PROCEEDINGS OF THE I.R.E.

Microwave Attenuators for Powers up to 1,000 Watts*H. J. CARLINt, MEMBER, IRE, AND E. N. TORGOWt, ASSOCIATE MEMBER, IRE

Summary-A new type of high-power, broad-band probe attenua-tor is described which operates on a capacitance-divider principle. Atypical design reduces input powers up to 1,000 watts, by a fixedratio, to a low level which can be conveniently measured by bolo-metric means. The prototype consists of a probe, a buffer-equalizerattenuator, and a lossy tapered matched load. Two designs are re-quired to cover the frequency band 1,000 to 10,000 Mc with relativelyconstant attenuation and voltage standing-wave ratio below 1.30.

I. MICROWAVE MEASUREMENTS AT HIGH POWERLEVELS

f HE TESTING of microwave equipment at highpower levels is most conveniently done by theuse of conventional test equipment which is de-

signed to operate at nominal power magnitudes. Thetvpe of device described here accepts powers up to 1,000watts average at its input terminals, and at its outputterminals provides a level which is down a fixed multipleof the input power. It has been possible to do this withan input voltage standing-wave ratio less than 1.30,over a broad band, and an attenuation swing which isquite nominal. Such a device is very useful, since itmay be calibrated as an attenuator at low power levels.

CMa2

20

4

I.-

k4

53152 -5150_-49 _

6 8 10 12 14 16 18 20

WAVELENGTH I N CM.

22 24 26 28

It is then applied to. the high-power measurement byterminating the device in a low-level power meter head,which may be a bolometer designed to operate up to,say, 1 mw. Accepting this latter figure as representative,a 100-watt attenuator should produce a power level re-duction of 50 db, and a 1,000-watt unit an attenuationof 60 db. Fig. 1 shows the performance characteristicsof a 50-db unit for the frequency band 1,000 to 4,000Mc. It will be noted that maximum voltage standing-wave ratio (either looking in at the input terminals orin at the output terminals) is less than 1.30, and theattenuation variation is + 1.85 db in 50 from 1,000 to4,000 Mc, or + 1.00 db in 50 from 1,000 to 3,000Mc. Characteristics of other units up to 60 db, andcovering frequencies up to 10,000 Mc are shown inFigs. 2, 3, and 4.

' X, ATi~~9~ZrTTENUATIONIi

so I 1.55dIIr !~bI-~~~ ~ ~ ~~~~~j1-.6 =5o--

7 8 10 12 14 16 18 20 22 24 26 28 30

WAVELENGTH - GM.

-5+IA.85 DB

30

WAVELENGTH -GM.

1.3 -12 JNPUT EN D1.2

Co I II>101 Nf Xa6 8 10 12 14 16 18 20 22 24 26 28 30

WAVELENGTH IN CM

Cr,n)

6 8 10 12 14 16 8 20 22 24 26 28 30

WAVELENGTH IN CM

Fig. 1-Voltage standing-wave ratio and attenuation characteristicsof the high-power 50-db probe-type j-inch line attenuator: fre-quency range, 1,000-4,000 Mc.

* Decimal classification: R396.9XR310. Original manuscript re-ceived by the Institute, October 11, 1949; revised manuscript re-ceived, February 6, 1950. Presented at the 1949 IRE National Con-vention, March 7, 1949, New York, N. Y. This paper was preparedin connection with Watson Laboratories Contract W33-038-ac-13848and Navy Bureau of Ships Contract NObs-28376 with the Poly-technic Institute of Brooklyn.

t Microwave Research Institute of the Polytechnic Institute ofBrooklyn, Brooklyn, N. Y.

1.3~~~~~fRONT END \00

1.0 7 8 10 12 14 16 18 20 22 24 26 28 30WAVELENGTH -CM.

Fig. 2 V;oltage standing-wave ratio and attenuation characteristicsof the high-power 60-db probe-type j-inch line attenuator: fre-quency range, 1,000-4,000 Mc.

