Research and Development Technical Report

2* ~ ECOM- 4395

AN ANTENNA FOR COMB INED SURVEILLANCE AND FOLIAGE:j . PENETRATION RADAR -COMSFOR

John J. Borowick

t Combat Surveillance & Target Acquisition Laboratory

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DITIUIN0TTMNAppove fo pulcrlaeditiuinu lmtd

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NOTICES

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Disclaimers

The findings in this report wre not to be construed as anofficial Department of the Army position, unless so desig-nated by other authtw ld documents.

The citation of trade names %nd ntmes of manufacturers inthis report is not to be cout*trued as official Governmentindousement or %pptoval of commercial produc ts or servicesreferenced herein.

Disposition

Ir Destroy this report when it is no longer needed. Do notf /return it to the originator.

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SE~~uRI5. CLASS1FI F1CATION/ DOWNGRAGE (hDING

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UNCLASSIFIEDI

SECURITY CLASSIFICATION OF THIS PAGE(Whe DMS 3.Aued)

frequency bands. A separate L-band corner reflector autenina design vbdch canbe mwwwted on a lightveigbt telescoping mnat is also described. The airteimahas demonstrated excellent g~in and pattern cbaracteristics for its siz~e.\

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2. 'M'-asu.reci radiat-on pattern, L,>-band Arr-avy, sin and d-ifferenceseed, 11230 'liz, ----Dane6-easured radiation Pattern, L-band ar-,15 i Hpne74.11eassured radiation pattern, single dipole/director elemant

5. * eas-ared z~pedan:we, single dipole/directcr element arer X-band

'7. ' pdance, stra.ip~na hvbr',-i -h5-h o~a

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M*easuired radiation pattern, ):-band antenna withou L-banda nt Ienn in place, 9.2 G;,z, E plane 12%!easur.Led adi-ati on patt-ern, X-.band antenna withou L-bandantenna in place, 9.2 Gi~z, H plane 1

Measued medanc , X-band antena withan withou L-bandJ

10 Measured radiation ptten -adcre rfetratnasunte and plaern 9feed~, E25 plzne p 14

11 easured radiation pattern, L-band cre elco antennawt Lbn1250nn :L- pae,93 z, H plane 19

1.Measured impedance, L-band conterna reletoh andwthonna -an

1.L-band Corner Reflector Antenna gi217

1-4."leslied aditionpatern L-andcornr rfletorantnnasu.adCfeecfed 20Mz ln 115. ..esure railaion attrn, -bad cmer eflctorantnna1250 *-iHpae1

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. ;X.a.e> fadtI oLr. pat"- ern, - id arraLy, 1250 L ?z,, H plane 7

4. raa,•ed._iatý_on pat-ýer-., single dipole/-director elements:=-m.etrica.ý~ placed over X-bad aperture, 1250 MHz, E, H plane 8. .epole/dlrectcr element o.er X-band

"..asured =.- nc, comee L-nd antenna over X- d -10ue 4:.s'as-,red _z.pedance, stri:iii.7 h'.-trid .•Lng with 50-ohm coaxialloaa* on ..port il

". .:asrd a<•aln ate ->-band a~nterina without L-adSanterma -n place 9."•L Gi.z, -_ plane 122. .easarec radiation pattern, X-band antenra without a -dand

MhznteD_, ;n place, £ .n z, H plane 13SI). :'easure-i radiation patterr, X-band w-te-na with L-band;;rnters in place, 9.o n GHz, U-- and 14

.. :easu.ird raediation pattern, X-band antenna with L-band-oantenna in place, 9.3 GHs, H plane t1

12. >:easured rmpedanace, X-band antenna, with and without L-bandarra:"te n place 16

13. L-band Corner .Reflector Antenna 171 :easured radiation pattern, L-band corner reflector antenna,

si. and difference feed 1250 !z, E planpe 1n15. ::easured radiation pattern, LX-band corner reflector antenna.

a2ntnn n' pc9z, H plane 1912). Measured impeda•ce, L-barnd coraer reflector antenna

a12r - 13pl acez 2013. L- nd corner reflector antenna gain 21

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One of the basic deficiencies of short range, microwave surveillance ra-dars is their Lnability to penetrate even moderate foliage. A program hasbeen Lmder•.my in the Radar Technical Area of the Combat Surveillance and Tar-

get Acquisition Laboratory, Fort 140mouth, N.J. to investigate the additionof an L- .'and foliage penetration array to an existing X-band surveillance ra-

dar antenna. The objective was to achieve this capability without apprecia-bl- addizg to the size or weight of the X-band antenna. The acronym CCHSFOR"•s coined to designate this Combined Surveillance and Foliage Pemetratioa Ra-dar, ".-Lch resulted from the experiment.

