13.1 INTRODUCTION

Theoretically, frequency independent (FI) antennas have near-field and far-field characteristics independent of frequency. Thus, an FI antenna must have an infinitely large aperture to eliminate the low-frequency limit. It must also have a very fine and infinitely small feed region to remove the high-frequency limit. To eliminate length-dependent scaling, an FI antenna is exclusively described by angles. To reduce and ideally eliminate the far-field contamination, the residual currents must decay to zero after passing through the active (principal or main) region and before entering the next-higher order radiating region. The complete radiation from a single active region, also known as constructive radiation, is however not possible. Finally, an ideal FI antenna must have a frequency independent feed.

Certainly, no practically realizable antenna can fulfill these requirements. Thus, realistic FI antennas are radiators having virtually invariant impedance and pattern characteristics over very wide instantaneous bandwidths or consecutive multiple logarithmically periodic bands. Consistent impedance and pattern characteristics over 100:1 bandwidths have been successfully demonstrated, so that the system bandwidth is commonly limited by the electronics, rather than by the antenna. Owing to the previously mentioned properties, FI antennas have become irreplaceable components of many, predominantly military, systems. This chapter outlines the fundamental principles of frequency independence and reviews many antenna structures belonging to this class.