a new precision x-band phase-shifter-enk

8/8/2019 A New Precision X-Band Phase-Shifter-EnK

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A NEW PRECISION X-BAND PHASE-SHIFTERE. F. BarnettHewlett-Packard CompanyPalo Alto, California

A microwave phase shifter having alinear calibration and capable of shiftingthe phase through any number of cycles con- -330 0tinuously was described by A. G. Fox in1947.1 This device consists of two 900differential phase sections (quarter-wave 06sections) in round waveguide with a ro- /tatable 1800 differential phase section(half-wave section) between them. An in -w.ocident linearly polarized wave is con- RTER*WAVE SLABverted to a circularly polarized wave bythe f irst quarter-wave section. The half-wave section reverses the sense of thecircular polarization and introduces aphase-shift which depends linearly on theangle through which this section is ro- ---825tated, each degree of rotation corres-ponding to two degrees of phase shift. /The second quarter-wave section converts ___e_______th e reversed circular polari'zation backto linear polarization. l_l___l

Although a number of phase shifters oof this type h1v3 been developed for spe- | (EE) (b) HALF-WAVE SLABcial purposes, *f there has been a needfo r a general purpose lnstrument of thistype for use in microwave test procedures. Fig. 1 - Poyethlene slabs used in differentialThe instrument described in this article phase sections.is designed to cover the entire usefulfrequency range of X-band waveguide, from8.2 to 12.41 kmi/s. With this object in forked taper at the other end. This de-view, quarter-wave and half-wave sections sign was chosen for convenience in mount-having good performance over this range ing the slab in the waveguide and to per-were designed. mit on e end of the sl ab to dovetail intothe adjacent forkld taper of the half-Differential Phase Sections wave slab, as will be shown later.One of the simplest types of differen- It was not possible to choose thetial phase sections which give good per- over-all dimensions of the quarter-waveformance over a fairly broad bandwidth con- and half-wave sections for optimum per-sists of a section of round waveguide formance, because these dimensions wereloaded with a slab of dielectric material determined in advance by practical con-across a diametral plane of th e guide. siderations. It was decided to use theA number of such slabs was made and tested, same mechanical components as were usedvarying the shape and the material to ob- in an availaple X-band attenuator of thetain optimum performance. As this inves- rotary type.4 This limited the totaltigation was mainly empirical, and as the length of round guide containing theresults did not differ significantly f om tbree differential phase sections tothose obtained by other investigators,5 about 8" and fixed the internal diameter

only the final design arrived at for the of th e round guide at 1".dielectric slabs will be described. Figure 2 shows the ellipticity of theFigure la shows the dielectric slab wave emerging from the quarter-wave sec-used in the quarter-wave section, and tion of Figure la when th e incident waveFigure lb, that used in the half-wave is linearly polarized at an angle of 450section. The material used in the final to the dielectr,ic slab. The ellipticitydesign was polyethylene (E 22.25), ratio is fairly low over most of th e fre-materials with low dielectric constants quency band from 8.2 to 12.4 kmc/s, buthaving been found to give the best per- there are sharp peaks at 10. 7 and 11,7formance over a broad frequency band. kmc/s. These peaks are due to the sec-TJhe quarter-wave slab has a single pyra- tion Of guide containing the dielectricmidal matching taper at one end and a slab acting: as a resonant cavity for150

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2.4 th e fundamental (TEj1) mode with the di-______ ____ - electric tapers. The field-patternso2.2 _ashown are those for the air-filled round>2.U - guide. Th e modes will, of course, beE2.0_,_. _ greatly modified by the dielectric alab,2 1_8 but the principle of excitation may never-theless be underatood qualitatively. IfE ______ ____ _______ the TE11 and TM1l field patterns are1E6 superimposed, the transverse components

IA _ l of the electric field will add in the re-LL | ,gions marked A, which are air-filled, ando 1.2_L _ _ subtract in th e regions marksd B, which< _ _ _ / [V ar e dielectric-filled. Tus, the resul-t2 V /tanttransverse component of the field*Vt3 10 11 12 13 will be relatively strong in the air an dFREOUENCY IN KILOMEGACYCLES PER SECOND weak in the dielectric, as required by the

