Abstract
An HF receiving array antenna, consisting of ten 12 to 25 MHz horizontally-polarized log-periodic-dipole element antennas, spaced vertically 16 meters apart on the side of a 152-meter (500-foot) tower, has been erected near Boulder, Colo. It is capable of electronically scanning a sector in elevation ranging from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.5\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">51\deg</tex> at 12 MHz, to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.7\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22\deg</tex> at 25 MHz. The sector was scanned with a single major lobe which had a vertical plane half-power beamwidth that varied from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7\deg</tex> at 12 MHz to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3\deg</tex> at 25 MHz. This array was a part of a system designed for studying the angle-of-arrival of downcoming radio waves from the ionosphere. A sinusoidal current distribution formed a single major lobe which was scanned in elevation by varying the period of the sinusoid. Scanning was accomplished electronically at an intermediate frequency; therefore, the increase in bandwidth of the received signal, caused by the rapid scanning, took place after conversion and did not necessitate an increase in bandwidth of the RF input circuitry. The scanning rate was 524 Hz with the sector scanned by an upgoing lobe and a downgoing lobe in each scanning cycle. Photographs of the oscilloscope display are presented which show the vertical angle-of-arrival of 10 and 20 MHz signals from WWV. Comparison of sinusoidal distribution with conventional shows that the sinusoidal distribution results in half the beamwidth of the latter, 3 dB higher gain, and a lower minimum angle above horizon.
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