[Physics] How is extremely low frequency (ELF) radiation collected by a submarine antenna

Question

The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to $\approx 76$ Hz (implying a wavelength of $\approx 3,945$ km!). I was curious, what kind of receiving antennas do submarines use to actually pick up these ELF signals? What are the size requirements? Do submarines just trail a very long cable that approximates a monopole or dipole antenna, use electrical lengthening to shorten the antenna to some length $\approx 3,945/k$ km, where $k \in \mathbb{Z}$, and then travel along an optimal vector to pick up a transmitted signal?

Answer 1

There exist a wiki article on this, project elf.

The scaled-down system the Navy eventually constructed, called Project ELF, began testing in 1982 and became operational in 1989. It consisted of two transmitter facilities, one at Clam Lake, Wisconsin and one at Republic, Michigan. with a total of 84 miles[ of above-ground transmission line antenna. The two transmitters normally operated synchronized together as one antenna for greater range, but could also operate independently.

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In 2004 the Navy shut down both transmitters, with the explanation that very low frequency (VLF) communication systems had improved to the point that the ELF system was unnecessary

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The Russian Navy operates an ELF transmitter facility, named ZEVS ("Zeus"), to communicate with its submarines, located 30 km southeast of Murmansk on the Kola peninsula in northern Russia. Signals from it were detected in the 1990s at Stanford University and elsewhere. It normally operates at 82 Hz, using MSK (minimum shift keying) modulation. although it reportedly can cover the frequency range from 20 to 250 Hz. It reportedly consists of two parallel ground dipole antennas 60 km long, driven at currents of 200 to 300 amperes.[ Calculations from intercepted signals indicate it is 10 dB more powerful than the U.S. transmitters.Unlike them it is used for geophysical research in addition to military communications

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As fore receiving antennas, they do not have to be large.It is the generation of a frequency modulated with a signal that needs the full wavelength length. The receiver if the signal is strong works with smaller dimensions.

Ground dipoles are not needed for reception of ELF signals. The requirements for receiving antennas at ELF frequencies are less stringent than transmitting antennas, because the signal to noise ratio in ELF receivers is dominated by the large atmospheric noise in the band. The level of signal strength at the antenna is far above the noise in the receiver circuit, so small inefficient receiving antennas can be used. Various types of coil and ferrite loop antennas have been used for reception.

This might help somebody designing an antenna.