in the immediate neighbour­

hood of (2/i + 1)~ (n = 0, 1, 2,...) except when

ql = qc. However, the latter region is again very small, of the second order, which is also due to neglecting <5.

As is seen, neglection of the imaginary part d of the wave resistance is permissible as a

sin 2 pl----------- becomes of the2 pl

This consequently applies when 2. (31 does not differ too much from zitc, (n = 1,2,...), in other words neglect of the imaginary part of the characteristic impedance can generally not be permitted, except in the special case qi^qc-

For the imaginary part of the impedance as approximations:

weakly damped lines,ch 2al = 1 2a2/2 may be put.

The following applies in this case:

with and sin 2pl^>d2l2.For a closed line, (28a) therefore applies when

pl does not approach (2n -|- 1)

(n= 0, 1, 2...).For an open line, (28b) therefore applies

does not approach 2/i — too closely; (n = 1,2,...).

Often the imaginary part <5 of the characteristic impedance is neglected. From (28a) and (28b) it is seen, that as regards the real part of the im­pedance, this is only admissible for:

2yal 4- d sin 2 pi ~~2^T2pi

§ 12. Next will be investigated how the lines behave in the neighbourhood of the points where pl

71a whole multiple of — •

These are the regions where resonance occurs.For a line it is obvious to take both uj and / as

variable, as these are the magnitudes which are the easiest to vary for purposes of measurement. If we take co in (26a) and (26b) as variable, it will be found by differentiation for both open and closed lines that Z attains an extreme at

1

1 _l ^(Jl ~ (Jc^8

1 1I. . (33)fo = Zo = R

4

. . (38a)I

Zo = R1O = (34)

1 +■I (‘11 + <le)

and for the open line:

fl - (n =

. . (38b)1

Zo = R' =

. . (36)V

(35a)

• • (37)= oil]L0C0 = 2ti

27

With both therefore resonance:

I

A

\co)

13qi2 + 2qiqc + 3cyc2

8

71

4 I LOCO

2ICO

1

\'Loco

n

^f\LfCo

71

M\LoCo

2

ICO

a whole number) .... (35)

the length of line at which the phase (phase resonance).a relation in the form of (32), with

difference in the correction factor.Practically (35) is then obtained.

2I — 71 -

4

= 1/^1 = I Co al

t ILO al = -

2

Now v represents the velocity of the electromag­netic waves in the said medium. With the aid of (36) it is also possible to write for (35)

\LO r C

For a dielectric with permeability p. and a dielec­tric constant £, the following applies by approxi­mation:

of- and /-variation the following applies by rough approximation at

c

]Z£p.

I / Lo 7171

1 COT(zi even: low impedance)

§ 13. Neglecting terms in qi and qc square, the fol­lowing now applies in resonance for the closed line:

= ]/L°

ch

Lo £Co al

<-) therefore represents the number of electrical de­grees of the length of line. Subsequently we shall be satisfied with the approximation (35) as regards resonance frequency and length of line.

1 _2

1

J LOCO

(c = light velocity = 3.1010 cm/sec.)

Taking / as variable, we find by differentiation of (26a) as well as of (26b), that Z attains an extreme at a length /o, for which there applies by approxi­mation:

The error is maximally a term which is quadratic in qi and qc. If it is a no-loss line, i.e. a = 0, (35) ap­plies exactly. In the case of an air-dielectric the approximation is:

If r and g should be independent of co, it then follows from the above that the resonance impedance would be equally high at all resonance frequencies.

If r and g are proportional to co '*, the high im­pedance becomes proportional to co-*'5 and the low impedance to co’/s.

The imaginary part of the impedance in resonance may always be neglected with respect to the real part, both at high and at low impedance.

The ratio between imaginary and real part in resonance both at variable co and I is expressed by a form of the first degree in qi and qc. The equa­tions will not be given, as they are of little practical value.

It must be observed that by resonance is always meant here the frequency or the length of line at which the absolute value of the impedance Z be­comes extreme. As resonance one also defines the frequency or angle is zero

This gives some

4 1

Co UTt qi + qc(?t even: high impedance)

71714 (<?/ + ?c)

(n odd : low impedance)

1htz: qi 4- qc

(n odd : high impedance) (

= ]/—1 Co

where by 2 is meant the wavelength in the sur­rounding medium. Practically p. is always 1, yet e can differ from it.

