Frequency-independent (FI) antennas are radiating structures capable of maintaining consistent impedance and pattern characteristics over multiple-decade bandwidths. Their finite size limits the lowest frequency of operation, and the finite precision of the center region bounds the highest frequency of operation. The geometry of a frequency-independent antenna must be specified by angles alone, and principles such as truncation and self-complementarity are necessary and desired, respectively. In this article, Rumsey's theory of FI antennas, developed in the mid-1950s, is thoroughly discussed, and historical highlights listing the most important contributions in this area are given. A mathematical derivation for a general FI structure is provided, and some special cases are considered both theoretically and practically. A description and an explanation of the underlying theory of operation for several FI antenna types, including equiangular spirals, conventional and unconventional sinuous antennas, and log-periodic folded slot arrays, are also provided. A view of frequency independence through the modal content and contamination of the radiated far field is emphasized. Considerations for flush/conformal mounting are briefly discussed, including the use of log-periodic lossless cavities and broadband conical cavities.
Keywords:
frequency-independent antenna;
equiangular spiral antenna;
logperiodic antenna;
dipole slot array;
sinuous antenna;
folded slot array
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