VHF antenna systems for spin-stabilized satellites

Abstract

Absfmcf-LincolnLaboratory has developed two circularly polarized VHF antenna systems for use on spin-stabilized satellites Both are described, and test results are discussed. One is a low-gain (3-dB) antenna with omnidirectional coverage in the equatorial plane. The second is a high-gain (IO-dB) antenna with a despun beam electronically scanned to always be Earth directed. Development of the component dipole and slot array elements is explained, and performance data are given for the final flight configuration. s I. INTRODCCTIOXATELLITES which are spin-stabilized require either omnidirectiond or despun antenna systems. Circular polarization is needed to overcome Faraday rotation and multipat,h effects which can cause severe reduct.ion in signal st.rengt.hs when t,ransnission is behveen t,he satellite and Earth. Two circularly polarized YHF antenna systems have been tested in space. One is omnidirectional, in a near-synchronous orbit,;the other is despun, in a synchronous orbit.. Both are reciprocal devices. transmit h g and receiving over several frequency bands simultmeously through a frequency multiplexer. Each system consists of two IinearlJ-pola,rized. orthogonal ant.enna arrays fed in phase quadrature to obtain circular polarizat.ion. For omnidirectional coverage a.11 the ant,enna elements in each array are fed in phase. In the despun system four elements in each arra-are fed at, any given t,ime to form a directive beam with nlaxinlum field normal to t,he spin a.xis. By elect.ronic sequentia,l swit.ching and the proper phasing, this beam is sca.nned t,hrough 16 equally spa.ced positions about the spin axis. Sensorcontrolled slvitching logic keeps the bea.m pointing to Earth as the sat,ellite rot.ates. GENERAL DESCRIPTIONAs can be seen in Figs. 1 and 2, bot.h satellites have t.he same general shape. This was maid. determined by t.he desired orbit,al behavior a,nd the a.vaila,ble volume on the launch vehicle. For a spin-stabilized satellite with spin axis perpendicular to t.he plane of a circular orbit., t,he most natural shape is a right-circular cylinder. Because of the relatively low operating frequencies t.he 1969. This work was sponsored by the U. S. Air Force.