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DL-M420 DA 36-039 SC-87475.. NO. OF 100 COPIES

273 131

Polarization Scanner Progress Report

BERNARD J. LAMBERTYm

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EDL-M420Addendum 1

28 February 1962

ELECTRONIC DEFENSE LABORATORIES

P. 0. Box 205

Mountain View, California

Addendum No. 1 to Technical Memorandum EDL-M420

ERRATA

The following change(s) should be made in the original publication.Each corrected page should be marked "Revised 28 February 1962"in the upper right-hand corner under "EDL-M420. "

Page 11: Change first sentence to read "The 1-milthickness is much greater than a skin depth at S-band,but much less than a skin depth at audio frequencies."

EDL-M420

ELECTRONIC DEFENSE LABORATORIES

P. 0. Box 205

Mountain View, California

TECHNICAL MEMORANDUMNo. EDL-M420

5 February 1962

POLARIZATION SCANNER PROGRESS REPORT

Bernard J. Lamberty

Approved for publication .... N. J. GamaraMangerAntenna Department

Prepared for the U.S. Army Signal Research and DevelopmentLaboratory under Signal Corps Contract DA 36-09 SC-67473.

SYLVANIA ELECTRIC PRODUCTS INC.

EDL-M420

CONTENTS

Section Title Page

1. ABSTRACT ......... ....... ............ .... 1

2. INTRODUCTION ....................... 1

2.1 The Polarization Scanning System Usinga Ferrite Rotator ..................... 2

2.2 Polarization Scanning Using Peak-holdingTechniques ........................ 6

3. DESCRIPTION OF THE FERRITE-ROTATORPOLARIZATION SCANNING SYSTEM ......... 6

3. 1 Technical Approach and Experimental

Program ........................... 8

4. SUGGESTIONS FOR FURTHER WORK ........ 18

4. 1 Difficulties Anticipated in Future Work ....... 18

5. SUMMARY AND CONC LUSIONS ............ 19

6. REFERENCES ............................ 20

- ii -

EDL-M420

ILLUSTRATIONS

Figure Title Page

1 A Polarization Scanning System Using a FerriteRotator -- Block Diagram ............... 3

2 Waveforms for Orientation of Angle Analysis ofa Linearly-polarized Incident Wave ........ 5

3 A Ferrite Polarization Scanning Modification toa Conical-scan Position-tracking Antenna --Block Diagram ...................... 7

4 Design Details of a Quadruple-ridged FerritePolarization Rotator ................... 9

5 Photographs of Plexiglas Waveguide Sections ... 12

6 Insertion Loss of Plated (0.001 inch) Copperand Silvered (paint) Rexolite Circular Wave-guide Section ....................... 13

7 Impedance Characteristic of Solenoid Woundon Plated and Unplated Rexolite WaveguideSections ........................... 14

8(a) Photograph of Coil Assembly -- UnassembledComponents ................... . .... 15

8(b) Photograph of Coil Assembly -- AssembledComponents .......... . ............. 16

9 Voltage Ratio of Primary (Solenoid) to Secondary(Coil) as a Function of Axial Position of CoilAssembly .......................... 17

TABLES

Table Title Page

1 Summary of Approximate Properties of theMotorola M-052 Ferrite ................ 10

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EDL-M420

POLARIZATION SCANNER PROGRESS REPORT

Bernard J. Lamberty

1. ABSTRACT.

This is a report of progress in developing apolarization scanner for use with an existingS-band conical scanning antenna. An objec-tive was to provide as simple a system aspossible, both in terms of modification to anexisting position tracking system and in termsof operational complexity. A block diagramof one such system, using a ferrite rotator, ispresented, and preliminary experimental re-sults are described. Some difficulties wereencountered, chiefly due to operation at S-band. Use of a ferrite rotator appears to bemore feasible for use at X-band.

Another system involving peak-holding tech-niques was investigated. This system wasused successfully to track a moving light source.The success achieved shows that the peak-hold-ing technique could be used for polarization scan-ning of microwave signals.

2. INTRODUCTION.

An objective of the work reported herein was to develop a polarizationscanner for use with an existing S-band,conical-scanning, position-tracking antenna system. Consequently, an attempt was made to devel-ope as simple a polarization scanning system as possible, involving aminimum of modifications to existing position-tracking equipment. Ina previous reportl several methods of analysis and synthesis of polar-ization of electromagnetic waves were suggested. Among these was ananalog system, using three receiver channels, and a system using aferrite polartzatior) rotator. In later reports, the pnalog method wasdescribed in detail' and evaluated experimentally.

