1967, No. 5/6/7 211

,A survey of coaxial and strip-line microwave components

S. J. Robinson and P. T. SaaIer

Introduetion

The universal demand for components having a widebandwidth and small size in the microwave field has ledto a move away from waveguide to coaxial and striptransmission line, and so although the work describedis based on specialized wideband requirements, it hasfollowed recent passive microwave component develop-ment. In particular, considerable recourse has beenmade to balanced circuits incorporating hybrid junc-tions and to the use of double-comb filters (usuallycalled interdigital filters), and in these areas a numberof innovations have been introduced. In surveyingthe work carried out over the last few years in theseLaboratories no attempt is made to list the com-ponents studied but rather a few items which typifythe general trends are described.Due to close tolerance requirements and the existence

of enclosed conductors, the constructional techniquesplay an important part in the design of transmissionline components. These aspects will, therefore, beconsidered briefly, following the discussion of theelectrical aspects.At the end of World War 11, waveguide was by far

the most universally used form of transmission line forconveying signals at microwave frequencies. The fre-quency bands used were usually around 1GHz, 3 GHzand 10GHz and these are still popular frequencies. Thechief application for microwaves in those days wasradar, and a simple radar system worked in general ata spot frequency. There was therefore little requirementfor a transmission line which would propagate a widefrequency range. In fact, rectangular waveguide is cap-able of working over a fractional band of approximately40% of the centre frequency without higher modesbeing set up and waveguides are still used extensively insystems requiring a fairly wide band, e.g. communica-tion links. Nevertheless, waveguides have some severedisadvantages. The working bandwidth is limited bythe phenomenon of cut-off at the lower frequency andovermoding (i.e. the setting up of higher modes) at theupper. Waveguide is large and heavy, especially at lowmicrowave frequencies; a guide capable of propagatingat 1 GHz (wavelength 30 c~) has to be approximately20 cm by 10cm. For these reasons alternative formsoftransmission line were investigated and three basictypes became popular; these were coaxialline, strip-

S. J. Robinson, M.A., and P. T. Saaier, Ph.D., B.Sc.(Eng.), arewith Mullard Research Laboratories, Redhill, Surrey, England.

line (or slab-line) and microstrip. Strip-line consists oftwo flat ground planes with a rectangular section innerconductor (fig. la), in slab-line the inner conductorhas a circular cross-section (fig.lh), and microstripconsists of one ground plane and an exposed "hot" line(fig. le). These lines, when operated in their dominantmode, have no cut-off frequency although the upperworking frequency is limited by overmoding. They arecapable ofworking over much greater bandwidths thanwaveguides and are considerably smaller and lighter.The disadvantages are their high attenuation comparedwith that of a waveguide and the difficulty of making

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Q sFig. 1. The three types of transmission line used for makingmicrowave components.a) Strip-line consists oftwo flat ground planes with a rectangular

section inner conductor.b) Slab-line - the inner conductor has. a circular cross-section.c) Microstrip consists of one ground plane and an exposed "hot"

line.

connections between one component and thenext. lnaddition, microstrip has a drawback which is not pres-ent in coaxial or strip-line. Since it is not entirelyenclosed, microstrip tends to radiate unless the lineis supported on a material having a high dielectricconstant. This causes power loss and interference withadjacent circuits. For this reason microstrip has notbeen studied extensively at the Mullard ResearchLaboratories and no further mention will be made ofit.·Coaxialline makes an excellent transmission medium

for frequencies at least up to 10 GHz, although flexiblecable has a high loss at this frequency. Air-dielectriccoaxial line has been extensively used for buildingmicrowave components, in particular the hybrid ringand devices based upon it, e.g. mixers and phase-discriminators. Work has also been done on microwavefilters which use coaxialline; mainly on low-pass, band-pass and band-rejection types.Strip transmission line did not become popular until

Bates [1] in 1956 resolved the difficulty of calculating

[1] R. H. T. Bates, The characteristic impedance of the shieldedslab line, IRE Trans. on microwave theory and techniquesMTT-4, 28-33, 1956.

