Properties of 4-arm conical equiangular spiral antenna over extended bandwidth

The 4-arm spiral antenna has been studied for detection, direction finding, tracking, and homing applications over an extended bandwidth and compared to 2-arm performance, down to low frequencies. Resistive loading and truncation effects are explained and mixed mode patterns due to the use of imperfect feed networks are shown. Results show agreement between theory and practice for the following features: (1) squint of the mode 2 null due to other mode corruption, (2) relative levels of mode 1 and 2 from the same antenna, (3) mode 1 beamwidth from corrupted excitation, and (4) mode 2 peak to peak beamwidth from corrupted excitation. It is concluded that the addition of a resistive loading network improves the performance of an antenna operating below its normal frequency-independent band for all modes of excitation. The deterioration of the loaded antenna's performance is then due to the truncation of the active regions. In addition, mixed mode degradations can be sufficiently predicted and high order modes can be deliberately generated and useful patterns produced.