Island as an antenna

Pieter-Tjerk de Boer, PA3FWM pa3fwm@amsat.org

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, February 2022.)

Not only radio amateurs come up with weird ideas, professionals also do this. In 1960, prof. Millett Morgan (who also was W1HDA) proposed using an entire island as a transmission antenna for studying VLF propagation [5]. For this he eyed Deception Island, near Antarctica.

Have a look at the map of Deception Island, copied from [6]. It consists of a ring-shaped mountain range surrounding a lagoon, which is connected to the sea at only one spot. The antenna's feedpoint is at the top-left on the map: from a point on the land wires are installed to the water on the "outside" of the island, and to Telefon Bay in the lagoon. The current should then flow through the sea water around both halves of the island.

Since the sea water forms a conductive surface, interrupted by the island as an insulator, this would form a natural slot antenna. By Booker's theory (see here), the radiation diagram should be comparable to that of a horizontal dipole bent to form a circle, but polarized vertically. Such an antenna has indeed been constructed on Deception Island in 1961 and its impedance measured [6], but the results did not match the expectations. In fact, more experiments have been done with antennas formed by spits of land surrounded by sea [7]. The conclusion seems to be that the land simply is not a good enough insulator, so the electric current flows back under the feedline, rather than taking the desired detour via the seawater.

B.t.w., the name Telefon Bay in this context is noteworthy. As it turns out, this bay has been named after a Norwegian ship called Telefon that was repaired there early in the 20th century. It also had a sistership called Telegraf [8]. Which such names, one might guess these were cable layer ships, but no, they were used for the whaling industry [9].

References

[5] M.G. Morgan: An Island as a Natural Very-Low-Frequency Transmitting Antenna. IRE Tr. on antennas and propagation, Sept. 1960.

[6] M.G. Morgan: Impedance measurements of Deception Island as a natural very-low-frequency antenna. Radio Science, Feb. 1979. (online)

[7] N.K. Uzunoglu, S.J. Kouridakis: Radiation of Very Low and Extremely Low Frequencies (VLF & ELF) by a Natural Antenna Based on an Island or a Peninsula Structure. Radio Science Bulletin, March 2004.

[8] https://munin-sandefjord.blogspot.com/2017/02/1834-12-peter-anton-hansen-grn.html

[9] https://www.sjohistorie.no/no/skip/1038685/ and https://www.sjohistorie.no/en/skip/29119/default