VHF-UHF Aircraft Antenna

1 NRL Report 801200 VHF-UHF Aircraft AntennaFREDERICK FINEA erospace Systems Branch4<. Space Systems DivisionJuly 23, 1976 OCT. 19 'il 70D.;NAVAL RESEARCH LABORATORYW dhiton, Appr .ove f ,or publi rek .dl,.,i,,t ..,ir,, COPYSECURITY CLASSIFICATION OF THIS PAGE fWhoen Date Entered)REPORT DOCUMENTATION4 PAGE BEFORE COMPLETING FORM111611,11111 --mollGOVT ACCESSION NO. S. RECIPIENT'S CATALOG (andE ) S YEO EOTAPRO OEE6 VHF-UHF Aircraft Antenna ,.7* AUYNpR~e) 4 OTATO RN _ute;, Frederick Fine 739. PERFORMING ORGANIZATION NAME AND ADDRESS Ia. PROGRAM ELEMENT. Research Laboratory I 4 V60Q!1 Washington, 20375/11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE .Naval Electronics Systems Command July 23, 197W'1/ -Navy Space Projects Office (PME-106)Is uetOFP_Washington, 203803214 MONITORING AGENCY NAME A ADOIRESS(It iletent tain Controlling Office) 15. SECURITY CLASfWKNp$a. DECLASSIFICATION/OOWNGRAO#NG* SCHEDULEIS.

2 DISTRIBUTION STATEMENT (of tis Report)Approved for public release, distribution unlimited. 7I?. DISTRIBUTION STATEMENT (of th,. abstract entered In Block 20. If different hasm Report)IS. SUPPLEMENTARY NOTESThis is a final report on this aspect of the overall problem; work on other aspects of the problemis KEY WORDS (Conthnue on revese aide If necesar and identity by, block numer.)Antennas Broadband antiennasAntenna arrays Circularly polarized Antenna elementsAircraft antennas VHF-UHF measurementsSO. ARCT (Conthm n uemrse sfide liecossavy, And identity by block numnber)broadband, circularly polarized Antenna system suitable for use on WP-3 Aircraft andoperating in the VHF-UTHF frequency region has been designed and fabricated. This airborneradiometer system will operate over a 3 to 1 frequency range without tuning. The installationcontains a high- and low-frequency hemispherical coverage Antenna and a high- and fan- beam array.

3 The entire installation is designed to fit into the standard APS-20(Continued)DD 'P 1473 EDITION OF I NOV 5 5 I OSSLETE6/1, 010 6" 1A ~ rn 7 SECURITY CLASSIPICATIOtI Of THIS MW--,_juITy CLASSIFICATION OF THIS PAGI(Whem Dola Ea11eradome of the WP-3. Use has been made of a novel Antenna element to achieve good patternperformance in such a confined space. Emphasis has been placed on low los and .OCT 19,"I 'S O _ -I ,L L".~( DSECUNITY CLASSIFICATION OF THIS PAG3lMMa, O 3 Uae1 CONTENTSINTRODUCTION LOCATION AND GENERAL DESCRIPTION ..1 THEORY .. ,.. 4 THE RADIATING ELEMENT ..6 Antenna SYSTEM RF NETW'ORK ..10 MECHANICAL STRUCTURE .. 15 MODEL MEASUREMENTS ..16 TESTING THE Antenna SYSTEM ..16 SUMMARY OF Antenna ARRAY PERFORMANCE ..28 ACKNOWLEDGMENTS .. 29 REFERENCES .. 29 VHF-UHF Aircraft Antenna system was designed and fabricated to be used in an airborne VHF andUHF radiometer system.))

4 This unusual radiometric Antenna will operate without tuningover a 3 to 1 frequency band. The design incorporates two relatively high gain arrayantennas (14 to 20 dB gain) and two single radiating elements (5 to 8 dB gain). A gen-eral difficulty in regard to this task was that of isolating the Antenna sufficiently fromthe Aircraft structure to produce stable and predictable increase the isolation from the Aircraft structure, the Antenna is tilted downwardrelative to the Aircraft wing and fuselage. Full azimuth coverage is obtained by flying theaircraft in a tight circle at a bank angle of 30 degrees. This places the peak of the antennabeam on the horizon. The type of radiating element employed in these antennas exhibitsintrinsic directivity so as to further isolate the Antenna beam from nearby LOCATION AND GENERAL DESCRIPTIONThe WP-3 Lockheed Electra Aircraft affords one reasonable location without airframemodification for the type of Antenna system described in this report.

5 The Antenna ismounted within the radome that normally encloses the APS-20 radar Antenna . Theradome is located approximately 7 meters aft of the Aircraft nose and immediately belowthe fuselage. Figures 1 through 3 depict the randome location. The radome is approxi-mately m in diameter, m high, and has an elliptical cross section in the Antenna system is designed to maximize its reflector surfaces within the confinesof the existing radome and still perform its intended function. Two elliptically shapedFig. 1 -WP-3 Aircraft , side viewManuscript submitted April 22. " ..IFREDERICK FINEZV#F -OUF ANTENNAFig. 2-WP-3 Aircraft , top viewV#F -OHF ANTENNAFig. 3-WP-3 Aircraft , front viewfiat reflectors (one facing port, the other facing starboard) are tilted 30 degrees from thevertical. The major axis of the Antenna assembly lies parallel with the axis of the aircraftfuselage. The port Antenna array operates over a range of from 200 to 350 megahertz(MHz) and the starboard array operates at from 342 to 600 MHz, affording an overlap of8 MHz.