II. DESIGN OF THE HIGH-POWER ATTENUATORA. General DescriptionThe high-power attenuator is essentially a combina-

tion of three components. Fig. 5 shows a schematic dia-gram of the device. The first component is a broad-bandprobe. This, as will be seen subsequently, operates as acapacitive voltage divider and effects the major por-tion of the attenuation. The second component is ametallized film attenuator which is connected in tandemwith the output of the probe. This device buffs out the

II -1 _r--

1950 777

PROCEEDINGS OF THE I.R.E.

A52a 51'65049148F- 471!a6

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L3

~~ ~ ~ ~ -ACK END VWR

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0,~~~~~~-

1.0

3 35 4 4.5 5 5.5 6 65 7 7.5

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POWER OUTPUT

, METALUZED GLASSBUFFER-EQUALIZER

IPOWER LOAb

POWER INPUT

L3-RONT END VS.WR.

1.0

3 35 4 45 5 5.5 6 65 7 7.5

WAVELENGTH -CM

Fig. 3-Voltage standing-wave ratio and attenuation characteristicsof the high-power 50-db probe-type 8-inch line attenuator; fre-quency range, 4,000-10,000 Mc.

63 TENUAT1ONZ62 IITe5b

61

~59/~58

3 3.5 4 4.5 5 55 6 6.5 7 75

WAVELENGTH - CM.

Fig. 5-Schematic diagram of broad-band high-power attenuator.

Fig. 9 is a photograph of three high-power attenuatorsfor different power and frequency ranges. The illustra-tion shows these attenuators terminated in broad-bandbolometer powermeters for use in the measurement ofhigh radio-frequency power levels.

B. The Attenuating Probe

Fig. 6(a) is a schematic diagram of the probe con-

struction, and Fig. 6(b) shows an approximate equiva-lent circuit. If the probe inductance is neglected, it can

be seen that the device functions as a capacitive voltagedivider. The probe-to-center-conductor capacitance is

01,. I

WAVELENGTH -CM.

FRONT END V.S.VS

1.0 3 35 4 45 5 55 6 65 7 7.

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Fig. 4-Voltage standing-wave ratio and attenuation characteristicsof the high-power 60-db probe-type 8-inch line attenuator: fre-quency range, 4,000-10,000 Mc.

high impedance mismatch which exists at the output ofthe proble. In addition, this attenuator has an attenua-tion variation with frequency which is in the oppositesense to that of the probe; thus partial attenuationequalization results. The third component of the at-tenuator is a broad-band load which absorbs most of thepower. As shown in Fig. 5, this load is permanentlyconnected in the main line. The operation of the high-power attenuator depends on the characteristics of eachof these components.

TO ZTERMINATIONOUTPUT POWEh

DIELECTRIC _,ROBE LINE

CAPACITANCE~~~~~~C2

RADIAL

LINE -

/INPUT POWER ,;: CAPACTANCE

pi 0~~~~~~,

|1 |TOZoLOAD

MAIN LINE IZI)

(a)

C, L, sPROBE INDUCTANCEo 1

1-i

C2 'lPUT

MAIN LINE LOADTERMINATION

(b)

>Zo

(c)

Fig. 6 (a) Schematic diagram of probe. (b) Approximate equivalentcircuit of probe. (c) Derived equivalent circuit.

small and is represented by C1. The shunt capacitanceconsists of discontinuity capacitance, and the capacitythat exists between the face of the probe line disk andthe milled flat surface of the outer conductor which sur-rounds the hole through which the probe enters the mainline. The length "h" is small compared to wavelength,so that all the capacities may be considered lumped to-gether. The end of the probe line is surrounded bydielectric to increase the shunt capacity, and this si-