212 PHILlPS TECHNICAL REVIEW VOLUME 28

the characteristic impedance of the line from its dimen-sions. Strip-line in itself has few advantages over coaxialline but it is much more convenient for making a classof microwave components which depends on electro-magnetic coupling for its operation. These devicesrequire the close proximity of two or more transmis-sion-line inner conductors so that the electromagneticfield set up on the input line couples across to the near-by conductors. Using this technique a variety of direc-tional couplers (hybrids) and microwave filters can bedesigned. Coaxialline, being totally enclosed, does notlend itself to this arrangement whereas a strip-line con-figuration allows a number of inner conductors to beplaced close together between a pair of common groundplanes. Strip-line has one further important advantageover waveguide or coaxial line. This lies in the facilityto print the inner conductor as a thin copper strip on asheet of solid dielectric. A second sheet of the samethickness is added, both the sheets being clad withcopper on their outer faces to form the ground planes.This technique, which is often simpler to apply than its"thick-line", air-dielectric, counterpart, is particularlyuseful in the production of complicated structures suchas filters and phase-discriminators, especially where a filarge number of the same device is req uired. In the pastfew years a number of microwave components usingthe above techniques have been built at MullardResearch Laboratories and a few typical examples willbe described in more detail.

Hybrid junctions and couplers

A simple hybrid junction is essentially a four portdevice in which the power input at one port splits(usually equally) between two ports, the fourth portbeing isolated from the input. The addition of a re-versed-polarity pair of microwave diodes to the isolatedports of such a device forms a balanced mixer.

The simple hybridjunction in coaxialline or in wave-guide consists of a closed ring with four ports spaced atquarter-wavelength and three-quarter-wavelength inter-vals. A hybrid with a broader bandwidth is obtained ifa phase reversal section is inserted between two ports,the four ports being then symmetrically placed aroundthe ring (fig.2). A signal input at port Pi dividesequally between ports P3 and P4, the outputs being inantiphase. This device has been used as the basis of arange of microwave mixers marketed by the M.E. L.Equipment Company Ltd. A mixer in the band2.5-4.1 GHz is shown in fig. 2.

A more recent mixer design employs the electro-magnetic directional coupler, a solid-line version ofwhich is shown in jig. 3. This type of coupler has theproperty that a signal at port PI divides equally (for3 dB coupling) between ports P3 and P4, port P2 being

R

Fig. 2. Coaxial phase reversal ring balanced mixer. Band 2.5-4.1GHz, isolation 15 dB. The signal input al port PI divides equallybetween ports P3 and P4, the outputs being in antiphase. Cr; andCr2 are crystal diodes. R phase reversal hybrid ring.

Fig. 3. Solid-line ninety-degree coupler (1.0-2.6 GHz). Thiscoupler divides a signal at PI equally (for 3 dB coupling) betweenP3 and P4, P2 being theoretically isolated. The unwanted signalin P2 is typically 25 dB down on the input signal. The outputsignals at P3 and P4 are in phase quadrature at all frequencies- hence ninety-degree coupler.

1967, No. 5/6/7 MICROWAVE COMPONENTS 213

theoretically isolated, although the unwanted signal inport P2 is typically 25 dB down on the input signal. Inrespect of isolation or directivity, coaxial and strip-linehybrids are inferior to their waveguide counterparts.The output signals at ports P3 and P4 (fig. 3) are inphase quadrature at all frequencies and this device istherefore known as a ninety-degree coupler. ]t has abroader working bandwidth than the phase reversalring and can easily cover a 3 : 1 or 4 : I frequencyrange. The application of one microwave diode to sucha coupler yields a single-ended mixer. Such a deviceemploying a printed coupler for use in the band7-11.5 GHz is shown in fig. 4. A balanced mixer isobtained by fitting a 3 dB coupler with two diodes.