Fig. 2 - Ellipticity produced by dielectric-slab boundary condition at the air-dielectricquarter-wave. interface.Some attempts were made to reduce thehigher-order waveguide modes. The small coupling int1oe M1l mode by changingpeak at 9.8 kmc/s is also due to a re- the shape of the matching tapers, butsonance of this type. These resonances without success. From Figure 3b, it canhave heretofore limited the useful band be seen that the component of the electricwidth of this type of differential phase field of the TM11 mode in the directionsection, of propagation has two maxima in the planeof th e dielectric slab. By insertingIn order to find ways of suppressing strips of resistive material in these po-these resonances, the modes responsible for sitions in the slab (Figure 3c) it wasthem were investigated. By examining the possible to suppress the resonance at 11.7field patterns of the first few round- kmc/s while introducing relatively littleguide modes in order of increasing cut-off attenuation for the TE1 mode. Neverthe-frequency, it is possible to make a rough less, the attenuation Introduced by thisguess as to how they will be modified bythe introduction of a dielectric slab.Taking into account the fact that the pre..sence of the slab removes the degeneracy r -which exists between different orientations A' &i8of some round-guide modes, it was found wthat there are just three modes which could -2account for the observed resonances. Two (a) Fundamentol(TE,,)Moe

of the-se are modifications of the TE2mode, and the third is a modified TMlmode. The order of their cut-off fre-fr- -quencies suggests that th e first two modescan be identified with the observed re-sonances at 9.8 and 10.7 kmc/s, while theTMI mode accounts for the resonance at11.l;|7 kmc/ s . (bl TM,, MODEStrips of Resistive MateriolIt can be seen from the field pat- Inserted Hereterns of thbse modes that the symmetry of /the TE 1 mode in any orientation is such /that i can be coupled to the fundamentalTE 1 mode only by asymmetrical discontin-uiies in the guide, wfiereas the TM Ic} Dielectric Slab

mode can be excited even by symxretrl aldiscontinuities, such as the tapered ends Fig. 3 - Excitation of TM,,-mode resonance by diel-of the dielectric slab. This suggests electric tapers.that the excitation of the resonances ob-served at 9.8 and 10.7 kmd/s is due to resistive loading was sufficient to in-slight accidental asymmetries. By more crease considerably the insertion loss ofcareful alignme t of the slab, it was the phase shifter constructed from thesepossible to suppress these two resonances components, and the decision to introducealmost entirely, while the strong reso- loss in this way was made only becausenance at 11.7 kmc/s was almost unaffected, most of the applications envisaged for thephase shifter are not affected by th e pre-Figure 3 s#hows how the TM11 mode sence of a small insertion loss8 (of th eresonance is excited by the interaction of b order of 1 db).151



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In addition to th e resonance peaks in tion because the latter were not practicalFigure 2, there is a gradual increase of the to make in this case.lli.pticity toward the high end of the band.This is due to an increase of the differen- Two pairs of posts were inserted intial phase shift of the dielectric-slab the wal3l of the quarter-wave section inquarter-wave section with frequency. One the plane of the electric field of the in-method of compensating for this ef^ct Is to cident wave. Their purpose was to improvechange the cross-section of the guide in the match of the section, and their designsuch a way as to lower the cut-off.fequen- was entirely empirical.cy of the mode polarized with its electricfield parallel to the slab relative to that The performance of these differentialof the perpendicular mode. A small change phase sections is shown in Figure 5. Thein the cutoff wavelength of any waveguide two solid curves in Figure 5a show themode produces a change in the propagation differential phase shift of the quarter-constant (i.e. in th e phase shift per unit wave section before an d after the additionlength) iwich is proportionaZ to the guide of the compensating circular recesses.wavelength, and hence decreases steadily (The resistive-strip resonance suppressorswith increasing frequency, as required. were used in both cases.) T1he range ofvariation of the differential phase shiftThe quarter-wave and half-wave sec- over the band from 8.2 to 12.4 kmi/s wastions were compensated by modifying their reduced from 170 to 110 by the addition ofguide cross-sections as shown in Figure the recesses. It appears that even more14. The quarter-wave section was loaded compensation might be obtained by increas-periodically with a series of circular ing the depth of the recesses and reducingrecesses in the guide wall lying in a the length of the slab. The differentialplane perpendicualr to that of the slab, phase shift of the half-wave section withThe half-wave section was loaded with a the compensating grooves is shown by thepair of grooves in this same plane. The broken curve.effect of the circular recesses ca n be cal-culated with the aid f available micro- Figure 5b shows the attenuation ofwave design formulas,5 and proves to be a the quarter-wave section for a wave withdifferential phase shift which decreases its electric field parallel to the dielec-with frequency even more rapidly than the tric slab. This attenuation increasesguide wavelength. The grooves in the cen- with frequency to a maximum of about 1 dbter section give a change in the differen- at 12.4 kmi/s. The attenuation of thetial phase shift proportional to the guide quarter-wave section for a wave polarizedwavelength, as explained above. They were perpendicular to the slab was less thanused in preference to circular recesses 0.2 db at any frequency in the band. Theof the type used in the quarter-wave sec- attenuation of the half-wave section was