The analog method, although technically satisfactory, is considered tobe too complex to satisfy the objectives of simplicity and compatibility.Therefore, an Investigation was made of less complex systems. This

- I -

EDL-M420

2. Continued.

report includes a description of progress on an experimental modelusing a ferrite polarization rotator. An additional method using peak-holding techniques, Is suggested. As in the analog method, thesemethods are only concerned with determining the orientation angle ofthe incident wave, and then synthesizing this parameter in a linearlypolarized antenna of a position-tracking system. Axial ratio and senseof rotation are not determined. A disadvantage of the methods describedherein, that was not encountered in the analog method, is the require-ment that the incident wave be CW or have a high pulse repetition fre-quency.

The following section presents a qualitative descritpion of a polarizationscanning system using a ferrite rotator. In subsequent sections, thescanning system is analyzed in greater detail, and its incorporation Intoa specific position-tracking system is described. Results of preliminaryexperimental data are presented.

The peak-holding method and results of some early experiments are

also described.

2.1 The Polarization Scanning System Using a Ferrite Rotator.

A block diagram of a polarization scanning system, using a ferritepolarization rotator, is shown in Figure 1. A conical feed hornand a circular waveguide, both of which support electronmagnetic wavesof any polarization, are connected to a ferrite device which can rotatethe polarization ellipse of an incident wave. The angle of rotation Is afunction of the magnetic field in the ferrite, which is In turn related tothe magnitude of a magnetizing current. Broadband designs of such adevice have been reported at X-band. 4 The ferrite polarization rotatoris terminated in rectangular waveguide. If an arbitrarily polarized waveis incident on the conical horn, and the magnetizing current is varied,the orientation angle of the polarization ellipse is rotated in the wave-guide. When the orientation angle Is normal to the wide dimension ofthe rectangular waveguide, maximum signal is present at the output. Con-versely, when the orientation angle Is parallel to the wide dimension,minimum signal is present at the output. The magnitude of the magnet-Izing current producing minimum output from the rectangular waveguideis then a measure of the polarization orientation angle of the incidentwave.

A sweep generator provides the ferrite with a sawtooth magnetizingcurrent. If used with a bruadband ferrite rotator, the sawtooth currentwaveform produces 180-degree continuous polarization rotation at allfrequencies n the band of operation. Polarization rotation is linear In

-2 -

EDL -M420

Cl-

maUcr,

Insto

II

-3-t

EDL-M420

2.1 Continued.

time, resulting in modulation of the received CW in the rectangularwaveguide. The resultant waveform resembles a carrier which is am-plitude-modulated by a rectified cosine signal. The modulation is100 per cent for linearly-polarized incoming waves and zero per centfor circularly-polarized waves. The relative time position of the mini-mum value of the modulated wave with respect to the flyback of the saw-tooth magnetizing current waveform is a measure of the original orien-tation angle of the incoming wave. Phase comparison is made at thedetector output where the waveform is half of the modulation envelope.These waveforms are illustrated in Figure 2 for a linearly-polarizedincoming signal. The polarization orientation angle of the incomingwave is given by:

a

x 180 " =180 af,

where x = the polarization orientation angle, indegrees (x = 0 corresponds to orientationparallel to the broad face of the terminatingrectangular waveguide),

a = the time from flyback to minimum detector

output, in seconds,

t = the period of the sawtooth, in seconds,

f = the frequency of the sawtooth in cps.

This method is applicable to CW-type signals or pulse-type signals hav-ing a high PRE If pulse-type signals are received, the frequency of themagnetizing sawtooth current should be about one-tenth of the PRF sothat several pulse bursts may be compared on each sweep. If the signalPRF is low compared to the sawtooth frequency, signalpolarization ormagnitude may change appreciably between pulses. The problem of slowsweep rate is compounded where azimuth and frequency scan (scan-on-scan) is necessary in addition to polarization scan.

Orientation of a linearly-polarized transmitting or tracking antenna canbe achieved by using the displacement in time information between fly-back of the magnetizing current and the minimum signal from the de-tector output. Rotation of polarization of a linearly-polarized antenna,In the polarization synthesis portion of the system, can be accomplishedeither by mechanical means or by another ferrite rotator.

