A compact HF antenna array using reactively-terminated parasitic elements for pattern control

Measurements have been made on two 7 element azimuthally-symmetric arrays, consisting of one active receiving element mounted at the center of six symmetrically positioned parasitic elements with variable reactive terminations. The measurements were made at frequencies between 15 and 25 MHz on an antenna range. Most of the data was taken on an array of diameter 80 cm (0.05 lambda at the center frequency of 18 MHz) with helically-wound monopole elements. Attempts were made to form the array pattern deterministically by using Harrington's theory to compute the reactive loads necessary to form a pattern lobe in a desired direction. Although the pattern lobe could be steered in the general desired direction, the directivity of the pattern was low and reproducibility poor. The majority of the measurements used the array in an adaptive mode in which the reactive terminations were adjusted manually to minimize one or two incident signals representing undesired interference. The manual adaptation consistently produced sharp spatial notches in the direction of the interference in an otherwise nearly omnidirectional pattern. The notches typically had a width of 50 degrees and depth of 25 to 30 dB below the pattern main lobe; the cancellation bandwidth was 40 kHz.