Design of Airborne S-Band Telemetry Antennas

Abstract

The change from VHF to UHF for telemetry requires new antenna designs rather than the scaling of the antennas now used for the UHF frequencies. The reason for this is that the vehicle dimensions at UHF, in particular the rocket diameters, will be of the order of several wavelengths. A common method to obtain a nearly omnidirectional radiation pattern at VHF is to mount two or four element antenna arrays on the vehicle. This is sufficient since the wavelength for the presently used telemetry frequencies is approximately 50 inches and the array spacing rarely exceeds a half wavelength. The radiation pattern from such closely spaced unit radiators exhibits only minor scalloping which does not present a problem in the data acquisition. At the UHF frequencies, the array spacing, in wavelength, is increased by a factor of ten resulting in an interference pattern with narrow lobes and deep nulls. If the mechanical design limitations permit it, it is possible to design unit radiators which will give cardioid or nearly omnidirectional patterns for a single polarization component. Two such antennas are described. They are the axially mounted turnstile and the radial waveguide antennas.