Another L-band array, designed to provide more gain, was to be used forfoliage penetration alone. This antenna was also designed to be lightweight,

f-' beiring mounted atop a man-portable mast, separate from the X-

-< band antenna.

The design of each antenna and the results obtained are described in thisreport.

DUAL .... U•" fh

The exLperimental model developed for the Combined Surveillance and Fo-liage Penetration Radar dual-frequency antenna is shovn in Figure 1. The X-ba-nd antenna is a flat plate array of broadwll waveguide slots. The arrV isrpoIunted on a lightweight tripod and is mechanically scanned in azimuth. Theante•_a ra diates with vertical polarization in the scan plane. There is ;ioelectrc:i-c or mechanical scan in elevation. The foliage penetration antennaa'I s designed for L-band (1220-1280 MHz) for somewhat lesser attenuation

through the foliage and therefore better radiation penetration than that pos-sible with the X-band array.

The L-band arra* consists of four stripline dipole elements, horizontal-ly polarized, each with a passive director. The feed network includes a hy-brid ring to provide for phase monopulse in azimuth. The X-band array servesas a reflector when the radar is in the foliage penetration mode. The dipole/ *director elements are arranged in two pairs, as shown in Figure 1, and arecollapsible over each other and in front of the X-band antenna for protectionand for convenience of transport. A mechanical lever arrangement is combinedwith a stripline-to-coax connector for rigidity of the dielectric materialand to permit collapsing of the antenna. The mechanical leverage does not in-"terfere with the performance of the antenna. When folding or unfolding theantenna, no RF connections or disconnections are necessary.

The stripline dipole/director elements art -".'dec on two sheets of0.062-inch thick copper-plated Duroid dielectric. It was found that addition-al rigidity could be given the dielectric sheets by placing small rivets along-side the radiators and transmission line. The rivets caused no change in theelement's radiation or impedance characteristics.

The coax-stripline connector at either short edge of the dielectric ex-cites a 50-ohm stripline trmismission line which runs along and close to thelong edge of the sheet, to a 90-degree radius band, then serves as the innerconductor of a two a=z balun to excite its dipole. The tranaidesion line is

11

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terminated at the dipole. Each coax-stripline connector has attached to ita flexible coaxial cable for continuation of the transmission line to the in-side of the radar console. In the final model there would then be an RF bulk-head connector for each of the four flexible cables. Inside the radar con-sole there is a semi-rigid coax cable which continues the transmission lineto a stripline hybrid ring "rat race." Integrated with the hybrid is a match-ing transformer which mates each of the upper and lower dipole/director ele-ments to one of the hybrid's antenna ports.

* The L-band array was designed to provide horizontally-polarized phasemonopulse in azimuth over a 1220-1280 MHz band; Figure 2 shows the sum anddifference excitation modes plotted on polar log paper for the center frequen-cy. The patterns at the band edges are essentially the same, maintaining botha deep null on the difference pattern and pattern symmetry. The elevation"pattern of the array is given in Figure 3. That the aperture has been effi-ciently used is seen by comparing the measured gain with the directivity of auniformly illuminated area the size of the X-band aperture. The X-band aper-"ture measures approximately 8 X 13.5 inches. If this area were uniformly il-luminated at L-band, the directivity would be 11.8 dB. The measured gain ofthe stripline dipole/director array over this area used as a reflector is 11d9'across the band. This measured gain includes RF losses in the hybrid ring,the semi-rigid cable from the bulkhead connector to the edge of the striplineboard, and the stripline transmission line to the dipole.

The pattern of a single dipole/director element by itself over the cen-ter of the X-band aperture is shown in Figure 4. The measured element gair8.4 dB with respect to an isotropic point source. It has been noted that thefull array of four elements provides only about 3 dB more in gain over that ofa single dipole/director element. This is due to the aperture sharing of theindividual elements, that is, doubling the number of these elements does notactually double the effective excitation area, as might be the case for a moredirective planar antenna. Figure 5 shows the impedance of the dipole/directorelement, which is essentially the same with the radiator in the environment ofthe other L-band elements. Input impedance of the entire L-band array, look-ing into both the sum and difference arms of the hybrid, is given in Figure 6.Figure 7 shows the input impedance of the same hybrid, but with matched co-axial loads as terminations of the antenna ports.

The X-band aperture, which serves as a reflector when in the L-band mode,has broadwall slots in horizontal waveguide for vertical polarization.

