Maximal Bandwidth of an Archimedean Spiral Antenna Above a Reflector

In this letter, the maximal achievable bandwidth of an Archimedean spiral antenna above a reflector is investigated. This bandwidth is defined by the frequency range where the gain in the broadside direction is higher than the case without a reflector. To determine this theoretical bandwidth, we propose an analysis consisting in replacing the Archimedean spiral antenna by a bidirectional elliptical plane-waves source above an infinite reflector. Thanks to this approximation, it is possible to evaluate this bandwidth with a simple analytical expression, which is defined by constructive interferences between two plane waves. The maximal bandwidth obtained from this formula is equal to 5:1 for a metallic reflector. Simulations and measurements are then presented in order to confirm this result.