With (36) one can write for the often occurring form 6) = col]^LoCo :

Lo T Cj

_L + ALo ' Co

I Lq ILO z- R' - \'c. - TI Lo n;= I' Co ~

oddn

> . (42b)'p

ii even»

• • (40)

n— : 2/

(qi 4- <jc 4- i(6a)

(43)also applies.al 4- i {pl-1

'O1

with (la),

(41a)|Z| = |Z,|

similar to that of a series circuit with L

(<// + 7c) •

a

4and rL

1171

L

7t odd• (42a) . (44a)

Open line:

. . (44b)

28

ql 4~ fjc

(Jl + fjc2

ql — fi ~ 2

ICo2 ’

2

§ 16. For the loss with (28) and (30)

parallel circuit, the is similar to that of

20 fg9?=^20

2/

11 (qi + (Jc 4- I4 x

2/

of the line. At a a closed line and

(<7< -I- >lcY'O

7l\71 2/

Z = Zkal + i{pl

i ( pl \

If the line behaves like resonance curve circuit with

al 4- i (pl — n

LoCo

al 4- I (pl — 11

<- and /-variation it is also possible to write

for the expression: al 4“ i (pl

a closed line at pl

2dco\ \

2'

— 71 —2

At co

ZkZ =

al 4- I {pl

y

1171r = -

4

§15. The fact that a line for pl n - behaves as a

series or parallel circuit follows from the behaviour of the impedance, for one finds with the aid of (25a) for This is seen by comparing (42), (43) and (35)

with (1), (7) and (3). From (42) and (43) follow once more the equations (38) and (40).

20tg V = shT 20

and for an open line

„ - •

y \al 4-

\/bI C<

7/ 4- qc

Now, too, the values of /‘and / of the correspond­ing resonance are indicated by index 0. Obviously

fo *o A co Af A/.

1^2At a low impedance, thus n even for a closed line

and n odd for an open line, this becomes:

|Z| = |ZJ = ZJ 2 . . . . (41b)

At a high impedance, corresponding to parallel resonance, (39) is at the same time the condition at which the imaginary part of the impedance at­tains an extreme. The magnitude of the latter is half the resonance impedance.

da) and ell represent the deviation of the value at resonance.

Comparison with (la), (lb) and (7) shows clearly the resemblance to a series and parallel circuit respectively. If the line behaves like a series circuit, the resonance curve at a co variation is

ILO similar to that of a series circuit with L - ■>

2

2ILO ICOT’c = T

-)!n 2<)

) 2?) " eVen'

a parallel

Z= Z*jaZ+4^-«|)(

angle cp of a line section one finds as a good approximation beyond

the regions where resonance occurs:

Closed line:

n % with good approximation:

/ 4 \2’c=tJ

The magnitude ------------- could be called the Qqi + qchigh impedance, thus n odd for n even for an open line, the im­

pedance corresponding to (39) is:

Zo

Zk z --- —al -V i (pl-n

____ Z—al i I £/ _

§ 14. For the purpose of measurement it is, moreover, important to find the points at which the real and the imaginary part of the impedance are evenly high. For this case the condition

2pl — nzi ziz2al...........................(39)

is found, which can be complied with by varying both co and 7. If we call Af and Al the absolute value of respectively the difference in frequency and the difference in length of line between the points on cither side of the resonance, where the real and the imaginary part of the impedance are equal, we then find from (39)

<')o lo 1zko / ! /

y

0,

ana

• • (46)26) ci 20=F Methods of measurement

and " are

ZJ £ C E C

E E

B D

a)Fig. 6

of figs. 6a and 6b

1

of its value

• • (48)

1 c■ ■ (49)

z/ = 2„ (1 - . . (50)

29

26)Zin 26)

generally consequence

Z>/23 B

b)

I LUCO

20 \26) ,-l 20 -

sin 26)/

when co and

r dJ?J6f

the current can

<// 7cqi + 7c

assumed to be independent of

— = vI

In reality the correction caused by the internal self-induction of the leads will be of greater impor­tance. The self-induction Lo per cm is built up out of an external self-induction Lu, which refers to the field outside the conductors, and an internal part Li, which refers to the field within the conduc­tors. Now for a sufficiently high frequency: coLj r (see ref. 4) applies, so that:

Lo = L«(l qi) . .

on the wavelength in the surrounding therefore finds as a good approxi-

In principle it would be possible, with the aid of (50), to determine the value of 7/ from the meas­ured length of line at resonance and the measured wavelength in the surrounding medium (air as rule).

This is, however, very inaccurate, because the distance and thickness of the leads can not be kept constant enough and as a of this greater errors may be made.

Moreover, coLi may no longer be taken as equal to r if r consists for a great part of radiation resist­ance, i.e. the leads must not be placed too far away from each other.

the length of line 1

be made to drop to

in resonance. In this case the fine impedance has thus become I 2 times higher. Now if coo and zlco or lo and Al are known, qi -p qc may be found with the aid of (40). The two methods can serve as a check on each other. In practice the constant e.m.f. E is generally obtained by coupling the line in the manner indicated in fig. 7 to a loop through which an HF current passes.