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EDL -M420

SWEEP GENERATOR

OUTPUT(MAGNETIZING CURRENT)

DETECTOR INPUT

DETECTOR OUTPUT

Figure 2

Waveforms for Orientation of Angle Analysisof a Linearly -polarized Incident Wave

II

-5

EDL-M420

2.1 Continued.

Minimum output from the rectangular waveguide is used as a referencesince its position is easier to determine than that of maximum output.For a small input signal condition, referencing may be required on themaximum output since the minimum may be obscured by noise.

2.2 Polarization Scanning using Peak-holding Techniques.

An alternate method of referencing to the maximum output value, usingpeak-holding techniques, is also being investigated. The peak-holdingmethod adjusts the polarization of the receiving antenna so that the re-ceived signal is continuously maximized. The controlling adjustmentcan be either electromechanical or purely electrical, as in the ferriterotator method.

An experimental model using the peak-holding technique was construc-ted for automatic tracking of a light source by a photo-sensitive cellmounted on a single-axis antenna control system. The basic principlesof this system are applicable to a polarization control system at lowerfrequencies. Preliminary experiments were successful.

3. DESCRIPTION OF THE FERRITE-ROTATOR POLARI-ZATION SCANNING SYSTEM.

The ferrite-rotator polarization scanning system is intended to scanpolarization of an incident S-band wave, while a conical-scanning an-tenna is tracking the position of its source. The polarization scanningtechnique, described herein, requires that the rotating beam resultingfrom conical scan be adjustable to any fixed linear plarization. Suchan adjustable polarization has been described by Epis" in a report on anovel design of a conical-scanning position tracker. The design des-cribed by Epis enables the position-trackIng antenna to track a sourcewhose emitted signals have an arbitrary polarization. The ferriterotator, to be described, extends the position tracking capability topolarization tracking. In the experimental work, the conical-scan sys-tem designed by Epis was used for convenience.

Figure 3 shows a system using a ferrite rotator with a conical-scan,position-tracking antenna. Note that the conical horn feeds an offsetparabolic reflector so aperture blocking Is minimized. The horn, whichrotates at 1800 rpm (30 cpe),is connected to a circular-waveguide rotatingjoint. The ferrite polarization rotator and circuitry is attached to theoutput of the rotating joint, and the ferrite magnetizing sawtooth currentwaveform is varied at a rate several times greater than 30 cps. Thus,

-6 -

EDL -M420

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EDL-M420

3. Continued.

if the position scanner is tracking a steady signal, the input to the fer-rite polarization rotator is a fixed polarization signal, amplitude modu-lated at the position-scanning rate of 30 cps. The amplitude and phaseof the 30-cps modulation provide position information about the signal.The task of the polarization analysis circuitry is to extract orientationangle information from this signal without destroying the position in-formation. This is accomplished by means of high and low-pass filtersafter signal amplification. The output of the high-pass ftlter containsincident-wave orientation angle information which may be used to con-trol the polarization of another (i.e., transmitting) antenna. The out-put of the low-pass filter contains position information from the conicalscanner which is used to control the tracking-antenna pedestal position.

3.1 Technical Approach and Experimental Program.

Since the modulation frequency of the position tracker is 30 cpa, selec-tion of the polarization-rotator sweep frequency must be chosen forleast interference with the position tracking. Epis has shown that thesignal from a properly-designed conical scanner contains yery littleenergy above the third harmonic of the scanning frequency'. This dic-tates a polarization-scanning frequency above 100 cps which is not anIntegral multiple of the position-scanning frequency. Choice ofscanning frequency is also dictated by other considerations whichwill be discussed later In this section.

The ferrite rotator itself is based on the design of a phase shifter re-ported in Reference 4. The rotator is a quadruple-ridged circularwaveguide shown in Figure 4. A ferrite Investigated for the S-bandapplication was the Motorola M-052 whose characteristics are sum-mar ized in Table 1.

It is difficult to apply an audio frequency magnetic field on the ferriteinside the waveguide. A convenient method of applying the magneticfield is by means of a coil wound around the outside of the wavegulde,since its presence will nct perturb the propagation of the electromag-netic wave within the waveguide. However, if the current In such acoil is varied at a rapid rate, 100 cps or more to produce polariza-tion scanning, the ferrite rotator becomes a transformer, with themagnetizing current coil acting as the primary winding and the wave-guide as a shorted secondary winding. Unacceptably large circulatingcurrents would result when an attempt is made to apply an appreciablemagnetic field on the ferrite. Consequently, it was decided to constructthe circular waveguide portion of the ridged-wavegulde section out of

