NASA Technical Memorandum 87357

Near-Field Testing of the 30-GHz TRWProof-of-Concept Multibeam Antenna

(NASA-TM-87357) NEAB-FIELD TESTING OF THE N86-2757830 GHz TEW FBCOf-CF-CCNCEPl MUITIEEAMANTENNA (NASA) 11 p HC A02/M* A01 CSCL 09C

UnclasG3/33 43238

Richard R. Kunath, Jr. and Robert J. ZakrajsekLewis Research CenterCleveland, Ohio

Prepared for theAntenna Measurements Techniques Association Eighth Annual Meetingand Symposiumcosponsored by the National Research Council Canada, David FloridaLaboratory, and the AMTAOttawa, Canada, September 23-25, 1986

NASA

https://ntrs.nasa.gov/search.jsp?R=19860018106 2020-04-03T00:44:52+00:00Z

NEAR-FIELD TESTING OF THE 30-GHz TRW PROOF-OF-CONCEPT MULTIBEAM ANTENNA

Richard R. Kunath, Jr. and Robert J. Zakrajsek

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

Near-field testing was conducted on the 30 GHzTRW proof-of-concept (POC) Multibeam Antenna(MBA). The TRW POC MBA is a dual offset casse-grain reflector system using a 2.7 m main reflec-tor. This configuration was selected to assessthe ability to create both multiple fixed andscanned spot beams. The POC configuration inves-tigated frequency reuse via spatial separation ofbeams, polarization selectivity and time divisionmultiple access scanning at 30 GHz.

Measurements of directivity, sidelobe level,and pattern were made at NASA Lewis ResearchCenter's Near-Field Antenna Test Facility. Pre-sented in this paper are complete results of thesemeasurements. Included is a detailed discussionof all testing procedures and parameters. Resultsof additional testing used to evaluate diffractioneffects of the subreflector and-distortions of themain reflector are also presented.

INTRODUCTION

As part of its satellite communications pro-gram, Lewis has been investigating advancedantenna concepts at Ka-band frequencies that sup-port greater frequency reuse. One concept usesmultiple fixed and scanned spot beams to allowfor multiple independent users. This paper willdiscuss the testing of a proof-of-concept (POC)antenna built to demonstrate the feasibility ofthe mulitple beam concept.

MULTIPLE BEAM ANTENNA (MBA) CONFIGURATION

The POC MBA is a dual offset reflector, line-arly polarized antenna using a 2.67 m diameterparabolic primary reflector and a hyperbolic sub-reflector (Fig. 1). This antenna was built insupport of the Lewis Space Communications POCTechnology Program under NASA contract NAS3-22499(Ref. 1). A polarization selective subreflectoris used in conjunction with orthogonally mountedfeeds to enhance frequency reuse.

The feed assemblies contain three types offeeds (Fig. 2). Seven circular aperture dualmode horns are used to create three horizontallypolarized (Cleveland, Atlanta, and Los Angeles)and four vertically polarized (Tampa, Houston,Chicago, and Denver) fixed spot beams. A clusterof 19 rectangular aperture horns are used intriplet combinations to create 22 scanned spotbeams to cover a one-sixth CONUS sector scanned

region. A diplexed cluster of 16 circular aper-ture horns is used to create three contoured spotbeams for Boston/New York/Washington D.C. coverage.This paper will discuss only the fixed spot andscanned spot beam feed testing results.

The fixed spot beam feed assemblies weredesigned to produce a -18 dB illumination taperto yield a 0.3° beamwidth with sidelobes less than-25 dB between 28.5 to 29.0 GHz. To achieve thelow sidelobe levels, the primary reflector surfacetolerance was to be 2 mils rms.

TEST PLAN

Characterization of the POC MBA performancewas accomplished at the Lewis Antenna Test Facil-ity (ATF). A test plan was written to outlinewhich feed configurations would be tested and atwhat frequencies. A complete list of all thetested configurations is given in Ref. 2. Thetested feeds represent those beams which are usedfor alignment purposes, which are aimed far fromboresight or which aid in evaluating minimumcrossover boundaries of beams in the scannedarray.

