FREQUENCY-INDEPENDENT ANTENNAS AND BROAD-BAND DERIVATIVES THEREOF

The principles used in the development of frequency-independent antennas are reviewed. Examples of log-periodic (LP) geometry and the additional requirements that must be met for practical success are given. The evolution of unidirectional, wire-outline, and dipole arrays is presented. Analytical results for log-periodic dipole arrays led to the concept of an active region that remains almost fixed in size relative to the wavelength. Periodic structure theory serves as an aid to understanding LP structures. The analysis of periodically loaded lines shows the problems that may arise from the existence of stop regions on LP antennas, and provides procedures for eliminating them. This has led to successful LP monopole and cavity-backed slot arrays. Planar and conical log-spiral antennas are discussed. Frequency-independent, circularly polarized patterns; bidirectional, unidirectional, omnidirectional, and conical, can be provided from log-spiral antennas with two or more arms. Recently, the log-periodic design principles have been applied to low-profile antennas: patches, thin cavities, and spirals over ground. They have also produced flush-mounted aircraft antennas that can respond simultaneously to two different linear or circular polarizations.