Each of the L-band array pairs is positioned so that the dielectric sheetsare centered between the horizontal rows of waveguide line sources of the X-band antenna. That is, there is no physical obstacle directly over any of theslots. Figures 8 and 9 show plotted in relative power (in dB) the unobstructedradiation patterns of the X-band apertures for the elevation and azimuthplanes, respectively, at the center frequency at 9.8 GHz. Figures 10 and 11show the same pattern cuts for the X-band antenna with the L-band antenna in

J place, also at 9.8 GHz. Observe that the azimuth scan plane is not affectedas much as the pattern of the elevation plane. These patterns are representa-tive of those measured at 9.7 and 9.9 GHz for both the obstructed and unob-structed pattern conditions. The measured input impedance of the X-band arrayfor these same two input conditions is shown in Figure 12.

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117

An indication or the optimum placement of the L-band radiators va~s dater-mined by measuring both impedance and antenna patterns for several E- and H-plane spacings. Gain variations were also noted for the X-band array for thevarious L-band configurations, for example:

1. Single set of 2 L-band elements placed symmetrically on the eleva-tion centerline of the X-band aperture: gain change -0.5dB :-

2. Two sets of 2 L-band elements placed symmetrically abovei-the X-bandaperture (3.8" vertical spacing): gain change -1.5 aB "

3. Two sets of 2 L-band elements placed symmetrically above the X-,bandaperture (5.6" vertical spacing): gain change -1.0 dB -

4. Three sets of 2 L-band elements placed above the X-band aperture,one set at the elevation center and the other two sets at the upper and- lower-edges of the aperture: gain change -0.8 dB

Case 1, although it had the least X-band gain degradation, was not chosen-because the one set of L-band radiators had a nominal gain of only 9 dB across,

K the band. Cases 3 and 4 were not used because they resulted in higher sideand back lobes than tolerable in the L-band elevation plane. Thus, Case 2,while having the greatest X-band gain degradation of the configurations at-tempted, resulted in the best overall tradeoff when dual frequency character-

V istics were considered.

L-BAND CORNER REFLECTOR ANTENNA

An L-band antenna for the Combined Surveillance & Foliage PenetrationRadar project which could be used separate from the X-band antenna was also'designed and built. This antenna would not actually have dual-imode capabili-ty, but would be used for the foliage penetration mode alone. The antenna was:!• •,to have more gain, minimum resistance to wind, and still be light weight so A• ithat it could be hoisted atop a lightweight man-portable mast. By raising the

antenna, the ground-lobing loss would also be lessened.

The antenna built is a 90-degree corner reflector with four of what are

essentially the same dipole/director elements as used in the dual aperture ai-tenna. The reflector is constructed of a grid of cylindrical aluminum rods tosatisfy the requirements of light weight and minimum wind resistance. Figure"13 is a photograph of the antenna mounted on a tripod supported mast. The re-flector is hinged along its apex, which permits folding of the reflector to

.' •protect the elements in transport. The radiators were adjusted in azimuth foroptimum gain-pattern-impedance characteristics. Each pair of elements isjoined by a 3-dB stripline power splitter which is in turn joined with flex-ible coaxial cable to a stripline hybrid ring. The reflector projects an areaof 30 inches in azimuth by 19 inches in elevation. An azimuth pattern display-ing the sum and difference modes at 1250 MHz is given in Figure 14. The pat-terns at 1220 and 1280 MHz are essentially the same. An elevation pattern isshown in Figure 15. The stripline hybrid ring for. this antenna, unlike that. 4of the dual-band antenna, has no additional matching network as part of thesame stripline board. This hybrid ring has a maximum VSWR of 1.15 with atleast 30 dB isolation across the band. For the complete antenna feed, the

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siphasle iduwl-hin~dges frombihibridwinpou ttode ach of the X-andaetenea Thnectiol-bns apoture a~nd thiine isepartted ione Fgrefl16.oGantena aribth renpet-t

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of-theDTechicatlo SericsAtiantenn far bhee designe ofo the folhaicage pene-

age used pioh teCMSO antexitng, Xand atorerna canley affCordedat Suveryld-

dual-band Tapgertrequndithen Leabrate ornesRdr refectrca AraAntenna aebThnerh-

aiue Team, for help in the RF measurements.

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*Fig. 2 Measured. radiation pattern,, L-band Array- ~Sum f'eed----- Diff~erence feed

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Fig. Mr easured radiation pattern, single dlipolel/d-irector elementsymmetrically placed over X ~band aperture., 1250 MzY

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Figure 11. Measured radiation pattern, X-bandantenna with L-band antenna in p~lace-

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