45 , , - ()C"8.3?5

(a) QUARTER-WAVE SECTION

~~~~~~ ~~~~~~~~~~~~~~.100 I ~

SECTION'A-A'L ~~~~425(b) HALF -WAVE SECTION

Pt.g. 4~- Compensated differential phase sections.152



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01 COMlPENSATED -:90 Tilhe maximum ellipticity ratio now occurs


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kmi/s. The cqlibration was made by a with th e center section rotatect 'o givebridge technioue s.milar to that describ- is shown in the upper curve in Pigure 7d.ed by P. G. Smith. 0The measured error The maximum value is about 1.5 db at 12.4was largest at 12.4 krnc/s. The cali- kmc/s. A residual peak near the resonancebration curve for this frequency is shown at 11.7 kmc/s is evident. The peak-to-in Figure 7a. The origin of the phase- peak variation of insertion loss withshift scale is arbitrary, so that the phase setting shown in the lower curve insignificant quantity for specifying the Figure 7d reached a maximum value of 0.25maximum error is the total range of db at 12.4 kc/s. Below 10 lc/s it wasvariation of the phase error, designated nowhere more than .05 db.It seems likely that the performanceERRORIN P1ASE.5URATON AT . .of this phase shifter could be furtheri ERRORIN PHiASE CALIBRATION AT 12.4 KMC/SEC0SEC impr6ved by refinement s in the component

z __lt differential phase sections, In par-F)0 /icular, the cifferential phase shift[C could be made still flatter over the fre-0 27 (a) quency band using the compensation tech-> 0 90 ISO 270 360 niques described. It mnight al180 beNOMINAL PHASE SHIFT IN DEGREES E2.0 possible to reduce th e insertion loss by__ designing the tapered ends of the quarter-0 .c- wave slabs so as to couple less stronglyto the undesired TM11 mode. However,0 9 032 13 practical experience in the quantity pro-13 1duction of this type of phase shifter has1.2 shown that some of the design parameters.2 ANY PHASE SETTING are difficult to control to the accuracy1.0l3- !i (C) 2 3 required, and these difficulties limit theperformance attainable at present.

MAXIMUM RiSERTON LOSS FOR ANY PHASE SETTING Biblio2j apbyA. G. Fox, "An Adjustable WaveguideWu~ ~ ~ OISRT2 OS IHPhase Changer"RANGE OF VARIATIO OF IWRTN LOSS wITH PHS SETT1 lPheGe___._ Proc. I.R.E., vol. 35, pp._ g > 10 11 12 1489-1498 (December 1947)FREQUENCY IN KILOMEGACYCLES PER SECOND 9(d) 2. E. M. Pur cell et al. U. S. Patent No.2, 07 849. Filed OctoberPig. 7 -Performance of phase shifter. 2, 1943

3. R. M. Brown and A. J. Simnons, "Di-by 1, in the figure. Tis is the maximum electric Quarter-wave anderror which can oc ur in measuring any Hlalf-wave Plates in Cir-phase difference., is slightly less cular Waveguide"than 20 at 12.4 kmc/s. The error varies Naval Research Lab. Reportfairly slowLy wit-h phase setting, so that #4218 (November, 1953)a smaller error can be specified forsmaller phase shifts. 4. B. P. Hand, "Broadband Rotar Wave-guide Attenuator'The maximum error is shown as a func- Electronics, vol. 27 pp.tion of frequency in Figure 7b. The error 184-5 (January, 19543is greatest at the high end of the bandwhich correlates with the performance of 5. C. G. Montgomery, R. H. Dicke and E. M.the components. Below 10 kmc/s, the meas- Purcell, "Principles of Microwaveured error is well under 10. It will be Circuits"noticed that the error in the calibration Radiation Lab. Series,of the phase-Shifter is considerably small- vol. 8, p. 296, McGrawer than the errors in the differential Hill Book Company, New Yorkphase shifts of th e individual components.This Is the result of a theorem pointed 6. P. G. Smith, M. I. T. Master's Thesis,out by Simmons which states that the de- 1948partures from linearity of the calibrationof a phase shifter of this type depend 7. A. J. Simmons, "Errors in a Microwaveonly in the second order on the errors in Rotary Phase Shifter"the individual components. Correspondence in Proc.I.R.EE., vol. 40, p. 869The input VSWR of th e phase-shifter (July, 1952)

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a new precision x-band phase-shifter-enk

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