778 July

Carlin and Torgow: Microwave Attenuators for Powers up to 1,000 Watts

multaneously acts as a mechanical bead support. Inthe initial calculations of C1 and C2, the discontinuitycapacitance formulas of Whinnery and Jamieson' wereused in addition to the simple parallel-plane capacitanceequation. Final attenuation values were obtained ex-perimentally by adjusting the probe length. Eventhough simplifying assumptions were made for calculat-ing the capacitances and the mechanical dimensionsof the probe line, comparatively little redesign was re-quired. To check the assumed form of the circuit shownin Fig. 6(b), an equivalent lossless four-pole was de-termined from the measured insertion loss character-istics of the probe. To do this, a rational approximatingfunction was determined by interpolation to character-istic curves of the probe, such as that shown in Fig. 7.A network was then realized by the Darlington2 react-ance four-pole synthesis procedure. The form of theresult is shown in Fig. 6(c) and this essentially agreeswith the circuit of Fig. 6(b). The antiresonance in theshunt branch occurs well outside the frequency rangeof operation and may be related to a resonance of theradial line section shown in Fig. 6(a).

ATTENUATION OF PROBE42_

3t_32

0

z0I-4zw

4 ,ATTENUATION OF BUFFER

12 ,

10

tpoo 2,000 3,000 4,000FREQUENCY ( MC/SEC)

Fig. 7-Attenuation characteristics of probe andmetallized buffer, (1-4 kMc).

Referring to Fig. 6(b) and neglecting the inductanceL1, the ratio of input to output power is given by

Pia = 10 log -

P0

= 10 log [i(C Z0)2 + C + 1)] db. (1)

If the normalized probe reactance, 17/ C1Zo, is small

1 J. R. Whinnery, H. W. Jamieson, and T. H. Robbins, "Coaxialline discontinuities," PROC. I.R.E., vol. 32, pp. 695-710; November,1944.

2 S. Darlington "Synthesis of reactance four-poles," Jour. Math.& Phys., vol. 28; September, 1939.

with respect to the ratio C2/C1, then the attenuation isapproximated by

a. = 10 log-= 20 log db. (2)

Thus for 40 db of probe attenuation, the designparameters at the high-frequency end of the band(4,000 Mc) may be chosen as

C2-= 100Cl

1-- = loz0,coC1

(3a)

where the probe impedance is made large with respectto the characteristic impedance of the main line toprevent the introduction of excessive reflection. Then

a4,000o- 40.14 db

al,ooo = 40. 72 db. (3b)

This ideal performance was actually not realized asseen in Fig. 7, which shows attenuation versus frequencyfor the probe alone. The attenuation drops sharply atthe high-frequency end of the band. This is essentiallydue to the probe inductance, which reduces the effectivecapacitive reactance of the probe, [1/c,C1-coL],markedly at the high-frequency end of the band. Thebuffer attenuator discussed in the next section, partiallycompensates for this.

C. The Buffer AttenuatorThe back end of the probe attenuator presents an im-

pedance which is badly mismatched to the characteristicimpedance of the probe line. If the probe is terminatedin a slightly mismatched load, a reflection factor resultswhich fluctuates with frequency (as the reflection mag-nitude and phase of the load vary) and which is noteasily corrected for.

If the load has a voltage standing-wave ratio of PL,and the probe a back-end voltage standing-wave ratioof pS, the maximum variation of output level (due to thepossible phasings of the load and probe impedances) isgiven by

max power levelmin power level1 Pg PL+ db

= 20 log db.PL (P,7/PL) + 1

(4)

To see the order of magnitude of this effect, observethat if p, becomes very large,

AD -20 log PL db (5)p>> 1.

Thus a load of arbitrary phase and of maximumvoltage standing-wave ratio equal to 1.30, may intro-duce a fluctuation in power level of as much as 2.28 db.

7791950

PROCEEDINGS OF THE I.R.E.J

In order to reduce this, a metallized glass attenuator,matched at both ends and designed according to meth-ods described by Carlin and Griemsmann', is placed intandem with the probe output. An attenuator nominallyvalued at 13 db, gives a resultant maximum back endvoltage standing-wave ratio of 1.30. In this case thevariation in attenuation, due to the junction reflectionwhere the buffer joins the probe line, is calibrated intothe over-all performance of the probe-buffer combina-tion. The maximum uncertainty in output level for a

load with a voltage standing-wave ratio of 1.30 termi-nating the buffer may now be calculated by lettingp, = 1.30 and PL= 1.30 in (4). The resultant maximumswing is 0.28 db. Since voltage standing-wave ratio ofload and buffer do not reach 1.30 simultaneously, thereflection error in practice is actually considerably lessthan the above figure.

