The measured performance of horizontal dipole transmitting arrays

Measurements of the performance of horizontal dipole arrays are described, and are compared with the theoretical performance under ideal conditions. The measurements include polar diagrams, the effect of ground slope and radiation through other arrays, and the performance over a frequency band of ?2% relative to the design frequency, which is typical of the width of existing short-wave broadcasting bands. One method of measurement consisted in elevating a calibrated frame receiving aerial using a captive balloon; by varying the height and position of the balloon, the field strengths in different directions from the array were measured. In the second method a frame aerial at ground level was used; this gave only relative values of field strength and was used mainly to determine the frequency characteristic. It was found that for an array radiating over a flat site free from obstacles there was good agreement between the theoretical and measured performance; the maximum field strength was of the order of 0.8?0.9 of the theoretical value. A sloping site or radiation through other near-by arrays, however, may cause appreciable departures from the theoretical characteristics. An array having no reflector curtain covers the frequency band of ?2% without appreciable loss in radiation efficiency. This is also true for an array with a parasitic reflector curtain which is tuned to the working frequency. If, however, the reflector is tuned always to the mid-band frequency, the radiation efficiency at other frequencies is reduced. The band-width can be increased by reducing the characteristic impedance of the dipoles, and measurements have been made on two types, one consisting of single wires and the other of two parallel wires spaced 6 in apart. For an array of four rows of single-wire dipoles, the band-width is ?1% for a 10% drop in field strength relative to the mid-band frequency; for a similar array of two-wire dipoles the corresponding band-width is ?2%. The band-width o- f arrays containing less than four rows is approximately the same.