Cryogenic loop antennas for VLF reception in seawater

The signal-to-noise ratio used to receive VLF transmissions will improve as the antenna temperature is lowered, even when the antenna is enclosed in an insulating radome and submerged in seawater. Noise (or losses) coupled into the antenna from the seawater will limit the improvement if the received atmospheric noise is small compared to the coupled sea noise. The coupled losses can be reduced by increasing the size of the radome. The loop does not have to be superconducting to achieve important gains in its signal-to-noise ratio. Several small 16-gauge copper wire loops were analyzed. Calculations showed that the signal-to-noise ratio of a 5.08-cm-diameter, untuned, single-turn loop, enclosed in a 30.48-cm-diameter radome and submerged in 4-mho/m water, will improve by a factor of about 104(40 db) as the temperature of the antenna is lowered from 295 K to 20 K. The calculation was made at 20 kHz, and the noise level was assumed to be established by the real part of the antenna impedance. The signal level was held constant. A preamplifier using a superconducting quantum interference device (SQUID) appears to be the best suited for use with a small cryogenic loop antenna.