In addition the attenuation loss of the buffer variesin a direction opposite to that of the probe.$ Fig. 7shows the attenuation curve of the buffer alone. Theresult of adding the buffer to the probe gives the overallattenuation curve shown in Fig. 1, and the total at-tenuation is close to 50 db over the band. The bufferintroduces approximately 0.9 db of equalization.

D. The High-Power Load and Power Ratings

It is desirable that the coaxial load which is placedat the termination of the main line be very well matchedover the frequency band. If the voltage standing-waveratio of this load is PT = 1.30, then the maximum possiblevariation in probe voltage is 1.30. Of course, the load ispermanently fixed to the line as an integral portion ofthe attenuator, so that the effect of this probe voltagevariation is included in the attenuator calibration. How-ever, a large voltage standing-wave ratio gives rise toto an erratic calibration curve of attenuation versus

frequency. The actual load used has a maximum voltagestanding-wave ratio of 1.08 over the band from 1,000-4,000 Mc, and a performance curve is shown in Fig. 8.To achieve this design, a tapered lossy dielectric is

used, as shown in Fig. 8. The dielectric material is a

mixture of carbon and X-pandotite which is capableof absorbing large powers. The design is based on a re-

port by Carlin and Blass4 which considers the solutionof a lossy tapered transmission line.5 From a thermalpoint of view it was found that this type of coaxial loadcould be rated at an average power capacity of 1,000watts if suitably supplied with cooling fins and a blower.Since high voltage tests on the probes indicated break-down powers of 324 kw peak for the i-inch line unit, and86 kw peak for the '-inch line unit, the load becomes the

l H. J. Carlin, and J. W. E. Griemsmann, 'A Bead SupportedCoaxial Attenuator," 1947 Proc. Nall. Elecronics Conf., pp. 79-89.

' H. J. Carlin, and J. Blass, Report R-167-48, "Theoretical At-tenuation Characteristics of a Tapered Dielectric Coaxial Attenua-tor," October 15, 1948. Prepared under Contract NObs-28376,sponsored by Bureau of Ships.

Detailed test procedures on the loads are given in PolytechnicInstitute of Brooklyn Report R-198-49, PIB-142, dated April 1949,Contract No. W33-038-ac-13848, by M. Tanenbaum.

deciding factor for average ratings, and the probe break-down determines the peak power rating of the entire at-tenuator assembly.

LOSSY TAPERED7DELEOTRIO

BEAD SUPPORT

3/8 LINE LOAD40-10,0 MO/SEC

Loa _1~_ y~~_7jcI3 4 5 6 7 a

WAVELENGTH-CM

L0*oo __

6 a 10 12 14 16 1 20 22

WAVELENGTH -CM24 26 28 30

Fig. 8-Characteristics of coaxial high-power loads.

Fig. 9-Photograph of high-power probe attenuators; B-Buffer-Equalizer, D-Bolometer Powermeter, I-Input to Attenuator,L-High Power Load, P-Probe.

CONCLUSIONThe units described in this paper have proven ex-

tremely useful as high-power attenuators, presentingsmall variation in attenuation over large frequencybands, and having input voltage standing-wave ratiosless than 1.30 at all frequencies. As can be seen fromFigs. 1, 2, 3, and 4, the 50-db unit has an attenuationvariation of 51.1 + 1.85 db from 1,000 to 4,000 Mc andthe 60-db unit 59.35 ± 1.55 db over this band. The 3-inch line units show swings of 49.0 ± 2.4 db and 60.751.85 db from 4,000 to 10,000 Mc. This constitutes ex-

cellent performance for attenuators in the microwaveregion.

1.15 -_ _ - 7/8 LINE LOAD_000- 4000 MOC/SEC

uCi _- II

780 July

,70

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