Experiments in hemispherical scanning

Abstract

A 01731Conventlonai phased arrays suffer seriously decreased efflciency when scanned to reglons near end-fire. Typical results for arrays with nominal 20-30dB galn mounted on cylinders to simulate an aircraft fuselage show gain reductions of 5 to 7dB at end-fire compared wlth broadslde radiation even when the array has been scan matched st angles near end-fire. 132 Typlcal aperture efflciencies for such arrays is between 10 and 15% at the horizon because these arrays l to 3dB additlonal mlsmatch loss at broadside. Recent efforts have lncluded the use of dielectric structures over or in the viclnity of the array, and have shown that these means also improve galn galn is always within about 6dB of the maximum radiation over a coverage wlthln the hemisphere so that the envelope of peak radiation narrow frequency range. Computations3 have shown that the coverage obtainable from an array with 23dB nominal galn presents maximum oscillations of 8dB over a 10-percent bandwidth. This data was obtained for an array covered wlth a layer of E, = 4 materlal 0.0751 thick extending over and beyond the array. , P eA new concept In wide-angle coverage arrays Is investigated in this experimental program and has appilcation to the problem of protidlng low cost efficlent antennas wlth hemlspherlcal coverage for sircraft to satel 1 ite .communication 1 Inks. This concept involves the use of a dual mode radlatlng structure consisting of a conventional waveguide phased array and the added feature that each waveguide can be short circuited to form a corrugated surface for end-fire radlatlon. The phased array Is therefore used over a wide angle scanning range, but is not required to provide radlation at end-fire. Endfire radiation is provided by exciting the shorted corrugated glve hemispherical coverage. structure wlth a separate waveguide feed. The array is rotated to Figure 1 shows an array of sixty four wavegulde elements excited by an eight element feed. Although not shown In the figure, the nrray 1s also excited by a sixty-four way power dlvider and 8-phase shifters to form a beam scanned In the elevation plane. In practice, the waveguides would be short clrcuited by diodes or mechanical shutter switches to form the corrugated surface for end-flre radiatlon, but the experlment was conducted uslng flxed short circuits.The ground plane, partially shown In Figure 1, measured six Feet wlde and had a four foot curved surface with 84 inch radius extendlng in front of the antenna structure. 40