Fig. 4. Printed single-ended mixer within the band 7-11.5 GHz.At 9.6 GHz the local oscillator signal isolation is 20 dB and thevoltage standing wave ratio is 1.3.

A variation of the solid-line ninety-degree coupler isshown infig. 5. The two inner conductors pass througha metallic block forming a re-entrant section. This con-figuration was devised by Cohn in America [2J and hasbeen developed in printed form at Mullard ResearchLaboratories. In this case the surrounding metallicblock is replaced by two printed copper fiat sheetswhich are not earthed. The advantage of the arrange-ment is that the small coupling gap between the lineswhich is otherwise required to achieve 3 dB coupling isavoided, hence the mechanical tolerances are lessstringent. The solid-line re-entrant coupler uses slab-line for the feed arms, and a manufacturing methodwhich takes advantage of this is shown by the powersplitter in fig. 6. Here the slab-line inner conductorsare made from ordinary copper wire and are sealedinto the dielectric by electric current heating.

[2] S. B. Cohn, The re-entrant cross section and wide-band 3-dBhybrid couplers, IEEE Trans. on microwave theory andtechniques MTT-ll, 254-258, 1963.

Fig. 5. Slab-line re-entrant ninety-degree coupler (1.0-2.6 GHz).Coupling 3 dB, isolation 23 dB. The two inner conductors passthrough a metallic block forming a re-entrant section.

Microwave bridge circuits as frequency and phase-discriminators

The 3 dB ninety-degree coupler or the phase reversalhybrid can be used as a phase comparator. Inter-connecting five couplers or hybrids with transmissionlines produces a twin-bridge circuit having four outputsfrom one input. The relative amplitudes of the outputsdepend on the line lengths and it can therefore bearranged that the outputs yield the input signal fre-quency without ambiguity. Such bridge circuits findapplications for frequency and phase measurement infrequency metering and interferometer systems. Highincremental accuracy « 1%) can be obtained over

o 3 4 5cm2

Fig. 6. Power splitter consisting ofthree re-entrant couplers. Herethe slab-line inner conductors are sealed into the dielectric byelectric current heating.

214 PHILIPS TECHNICAL REVIEW VOLUME 28

I ! I I I

o 2 3 .4 Scm

Fig. 7. Coaxial phase-discriminator (1.7-2.6GHz) using solid-line,coaxial phase reversal ring hybrids.

wide frequency ranges. Fig. 7 shows a discriminatorfor the 2 GHz band which uses solid-line, coaxial phasereversal ring hybrids. Such a discriminator is extremelydifficult and expensive to produce and in recent yearsprinted techniques and ninety-degree couplers havetherefore been employed. Fig. 8 shows a printeddiscriminator (within the band 2.5-4.1 GHz) with theground plane removed. The inner conductors of there-entrant couplers can be seen, as the auxiliary planeshave also been removed.

Modulators, switches and limiters

The association of diodes with directional couplersleads to a num ber of other wideband balanced circuits.Among these are carrier suppressed modulators, micro-wave limiters and switches. A three-element passivelimiter can be made by fitting diodes to six ports of thepower splitter arrangement shown in fig. 6. The diodesare heavily reflecting at low signal levels but becomematched to the incident signal power at a level of aboutImW. Thus the insertion loss through the device is lowat low level but will rise to about 20 dB at high level.This type of component may be used to protect asensitive receiver from overload by large signals.

A P-I-N diode does not rectify microwave signals butacts as an electrically variable impedance. Circuits in-corporating a pair of such diodes and a 3 dB couplermay be designed to operate as switches, attenuators andmodulators. Such circuits have been constructed and

have achieved a rejection of 20 dB with good inputmatch over the frequency range 2-6 GHz.

Microwave filters

Very broad band filters are difficult to construct inwaveguide, whereas strip and coaxial line lend them-selves well to filter design. A low-pass filter can beformed as a cascade of transmission-line sections whichhave alternatively high and low characteristic impe-dance (stepped-impedance filter).