8 -

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-9-

EDL-M420

TABLE 1

SUMMARY OF APPROXIMATE PROPERTIESOF THE MOTOROLA M-052 FERRITE

Ferrimagnetic resonance line

width at 9300 Mc AH 210 oersteds

Effective g-factor at 9300 Mc geff 2.27

Saturation magnetization at Hdc= 2500 oersteds (200 C) 4xM s 3150 gauss

Relative dielectric constant at 20 Mc er 12

Dielectric power factor at 20 Mc tan 6 0.002

Curie Temperature Te 590" C

- 10 -

EDL-M420

3.1 Continued.

Plexiglas, to insert metal ridges, and to plate the inside of the sectionwith about 1-mi (0.001 inch) of copper. The 1-mil thickness is muchless than a skin depth at S-band, but much greater than a skin depth ataudio frequencies. Thus, the waveguide propagates electromagneticwaves at S-band, but is transparent to the audio frequency megnettzingcurrent. (I -ml of copper is equal to the skin depth at about 100, 000cps. )7 Figure 5 Is a photograph of the Plextglas section before andafter plating.

The characteristics of plated Plexiglas wavegutde were evaluated inan experimental program. Short ctrcular-wavegutde sections wereconstructed of Plexiglas. One was plated with 1-mil copper and anotherwith a thin layer of silver impregnated epoxy. Insertion loss wasmeasured, and the results of these measurements are shown in Figure6. Figure 6 shows that an average insertion loss of about 0. 1 db wasmeasured over most of the frequency band for the copper-plated Plexi-glas.

Next, both an unplated Plexiglas circular-waveguide section and aPlexiglas section plated with 1-mil copper were wound with severalhundred turns of insulated wire to simulate the magnetizing currentcoil on the full-size ferrite rotator. Magnitude of input impedance wasmeasured on the coils of both waveguide sections over the audio fre-quency range, and was found to be nearly identical in both cases (Figure7). A secadary coil was then wound around a smaller phenolic tube.This coil sampled the magnetic field produced by a current in the pri-mary coils along the axis of the waveguides of both the plated and un-plated waveguide sections (Figures 8(a) and 8(b)). The magnetic fieldwas sampled at several audio frequencies to determine whether theplated wavegutde caused appreciably more shielding of the field than theunplated waveguide. Results of the test are given in Figure 9. Figure 9shows the ratio of primary to secondary voltage, as a function of axialpositionof the secondary coil at several frequencies from 30 to 5000 cps.This figure shows that the shielding effects of the 1-mil copper plating arenegligible up to 5000 cpa, as predicted from theory. In fact, differ-ences noted between the plated and unplated sections in Figure 7 and 9,may be attributed to differences in the two primary coils themselves.

Many difficulties were encountered in plating the Plexigisa circular-rided wavegutde. A satisfactory component has been fabricated, andwill be evaluated in the near future. In the interim, an all-metalquadrupally-ridged waveguide was constructed and tested with staticcurrents applied to the magnetizing coils. The purpose of the tsts

- 11 -

EDL-M420

(a) Before Plating 61021402 (b) After Plating 61092811

Figuare 5

Photographs of Plexiglas Waveguid. Sections

- 12-

EDL-M420

0.9 - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

0.8

0.7

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FREQUENCY -- Mc

Figure 6Insertion Loss of Plated (0. 001 inch) Copper and

Silvered (paint) Rexolite Circular Waveguide Section

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EDL-M420

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EDL -M42O

LEGEND

PRIMARY (SOLENOID): 3"' IA,6"t LONG, 100 TURNS/INCHSECONDARY (COIL) :2.5"1 1.0., 0.75" LONG, 100 TURNS/INCH

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Voltage Ratio Of Primary (Solenoid) to Secoadary(Coil) as a FunCtion of AMdal Position of Coil Assembly

-17 -

EDL-M420

3.1 Continued.

was to determine the phase shift as a function of magnetizing currentover the intended frequency range of operation. Unfortunately, theferrite selected (M-052), produced an extremely high zero-field inner-tion loss (about 30 db for 12 inches of ferrite rod). An applied field ofabout 75 oersteds reduced this loss by only 3 db.

Existence of the high zero-field loss is disconcerting. Similar experi-ments at this laboratory and elsewhere 4 have indicated that such a highloss is not usually encountered. Although the other referenced experi-ments were performed at X-band, it seems reasonable to expect thatferrite material should be available which would provide satisfactoryoperation at S-band. Consequently, use of other ferrites for this ap-plication is under investigation. Previous experiments have also shownthat tolerances in ferrite devices are extremely critical. Tolerancein ferrite devices is also being investigated.

