2ND-MODE OPERATION OF AN ARCHIMEDEAN SPIRAL ANTENNA BACKED BY A CONDUCTING PLANE REFLECTOR

A two-arm Archimedean spiral antenna operating in the second mode is investigated numerically and experimentally. The effects of the presence of the infinite plane conducting reflector on the radiation characteristics are studied using an integral equation technique. A traveling-wave type current is realized due to the mutual coupling between the real and image spirals, provided the antenna height is appropriately chosen, in spite of the fact that in the absence of the reflector a standing wave is appreciable in the current distribution. The operating bandwidth of the spiral antenna is discussed in detail. Experimental results are also presented to verify the numerical results.