Band-pass filters are formed by coupling a number ofmicrowave cavities or resonant sections together. Inthese components strip and slab-line find their mostpowerful applications. The most popular forms oftransmission-line filters are:1) Half-wave resonant sections, quarter-wave coupled.2) Double-comb (interdigital) transmission line.3) Comb transmission line.

Fig. 8. Printed phase-discriminator (2.5-4.1 GHz), shown withthe ground plane removed. The inner conductors of the re-entrant couplers can be seen, as the auxiliary planes have alsobeen removed.

The dou ble-cornb (interdigital) filter was derived fromthe half-wave filter by Matthaei [3] and is now extensivelyused. It consists of two sets of resonant fingers (combs),each finger being capacitively coupled to its neighbourand ideally short-circuited at one end and open-circuitedat the other (jig. 9a). This type of filter can be made

[3] G. L. Matthaei, lnterdigital band-pass filters, IRE Trans.on microwave theory and techniques MTT-IO, 479-491,1962.

1967, No. 5/6/7 MICROWAVE COMPONENTS 215

Fig. 9. a) Solid-line double-comb (interdigitaJ) filter. Pass-band 4-8 GHz. Insertion loss 0.5 dB.Voltage standing wave ratio 2.0. It consists of two sets of resonant fingers (combs), eachfinger being capacitively coupled to its neighbour and ideally short-circuited at one end andopen-circuited at the other.b) Typical printed double-comb filter.

a i!ii~ __~ +-__~~ __~ __~.o 2 3 4 5cmIl.. ...

broadband (3 : 1 frequency range) or narrowband (lessthan 10% ofcentre frequency). The limits are imposedby the narrowness of the coupling gaps between thefingers and the width of the fingers themselves. Thelength of the fingers is approximately a quarter-wave-length at midband and this at high frequencies becomesvery short. Since for good operation the length-to-width ratio of the fingers should be high (3 ormore) the ground plane spacing has to be madesmall, which scales down all the dimensions exceptthe fingerlength. Thus it is difficult to make double-comb filters work satisfactorily much above 10 GHz.The situation is improved by printing on highdielectric-constant material since for the same fingerimpedance the width is reduced more than the length(fig. 9b). Matthaei's design equations are approximateand it has been found useful to make these filters (whenprinted) as separate combs of beryllium-copper foilwhich are mounted between sheets of unclad dielectric.It is then fairly simple to move one comb relative to theother to provide some experimental adjustment of thecoupling gaps. The foreshortening of the fingers at theiropen-circuit ends has to be found experimentally; fornarrowband filters, especially above 3 GHz, this is verycritical.

Also due to Matthaei and closely allied to the dou ble-comb filter is the comb filter. This has only one comb,all the fingers being short-circuited at the same end.The other end of each finger must be loaded by capacit-ance in order to produce a pass-band so that combfilters are really only suitable for narrow bandwidths.They have the advantage over double-comb filters thatthey can be tuned over a wide frequency range byvarying the capacitance. One comb filter can, for

b

example, be made to tune to anyone of twenty UHFtelevision channels. The comb filter is particularly wellsuited to the UHF range since the fingers can be madequite short, usually an eighth-wavelength at midband.

Other forms of coaxial and strip-line filter have beenstudied. A line with shunt open- or short-circuit stubsat quarter-wavelength intervals along its length formsa band-pass filter which is particularly useful for braad-band operation. For narrowband performance theimpedance values required for the stubs may be toohigh to be mechanically practicable. A printed band-pass filter (2.5-4.1 GHz) of this form is shown infig. 10,together with a printed low-pass stepped-impedancefilter of the type mentioned at the beginning of thissection.

o 2 J 4 5cm

Fig. 10. Printed stubs and line band-pass filter (left) and printedstepped-impedance low-pass filter (right). Pass-band 2.5-4.1 GHz.Insertion loss 1 dB.

