Superdirective aerial array containing only one fed element

Abstract

The maximum-gain theorem developed by Bloch, Medhurst and Pool in 1953 has been applied to an endfire array of four unequally spaced half-wave dipoles, an extensive range of element spacings being taken. A configuration has been found where, for maximum gain, one element is parasitic and two may be joined passively to form a parasitic pair, leaving one element to be fed. Two such arrays designed to operate at 275MHz have been built with ¼in- and ½in-diameter elements, respectively. The theoretical maximum gain for such arrays is about lOdB, and gains of 9.0dB and 8.8dB were measured for the two experimental arrays. Of the discrepancy, 0.2dB and 0.1 dB, respectively, were calculated to be due to losses in the metal structure (i.e. the dipoles and the section of transmission line connecting the parasitic pair). An orthodox endfire array (having currents of equal amplitudes, phased so that the distant fields add in the direction of maximum gain) of the same size would have a gain 5½dB less (3½: 1 in power). A Yagi array with the same gain would need to be between 2¼ and 3½ times longer, depending on its design.