Evaluating the impedance broadbanding potential of antennas

One of the principal concerns of the aircraft antenna designer is to determine whether an antenna having some known terminal impedance characteristics can be compensated to meet a specified, voltage standing-wave ratio (VSWR)-bandwidth relation. Alternatively, the problem may be to select from among possible antenna configurations that one which possesses the greatest impedance broadbanding potential. In this paper the problem is examined by means of modern circuit theory. Antenna impedances are approximated by rational algebraic fractions and the impedances represented by these functions are treated analytically to determine the VSWR-bandwidth relationship which may be obtained by optimum compensation. The functions that can be considered by the techniques developed are sufficiently complex to adequately represent the basic impedance characteristics obtained with typical airborne flush-mounted antennas over a frequency range that causes the dimensions of the antenna to vary between a small fraction of a wavelength and approximately a wavelength. Examples are given.