Abstract :
An array of four driven antennas, located at the corners of a square, around a central parasitic antenna, is analysed. All antennas are taken to be of one-quarter wavelength and grounded to a perfectly conducting plane. It can be easily proved that, with the antennas shorter than one-quarter wavelength, grounded to an imperfectly conducting earth, the shape of the radiation patterns and the field strength gain remain practically unaffected because the field strength is then reduced uniformly in the same ratio. The procedure followed in this analysis is, first, to calculate the total impedance at the base of each antenna as a function of the antenna diagonal spacing. This spacing is the fundamental variable used in all subsequent calculations. Next, the ratio of the parasitic antenna induction current to the current fed into each of the four driven antennas is determined. An equation is obtained, giving the field strength produced at an arbitrary point in space. This equation gives immediately the horizontal and vertical radiation patterns in terms of the diagonal spacing of the array. Under certain conditions it exhibits pronounced directive characteristics. Finally, a formula giving the root mean square field strength in the horizontal plane is obtained. This formula is transformed later in order to give the root mean square field strength which is produced from the total power fed into the array. The same total power fed into a single antenna of the same design produces a different horizontal field. Comparing the two fields produced by the array and the single antenna respectively, a relation giving the average field strength gain is obtained. It is confirmed that this average gain, under certain favourable conditions, having regard to the diagonal spacing, is increased as much as 10 to 20 per cent. An even greater field strength gain, up to 60 per cent., is obtained in certain directions, with a given diagonal spacing of the array.