The bandwidth of the antenna system is 27.5to 30.0 GHz. The fixed spot beam horns weretested at 28.75 GHz, while the scanned spot beamhorns were tested at 29.25 GHz. These frequenciesrepresent the mid-band frequency for each feedelement. Also, pattern change as a function offrequency was tested for three frequencies (29.00,29.25, and 29.50 GHz) for a single scanned spotbeam triplet.

MBA ASSEMBLY AND ALIGNMENT

After the POC MBA was tested at the contrac-tor's site, it was disassembled, packaged, andshipped to Lewis. The antenna was broken intothree parts: the antenna platform, the primaryreflector with its supporting truss, and the feedassemblies and subreflector. The antenna was thenreassembled at Lewis on the near-field range.

After reassembly, mechanical alignmentmeasurements were made. Tooling balls had beenattached to the platform, primary reflector, sub-reflector, and to the boresite mounting fixtureby the manufacturer previous to shipment. Toolingdimensions from platform to subreflector, platformto primary reflector, and primary reflector to

subreflector were measured and checked againstpreshipment measurements. After careful assemblyand adjustment, all of the tooling ball readingswere determined to be consistant with the previousvalues.

The prime focus feed phase pattern was used toelectrically align the antenna perpendicular tothe near-field plane. The horizontal and verticalfeed assemblies were installed on the platformafter boresite alignment. No alignment checkswere necessary as the feed assembly mounting fix-tures were pinned and bolted in place on theantenna platform.

DATA PROCESSING AND RESULTS

Two types of data processing are available foranalyzing antenna data sets at the ATF. Subsetsof the complete data set can be processed by theATF Perkin-Elmer 8/32 minicomputer. Complete datasets, due to the large number of points, are pro-cessed by the IBM 370 mainframe in the LewisResearch Analysis Center (RAC).

CENTERLINE DIAGNOSTICS

For diagnostic purposes during antenna align-ment, a single line of data (usually a centerline)is processed. The output of this processing is aset of three plots: near-field phase, near-fieldamplitude, and far-field relative power (Figs. 3to 5). From the near-field phase, antenna point-ing and feed focus can be determined. From near-field amplitude, aperture illumination can bedetermined. From far-field relative power,antenna pointing can be cross checked and majorantenna distortions can be seen by their effectson the sidelobe structure.

Near-field phase and amplitude measurementsare valuable diagnostic tools. For an idealantenna (i.e., perfectly focused, symmetric aper-ture illumination, perfect feed position, andnegligible surface distortion) a prime focus feedwould produce a flat phase distribution over theaperature and a symmetric (typically cos2) aper-ture illumination. As can be seen from Fig. 3,the phase distribution is an uneven concave curveover the aperture. This suggested an unfocusedfeed as well as a poor surface tolerance. Surfacemeasurements showed that the surface tolerancewas only 5.5 mils rms with a 35 mil peak-to-peakdeviation. Figure 4 shows a symmetric apertureillumination with an edge taper of approximately-20 dB. This coincides with the low sidelobelevel desired. The far-field centerline patterngives an accurate representation of the mainlobe,first one or two sidelobes, and the overall side-lobe envelope. However, the sidelobe magnitudeis generally a few dB higher for a centerlineproduced far-field due to the limited amount ofdata processed.

The complete NxN set of near-field data iscollected using the automated near-field range.A complete description of both the rf and mechan-ical capabilities and performance of the near-field range can be found in Ref. 3. A typical

data set for this antenna system was 200x200points spaced at 2\ centers. Data collectiontimes for these scans were approximately 3 hr/polarization. The probe used was a 15 dB cali-brated horn. From previous sample spacing andprobe tests (Ref. 4), it was shown that theresultant far-field calculated would be insensi-tive to probe effects over the angles of interest(±8°) and therefore probe corrections wereunnecessary.