4. SUGGESTIONS FOR FURTHER WORK.

Use of ferrites other than the M-052 will be investigated for the polari-zation rotator application. Particular attention will be given to thosehaving low zero-field loss.

In this experimental work, the solenoid used to apply the magnetic fieldto the ferrite had a large air gap. Consequently, use of a shorter coilwith a higher number of turns should be investigated to determine ifthe higher resultant magnetic field intensity, and a smaller piece offerrite, will be more successful in reducing zero-field loss, and stillprovide sufficient rotation to waves passing through the polarizationrotator.

Effects of ferrite tolerance on operation will also be Investigated. Tol-erance has been found to be an important factor in other ferrite applica-tions.

Once a satisfactory ferrite-solenotd combination Is developed, frequencydependence must be evaluated. Following this, the necessary ac waveformsto produce a linear 180-degree polarization rotation must be determined.and evaluated.

Development of the peak-holding technique will continue.

4.1 Difficulties Anticipated in Future Work.

Two main difficulties are anticipated; both are associated with the inten-ded application of the system in S-band. These difficulties are:

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EDL -M420

4.1 Continued.

(a) Characteristics of ferrites are not the sameat this frequency as they are at higher fre-quencies. Insertion losses are usually higherand polarization rotation with magnetic fieldis usually smaller at S-band than at X-band.

(b) The solenoid required to impose a magneticfield on the ferrite has a large air gap. Thisis the consequence of the large diameter ofS-band waveguldes. Consequently, it is dif-ficult to apply a magnetic field sufficientlylarge to cause an adequate polarization rotation.

In spite of these difficulties, the ferrite rotator method for polarizationscannng has the great advantage of being simple in design and operation,and should be brought to a successful completion.

5. SUMMARY AND CONCUSIONS.

The foregoing has been a progress report on a ferrite polarizationscanner to be used with an S-band position tracker. Experimentalinvestigations have reached a stage where a further investigation ofavailable ferrites is required.

The proposed method to achieve polarization scanning appears theoreti-cally feasible. However, certain difficulties have precluded an evalua-tion at this time. These difficulties seem directly related to the attemptto build the system at S-band. The described system could be construc-ted with relatively little difficulty at higher frequencies, for example, atX-band, but considerably more work is required before satisfactoryoperation at S-band is achieved.

This report has been concerned with two methods of polarization scanning,namely the ferrite rotator and the peak-holding method. Other methodscould also be used; in particular, the analog method. A disadvantage ofthe analog method, however, is its requirement of three nearly-identicalreceiver channels for effective operation. The analog system has beenevaluated elsewhere. 2 In contrast, the systems described herein aresimple, both in terms of operation and in terms of required modifica-tions to the system for which they are intended.

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EDL-M420

6. REFERENCES.

I. B. J. Lamberty, "Study of Polarization Analysis and Synthesisof Electromagnetic Waves", Technical Report No. EDL-M250,Electronic Defense Laboratories, Mountain View, California,29 February 1960.

2. B. J. Lamberty, "The Analog Method of Polarization Analysisand Synthesis for Arbitrarily Polarized Incident Waves", Tech-nical Report No. EDL-M366, Electronic Defense Laboratories,Mountain View, California, 11 September 1961.

3. C. Elfving, R. Franks, B. Lamberty, "Methods of AutomaticAnalysis and Synthesis of Polarization of ElectromagneticWaves", The Microwave Journal, Vol. 3, No. 11, November 1960,pp. 57-63.

4. H. N. Chait and N. G. Saklotis, "Broadband Ferrite RotatorsUsing Quadrupally Ridged Circular Waveguides", IRE Transac-tions on Microwave Theory and Techniques, Vol. 7, No. I,January 1959, pp. 38-41.

5. J. J. Epts, "A Conical Scanning Antenna of Novel Design", Tech-nical Report No. EDL-M348, Electronic Defense Laboratories,Mountain View, California, 27 February 1960.

6. J. J. Epis, "Fourier Analysis of Error Signals of ConicallyScanning Antennas and Applications to J*mpulse", TechnicalReport No. EDL-M273, Electronic Defense Laboratories,Mountain View, California, 10 June 1960.

7. S. Ramo and J. Whinnery, "Fields and Waves in Modern Radio",Second Edition, John Wiley and Sons, New York, February 1958,p. 238.

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