Design of an antenna subsystem for a semi active radar target

The paper describes the development of an antenna subsystem for airborne applications under the special conditions pertaining to the development and operation of a semi active radar target or SART. For this application the antenna system, together with the onboard electronics, must be capable of receiving the incoming signal, amplifying it to a specified level and retransmitting the signal, at the same frequency, in the direction of the original signal. Additionally the present application requires the antenna subsystem to operate over a bandwidth approaching 1 GHz. and with beamwidths exceeding 60 degrees. An antenna system is described which has been found to meet the specifications laid down. This antenna is based upon a helical antenna axially located within a circular waveguide. The basic helical wire antenna has been modified to improve its mechanical and electrical properties, culminating in a rugged design suitable for flight trials. Results are presented which show the antenna´s performance over the required frequency range for axial ratio and beamwidth. A further requirement for this system is high isolation between antennas as the receive and transmit frequencies are the same. Results are also included to indicate the high isolation levels achieved by the inclusion of chokes in the metal antenna supporting structure