NxN DATA SET RESULTS

Processing of the complete near-field dataset is accomplished remotely at the RAC. A mag-netic tape of the near-field data is made at theATF and is sent to the RAC. Execution of theprocessing program is performed remotely in theATF via an interactive graphics terminal. Theoutput of the complete near-field data set pro-cessing is a set of near-field and far-fieldplots.

Two types of plots are available after pro-cessing. Contour plots of the near-field phasean amplitude, as well as far-field relative powerare options. Also, cuts through the far-fielddata set in both principle axes and from highestsidelobe peak to mainbeam peak are options.

Figures 6 and 7 are examples of the far-fieldrelative power contours calculated for both thecopolarized and crosspolarized data sets of theTampa fixed spot beam. Highlighted are the -3 dBand -20 dB contours. From the -3 dB contour, abeamwidth of 0.3° is evident, while the -20 dBcontour bounds the highest sidelobe. The magni-tude of this sidelobe is approximately -17 dB.These results are comparable to the results forall of the beams tested. All results were con-sistent with near-field results obtained by thecontractor in terms of beamwidth, sidelobe struc-ture, and gain.

Relative pointing between beams was calculatedand measured. Pointing azimuth and elevationangles for all of the fixed spot beams tested werecalculated from known latitude and longitudecoordinates of the corresponding cities. Beforethe selected fixed spot beams were tested, theantenna was positioned with the Houston beam per-pendicular to the scan plane so it could be usedas an alignment reference to determine the rela-tive beam pointings. Before each beam was tested,the alignment of the Houston beam was recheckedand corrected if necessary. Calculated andmeasured relative beam pointing angles were deter-mined. Using this technique, it was determinedthat the Houston beam was mispointed by approxi-mately -0.4° in elevation.

ADDITIONAL TESTING

Upon completion of the initial test plan,Lewis took advantage of the opportunity to conductinvestigations beyond the scope of the test plan.A brief discussion of this work follows.

Special testing was conducted to improvefocusing, measure reflector surface distortions,and predict diffraction effects of the sub-reflector. To improve focusing, the prime focusfeed assembly position was varied towards andaway from the reflector with respect to its orig-inal position. The best overall focussing wasobtained by moving the feed assembly 3.2 in.toward the reflector from its original position.

Photogrammetry measurements were made toevaluate the reflector surface accuracy. A set of621 targets were applied to the primary reflector.By a triangulation of target position from mul-tiple photographs, the target positions were foundto the nearest mil. The best fit parabola surfaceerror calculated from this measurement was 8 milsrms. An rf analysis of this surface showed side-lobes around -15 dB, consistent with the previ-ously measured sidelobe performance.

Additional testing was conducted to verifytheoretical diffraction predictions. Figure 8shows the predicted and measured response formicrowave radiation diffracted off the MBA POCsubreflector. A feed horn was mounted so that itsradiated field would impinge on the subreflectoredge. A wall of absorber was positioned in frontof the primary reflector so that only the spill-over and diffracted radiation would be measuredon the near-field plane. A complete discussionof this testing can be found in Ref. 4.

CONCLUSIONS

Near-field testing of the POC MBA was con-ducted. Testing revealed that large, high gainantennas can be measured with high accuracy. Inaddition to producing detailed far-field patterns,the near-field approach provides significant diag-nostic capabilities. The alignment of the antennausing the near-field phase pattern is far superiorto any current mechanical alignment technique. Theadvantage of using near-field data to determinefocusing, beam pointing, and aperture illuminationis extremely useful for accurately characterizingadvanced complex antenna performance.

REFERENCES

1. Wong, W.C.; Chen, C.C.; and Duncan, J.W.:30/20 GHz Multiple Beam Antenna for Space Com-munications Satellites. NASA CR-174643, 1984.

