Frequency-Independent Antenna

an antenna with primary electrical characteristics that vary insignificantly with frequency over an extremely wide range; the various types of such antennas constitute a group of broadband antennas for which the ratio of maximum operating frequency to minimum ranges to 20:1 or more. The theoretical and technological foundations for frequency-independent antennas were established between 1957 and 1965 by the American scientists V. H. Rumsey, D. Dyson, and others.

The weak dependence of the antennas’ characteristics (the shape of the directive pattern, the front-to-rear ratio, the input impedance, and so on) on frequency stems from the fact that the radiation field is created by currents distributed over a finite portion of the antenna surface, known as the active region, beyond whose limits the currents decrease sharply; as the frequency varies, the active region changes in such a way that its relative dimensions expressed in terms of the wavelength λ corresponding to the frequency remain unchanged. The lower wavelength limit λ_max of the antenna’s operating range is determined by the frequency at which the active region is shifted to the edge of the antenna. The antenna’s operating range can, in principle, be extended toward the shorter wavelengths as far as desired, but in practice the limit is determined by a number of incidental factors, such as the cross-sectional dimensions of the power feeder that are acceptable for given values of the losses introduced, the breakdown voltage, the transmitted power, and so on.

The most common frequency-independent antennas are in the form of two-arm spiral and conical helical antennas, log-periodic antennas, and sickle-shaped dipoles. There are also multiarm spiral and helical antennas that have several independent inputs; a well-known type is in the form of a conical dipole with an ultra-wide range of input impedances.

Frequency-independent antennas are used for shortwave radio communications, telemetry, and radio astronomy. During the 1970’s lightweight types of relatively simple design were developed for various frequency ranges: log-periodic wire antennas were developed for decameter waves, and spiral and helical antennas were created for centimeter and millimeter waves from strip conductors deposited on a fiberglass substrate by a photochemical process. Highly directional frequency-independent antennas are being designed as horn antennas with walls having transverse ribs and antenna arrays composed of log-periodic or conical helical radiators positioned along radii in a specific sector of a circle.