Antenna Design for Field-Strength Gain

An analysis is made of obtainable root-mean-square field strength in the horizontal plane with a given radio-frequency power fed into four identical, short vertical linear antennas which are located in the corners of a square. This field strength is compared to that produced with the same power fed into a single antenna of the same design. The resistance coupled into each antenna by the other three is computed. A formula giving the gain in field strength for four antennas over that obtained from one antenna is derived, and families of gain curves are plotted. The theory presented is in essential agreement with measurements of gain in field strength made at 200 and 400 kilocycles at the Pittsburgh, Pennsylvania, radio range. A gain in field strength of the order of 1.8/1 can be realized at the lower frequencies. The gain increases as the coil and ground resistance of the antenna circuit becomes greater, and it also increases as the antenna spacing is reduced. At very small antenna spacings where the coupled reactance becomes comparable to the self-reactance of a single antenna, the field-strength gain decreases from the maximum value obtainable since the larger antenna tuning coils required in this case increase the losses.