216 PHILlPS TECHNICAL REVIEW VOLUME 28

Construction techniques

High dimensional tolerances can lead to construe-tional difficulties with small microwave components,and careful mechanical design is necessary if they are tobe made using normal machine shop practice.

As far as printing is concerned, the main problem isthat the dimensional tolerances are a smaller fractionof the pattern size than is usual in printed wiring. Theoptical systems in the printing process and dielectricmaterial stability impose undesirable limitations onwhat can be easily achieved.Three techniques for printing strip-line microwave

components have been useful. In the first method, mostconventional but least accurate, a photographic nega-tive is made of a drawing and the print then producedfrom the negative on copper-clad dielectric which hasbeen coated with photoresist material.

In the second method, which is used for the mostaccurate work, a negative is cut by a precision machinein a thick acrylic sheet which has been coated with aphoto-opaque material. An accuracy of a few micronscan be achieved but the master negative may sufferdimensional changes with time and temperature.The third method is very simple; a speciallaminated

material is used in which the negative is prepared bycutting the top layer which is opaque and peeling it off.The accuracy obtained is of the order of 100 (.Lm.

For all methods, considerable experience in etchingthe copper pattern is required for good and consistentresults. It has been found that the copper patterns fordouble-comb filters can be reproduced to an accuracybetter than 25 (.Lmwhere the combs are made as a freefoil and not on clad dielectric. In this case beryllium-copper is used to give the necessary mechanicalstrength.The dielectrics are low-loss irradiated cross-linked

polystyrene or polyolefin. These materials have dissi-pation factors of the order of 0.0001 in the microwaveregion. For narrowband filters the polyolefins arepreferred. as, apart from their somewhat lower loss,they are softer, a feature which is useful if small tuningscrews have to be inserted.

Future objectives

Good performance much above 10 GHz is difficultto obtain in coaxial and strip-line devices. The reason

for this lies in the manufacturing techniques since thesize (diameter or ground-plane spacing) has to be madesmall to prevent overmoding and the reduction in sizeincreases the power loss in the device. In addition, anysmall discontinuities present have a greater effect athigh frequencies than at low since they are not usuallyscaled down in proportion. The situation is aggravatedby the poor length-to-width ratios ofthe line which areinevitable in components in which the line is resonant,i.e. when it is a quarter-wavelength long. One furtherdifficulty, which is inherent in the use of conventionalstrip and coaxial line, is the comparatively poor per-formance of coaxial to strip-line transitions and coaxialconnectors which have to be used. Even subminiatureconnectors become highly reflective at frequenciesaround 20 GHz. Nevertheless, some work at these fre-quencies is now being carried out using conventionalprinting techniques.Some of these basic difficulties are now being tackled

by building miniaturized components on ceramicmaterial. Alumina materials having very low losses anddielectric-constants of about 10 are now available andon these strip-line can be laid down by evaporation.Using this technique the hybrid for a mixer within theband 7-11.5 GHz measures approx. 2.5 mmê. It isintended to build arrays of integrated components inthis way, i.e. mixers, filters etc. laid down on one pieceof dielectric material without the need for separateconnectors.It is a logical further step to adopt a high-resistivity

semiconductor substrate as the dielectric and to adddevices such as diodes and transistors to the circuit bystandard integrated circuit methods. Work is nowgoing on, in co-operation with the microwave devicegroups of Associated Semiconductor ManufacturersLimited, to exploit this approach.

Summary. A short survey is presented of work on coaxial andstrip-line passive microwave components. Most interest has beencentred on the design ofhybrid junctions and directional couplers,microwave mixers, filters and phase-discriminators. Some workhas also been carried out on microwave modulators, switches andlimiters. General considerations for good design technique arepresented and reference is made to methods of manufacture.Results show that with modern printing techniques it is possibleto build smaller and cheaper strip-line components comparablein performance with their coaxial counterparts.