That the indicated voltage is proportional to co in the small frequency region in which (with the Aco method) measurement is effected, is of no con­sequence. The objection to this method lies mainly in the resistance and the self-induction of the measuring instrument. Both influences may be reduced by shifting this instrument as far as pos-

§ 18. The equation (40) suggests methods of measurement for a transmission line similar to those formerly indicated for circuits.

The cases of figs. 6a and 6b arc analogous to those of figs. 2a and 2b. To a closed line (fig. 6a) of an even number of quarter wavelengths (or an open line of an odd number of quarter waves) a current is applied by means of a constant e.m.f. E with as low an internal resistance as possible. This is meas­ured for instance with an HF ammeter at points A and B or a corresponding value at another place on this line, for example at points E and F. If the line is in resonance the current becomes a maximum.

By varying the frequency or

. (47) 7/ + 7c

made proportional to co'-'*. The a closed line and the lower

extreme at

. . . (45)

as variable, the conditions for an of tgcp then become:

26)

sin 26)

when r and g arc upper sign refers to one to an open line.

whilst furthermore there applies exactly in the place of(36)

I his gives respectively qi and qc for 6) which is as it should be.

As function of /, tg<p attains

tg 20 = 20 .

The first extreme lies at O = 0. For g = 0 and therefore qc 0, a short section of line has also the smallest loss angle in absolute value.

As lines become longer the extremes of tgcp come more and more in between two successive resonance line lengths.

If co is takenextreme

§ 17. From (34) and (36) one might deduce that the wavelength on the line differs only very little from the wavelength in the surrounding medium. The wavelength on the line would namely be slightly shorter than in the insulator in which the line is situated and this is indicated by the factor:

13qi2 + 27/7, 3gc2

8

For the ratio between the wavelength Al line and the medium, one mation:

E C

Fig. 7

3 736 9

b)Fig. 8

of the value in resonance. The impedance at AB

has then therefore dropped to

For the impedances

*

r

(To be continued)

EMISSIONPHILIPS

30

k

reduced by current loop,

A

Z)B

a;

small variable capacities can

best be taken.In the place of the symmetrical circuit of fig- 6b

it is of course also possible to use one single imped­ance Zi at one of the two extremities in series with an e.m.f. E. However, the symmetrical circuit will generally be preferred.

7 o 37366

sible towards a voltage loop. But in this case the capacity of the instrument becomes a source of error, whilst moreover the currents become smaller.

It is now also possible to measure the current instead of the voltage, provided this is not done near a voltage loop.

It is usually advisable to place the voltmeter (for maximum deflection) in the neighbourhood of a voltage loop, and the ammeter near the current loop, whilst in between these positions there is a region where both methods are serviceable.

A current maximum can be determined more indirectly by coupling a voltmeter inductively with the line, a voltage maximum by coupling an ammeter capacitively (figs. 8a and 8b). The ad­vantage of this is an easily variable coupling.

1~ of the resonance Y2

value. With the aid of these measurements qi -p (]c can be found as before. The same method can be applied for an open line an even number of quarter waves long.

The objection to this method now lies mainly in the capacity and losses of the measuring instrument used. These influences are measuring as close as possible to a but at the same time the deflection becomes smaller.

Zi2

In order to load the line as little as possible the coupling in both cases is made as loose as is per- missable in connection with the sensitivity of the measuring instrument. The aim will therefore be to always endeavour to excite the line by as powerful a transmitter as available and to measure with as sensitive an instrument as possible.

Now the power required for a sensitive thermo­couple is of the order of 1 mW, whilst a D.V.M. with 0 01 mW can still give a sufficiently high de­flection. A T.V.M. has a complicated power supply. For this reason we generally use a D.V.M. with button diodes for ultra shortwave measurements. Coupling with the transmitter and the D.V.M. is preferably effected inductively and therefore in the neighbourhood of a current loop. In this way the line or circuit to be measured is influenced to the least possible degree. With all these measurements only the ratios between currents and voltages are of importance; it is not necessary to know the absolute values.

Now with the substitution method the min­imum and maximum impedance of a line can be determined in a similar way to that of § 7. To this end a known resistance rs must be inserted in series with the e.m.f. E in fig. 6a, and a known resistance Rp in parallel at AB in fig. 6b.

Instead of measuring the currents at for instance E and F, the voltage can also be measured over the same points provided they are not situated too close to a current loop, because the maximums of current and voltage practically coincide. This can be proved from the line equations.

With the closed line of fig. 6b of an odd number of quarter wavelengths, a voltage originating from the voltage source E delivering a constant voltage is applied via the impedances Zj/2, which are high with respect to the line impedance at points AB. After tuning, the voltage is measured at points AB with a suitable instrument (for instance a T.V.M. or D.V.M.) The voltage may also be measured at another place, EF for instance, provided it is pro­portional to the voltage at AB. Now by varying co or 1 the measured voltage can be made to drop to

1I 2