2. Reinhart, R.: Test Plan for the TRW MultibeamAntenna. NASA Lewis Research Center, InternalReport, July 1984.

3. Sharp, G.R., et al.: Characteristics and Cap-acities of the NASA Lewis Research Center HighPrecision 6.7-by 6.7-m Planar Near-Field Scan-ner. NASA TM-83785, 1984.

4. Acosta, R.J.; and Lee, R.Q.: Case Study ofSample Spacing in Planar Near-Field Measure-ment of High Gain Antennas. NASA TM-86872,1984.

5. Lee, S.W., et al.: Near-Field Spillover froma Subreflector: Theory and Experiment.University of Illinois, Electromagnetics LabReport No. UILU-ENG-86-2547, May 1986.

ORIGINAL FASt iSOF POOR QUALITY

C-84-0746

Figure 1. - 30 GHz POC MBA in the NASA-Lewis ATF.

ORIGINAL PAGE-ISOF POOR QUAUTY

Figure 2. - Feed assemblies for the 30 GHz POCMBA.

ISO

120-

60-

-§

-60-

-120-

-180.1000 2000 3000 4000

POSITION OF PROBE, mm5000

Figure 3. - Centerline near-field phase for the prime focus feed.

-601000 2000 3000 4000

POSITION OF PROBE, mm5000

Figure 4. - Centerline near-field amplitude for the primefocus feed.

-10 -5 0THETA, deg

Figure 5. - Far-field relative power from centerline near-fieldphase and amplitude for the prime focus feed.

•z.o

2.556 1—7*

2.156

1.756

1.356

.956-.2 .2 .6

AZIMUTH ANGLE, deg1.0

CONTOURVALUE

-1.000-2.000-3.000-4.000-5.000

-10.000-15.000-20.000-25.000-30.000-35.000-40.000

Figure 6. - Far-field copolarized relative power contours for theTampa fixed spot beam.

2.556

-.2 .2 .6AZIMUTH ANGLE, deg

1.0

Figure 7. - Far-field crosspolarized relative power contours forthe Tampa fixed spot beam.

CONTOURVALUE

-29.000-30.000-31.000-32.000-33.000-38.000-43.000-48.000-53.000-58.000-63.000-68.000

FEED

PLANE OF CALCULATIONAND MEASUREMENT

MEASURED

THEORY

Figure 8. - Near-field diffraction pattern of the POC MBA subreflector.

1. Report No.

NASA TM-873572. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle 5. Report Date

Near-Field Testing of the 30 GHz TRW Proof-of-ConceptMultlbeam Antenna 6. Performing Organization Code

505-41-21

7. Authorfs)

Richard R. Kunath, Jr. and Robert J. Zakrajsek8. Performing Organization Report Np.

E-3115

10. Work Unit No.

9. Performing Organization Name and Address

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

National Aeronautics and Space AdministrationWashington, O.C. 20546

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared for the Antenna Measurements Techniques Association Eighth AnnualMeeting and Symposium cosponsored by the National Research Council Canada, DavidFlorida Laboratory, and the AMTA, Ottawa, Canada, September 23-25, 1986.

16. Abstract

Near-field testing was conducted on the 30 GHz TRW proof-of-concept (POC) Multi-beam Antenna (MBA). The TRW POC MBA 1s a dual offset cassegraln reflector systemusing a 2.7 m main reflector. This configuration was selected to assess theability to create both multiple fixed and scanned spot beams. The POC configur-ation Investigated frequency reuse via spatial separation of beams, polarizationselectivity and time division multiple access scanning at 30 GHz. Measurementsof directivity, sldelobe level, and pattern were made at NASA Lewis ResearchCenter's Near-Field Antenna Test Facility. Presented 1n this paper are completeresults of these measurements. Included 1s a detailed discussion of all testingprocedures and parameters. Results of additional testing used to evaluate dif-fraction effects of the subreflector and distortions of the main reflector arealso presented.

17. Key Words (Suggested by Authors))

30 GHz Multlbeam antenna testing

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