6 These two arrays are separated by m. Two single radiating elements aretilted downward by 45 degrees, one looking aft and the other looking forward. The aftradiating element covers the 200- to 350-MHz band; the forward-looking element coversthe 342- to 600-MHz band. The port Antenna array comprises one row of six circularlypolarized crossed-dipole elements spaced cm apart and mounted cm above thereflector surface. The starboard Antenna array comprises two rows of eight circularlypolarized crossed-dipole elements spaced cm apart, vertically and horizontally, andmounted cm above the reflecting surface. Each radiating element is oriented 45degrees with respect to the horizontal axis. This unusual orientation allows the elementsto be more closely spaced than if they were arrayed with one arm of each element col-linear with those of the adjacent elements. This close spacing makes possible proper con-trol of the far out sidelobes for the required wide bandwidth.

7 Figure 4 through 6 aresketches of the complete Antenna REPORT 8012 PORT Antenna ARRAYSTARBOARD Antenna ARRAYFig. 4 -Port anid starboard arraysFORWARD Antenna ELEMENTAFT Antenna ELEMENTrig. 5-Forward and aft Antenna elernents3kFREDERICK FINEArT'"Fig. 6- Antenna asembly, cros-sectional views:(a) side, (b) front, A-ATHEORYA uniform feed amplitude is employed in the port and starboard Antenna arrays inorder to minimize the pattern beamwidth. Since the available space for radiating thesefrequencies is considerably less than ideal, minimizing the beamwidth will not only re-duce reflection from the Aircraft but will most efficiently make use of the available an-tenna area. The spacing between adjacent elements is the greatest possible without allow-ing a significant increase in the near or farout sidelobe structure. This considerationresulted in a spacing of ) at the highest operating frequency and an electrical spacingof ) at the lowest operating frequency for each array.

8 The physical spacings arethus and cm for the starboard and port arrays, respectively. The close elec-trical spacing at the lower frequencies dictates that some measure of control over themutual coupling must be exercised. This control is provided by a number of isolatedhybrid ports, distributed throughout the feed network, which absorb out-of-phase re-flected low band Antenna army, with a spacing, provides enough area for asingle row of six crossed dipole radiators. The far-field Antenna pattern in the horizontalplane may be represented mathematically by the following equation:G(o) = A(6) X E(9), (1)where E(8) represents the element pattern as a function of azimuth angle, and A(8) repre-sents the array factor a a function of azimuth angle. Furthermore, the array factor is given by4 NRL REPORT 8012A(O) = cos(Vi/2) + cos(30/2) + cos(50/2) (2)/2 180d (sin 0),where0 = azimuth angle measured from broadside,d = element spacing= operating element pattern must account for the feed gap in each set of dipole elements and forthe effect of the spacing of the dipole elements from the reflector.

9 Thus,E()= cos !(- sin) X sin (27r Cos) 0 X cos (w/2 sin ) (3)(X ) k XCos 0 ) 3in which g = distance between the gaps, and R = distance above the reflector (1) is plotted in Figs. 7 and 8 to show the calculated horizontal and verticalpatterns, respectively, of the low-band array. Since this Antenna is only one element highin the vertical plane, it has an array factor of unity in that plane. Therefore, Fig. 8 isalso a plot of Eq. (3) and represents the horizontal or vertical pattern of a single will be noted in Fig. 8 that the element beam is broader at 350 MHz than at 200 is to be expected after examination of the second factor in Eq. (3).9- M MNlI (24)J ---3 A , r32 --J131dII0 Is 5 31 48 U so 70 W 5 AMlUTH# AWLf W1 KsPig. 7-Horixontal Antenna pattern, low-bend fan beam fromI X 6-element array of crossed .,nFREDERICK FINE10M Hz (76'-).3-2-ELEVATION ANGLE (DEB)Fig. 8-Vertical Antenna pattern, low-band fan beam from1 x 6-element array of crossed dipolesThe horizontal array factor for the high-frequency array isA(O) = cos (/2) + cos (30/2) + cos (50/2) + cos (7tp/2), (4)and the element pattern is represented by Eq.

10 (3) with different values for g and vertical array factor for this Antenna isA(0) = cos (4 /2). (5)Equation (1) is plotted in Figs. 9 and 10 showing the horizontal and vertical patterns forthe high-band array. In this array the reflector provides sufficient area for two rows ofeight elements. Notice that the change of beamwidth with frequency in the vertical planeof this array is opposite to that of the low-band array shown in Fig. 8. This is becausethe array factor overpowers the effect of ground-plane RADIATING ELEMENTThe radiating element chosen for this Antenna system was designed under ContractN00173-75-C-0613. It is a dual-polarized broadband dipole element containing fourwidely separated gaps. Each collinear pair of arms is fed by a 180-degree balun. Figures11 and 12 are photographs of the low-band and high-band dipoles used in the antennasystem. The widely separated dipole gaps provide inherent element directivity not presentin standard dipole designs.