An electronically steerable antenna array for long-distance radio propagation studies

Abstract

The theory and construction of an electronically steerable antenna array and its use for long-distance radio propagation studies are described. A complete description of the electronic equipment and computer programs necessary to operate the system is given. The general theory of non-uniformly spaced, planar, phased arrays, containing a small number of antenna elements, is reviewed and a procedure for designing such arrays with a minimum value of maximum sidelobe level is given. This procedure is used to design an array of 30 antennas distributed over a circular aperture of diameter 160 metres for use at frequencies from 3 to 30 MHz. A new formulation of the probability distribution function of the maximum sidelobe level of such arrays is developed. The effect of amplitude and phase errors in the weights applied to the outputs of the antennas in the array, and the effect of mutual coupling between the antennas are examined. These effects are both shown to be small compared with interference caused by reradiation from metal objects near the site of the array. A maximum likelihood method for analysing the data obtained from the phased array is developed and applied to a number of relatively simple data records to demonstrate the performance capabilities of the system constructed here. Several days of direction of arrival data obtained using near-antipodal BBC transmissions from the United Kingdom are examined. Diurnal and seasonal variations in direction of arrival and signal strength are explained in terms of the low absorption of radio waves on the night side of the earth and, to a lesser extent, in the winter hemisphere. The high absorption which occurs in the auroral regions is also shown to affect the results significantly. Signal is often received from the directions of the edges of the auroral regions, but rarely from the directions of the auroral regions themselves. The effect of the transmitter beam direction on the direction of arrival is also considered. When absorption is low along a number of propagation paths, then the direction of arrival of the strongest received components tend to be along those paths nearest the transmitter beam direction.