Active region and higher-order modes of spiral antennas

The paper discusses characteristics of radiation, current distribution and excitation of the higher-order modes of the typical self-complementary spiral antennas. The investigated antennas are some of the leaky-wave antennas where the energy leaks during propagation through the spiral arms to produce radiated field. Radiation of the dominant first mode is formed in the specific area described as "the active region". This area depends on the rate of “wrap of the arms”. The investigated log-spiral and spiral Archimedean antennas have expansion of the arms exponential and linear, respectively. Expansion of the arms is only the function of angle and therefore these antennas belong to the class of frequency independent antennas.