Extremely Low Frequency (ELF) for Submarine Communication

Johnson Francis | June 23, 2023 | Radio Waves | No Comments

Extremely Low Frequency (ELF) for Submarine Communication

The lowest frequency allocated to Radio Amateurs in India is the 135.7-137.8 kHz in the Low Frequency or LF band from 30 to 300 kHz by National Frequency Allocation Plan 2022. I wonder whether anyone is using that band at all. Now there are radio waves even below that, of which Extremely Low Frequency or ELF has catched attention recently regarding submarine communication. According to International Telecommunication Union definition, it is between 3 to 30 Hz while in atmospheric science it is between 3 Hz to 3 KHz, as per the Wikipedia page on ELF.

Lightning and natural disturbances in earth’s magnetic field can produce ELF. Extremely long wavelengths of ELF necessitates very large sized antenna installations, virtually ruling out the possibility of amateur radio operations even if a band is allocated!

The importance of ELF is the ability to penetrate seawater which is rich in salt and can be used for communication with submarines. It may be noted that conventional radio waves cannot penetrate good conductors like seawater. But it requires antennas of lengths ranging from 15 to 60 km known as ground dipoles and megawatts of power, restricting it to only few National Governments, including India to have such facilities.

Wavelengths at ELF are in the range of 100,000 to 10,000 kilometers! They can diffract around large obstacles, are not blocked by mountain ranges and can travel around the curvature of the earth. Obviously, ELF transmissions to submarines can only be one-way as no submarine can have an antenna system large enough to transmit ELF. Data transmissions are extremely slow as well.

Alternate mode of radio communication by submarines is by surfacing their antennas intermittently, though at risk of being targeted. Another method is floating a tethered antenna buoy on the surface. Still other options for communications with submarines are underwater loudspeakers and hydrophones as sound waves travel well in water.