The paper elaborates the current distribution of the Archimedean Spiral Antenna (ASA), demonstrating the concept of frequency dependent active region and this determines the effective radiation area on the spiral arm. The band theory is used to explain the theoretical principles of the operation of the spiral antenna. In this paper, a two arm Archimedean spiral antenna is designed using Computer Simulation Technology, (CST). The properties of the active region of the spiral antenna are analyzed in different types of dielectric substrates. The calculated and simulations results show the position of the active region is very dependent to the operating frequency and dielectric permittivity of the substrates. The maximum surface current induced to the spiral arm increases as the operating frequency and permittivity of the dielectric substrates reduces.
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