Impedance measurements of Deception Island as a natural very‐low‐frequency antenna

Abstract

In 1960 I suggested that Deception Island, near the northern end of the Antarctic Peninsula, might serve as a useful naturally resonant VLF slot antenna. Connection across the island was subsequently made and impedance measurements were obtained. Contrary to theoretical predictions by Galejs and by Staras, a strong resonance was obtained. However, it occurred at 9.5 kHz, rather than at 5 kHz as anticipated. This is ascribed to a short, low cut through one of the arms of the island which otherwise range from 500 to 1000 ft high. A composite transmission line model is constructed to represent the data. A first approximation to the efficiency suggests that it is in the range of

On natural slot antennas

“…It is probable that the impedance resonances, observed by Gould [ 1961 ] in Scotland and reported by Morgan [ 1979] for the volcanic Deception Island in sub-Antarctica, were caused by nothing more than the interference of the traveling currents waves on their grounded cables. The propagation constant of such a thin insulated wire 'laid on the ground' can be considerably greater than the value for free space [Chang and Wait, 1974;Burrows, 1978].…”

“…Some similar suggestions were put forth by Morgan [1960] and Gould [1961], who both had the idea that a long island or peninsula might be excited in such a manner for it to act as a slot antenna cut in a conducting sheet. Both Gould [ 1961] and Morgan [ 1979] presented impedance measurements seen at the terminals of a cable that was terminated in the sea. The observed resonances were attributed to the same mechanism that caused a slot to resonate when it is cut in a metal sheet.…”

“…The observed resonances were attributed to the same mechanism that caused a slot to resonate when it is cut in a metal sheet. Unfortunately, neither GouM [ 1961 ] nor Morgan [1960Morgan [ , 1979 gave any independent measured data on the effective earth conductivity.…”

‘Peninsula impedance measurements’ in the Marlborough Sounds, New Zealand

“…To determine the effect of using a series of parallel transmission lines similar to the five lines used by Morgan [1979], the three single conductors were untaped and separated by about 8 m along the 900-m length of the line. The input impedance of this three-parallel-line configuration was then measured from Cissy's Bay.…”

“…and Barr [1980] have suggested that the resonance at 9.5 kHz found in the impedance measurements on Deception Island [Morgan, 1979] was due to the resonance of the feeder line connected to the island to facilitate the impedance measurements, the phase velocity on the line having been reduced by its proximity to the ground [Chang and Wait, 1974;Burrows, 1978;Barr, 1980;Wait, 1980]. This is contested by Morgan [1980], who suggests that the line in fact exhibited a resonant frequency of 60 kHz, indicating essentially a free space propagation velocity, and that the 9.5-kHz resonance was due to the island.…”