VLF radio waves (3–30 kHz) can penetrate seawater to a depth of approximately 20 meters. Hence a submarine at shallow depth can use these frequencies. A vessel more deeply submerged might use a buoy on a long cable equipped with an antenna. The buoy rises to a few meters below the surface, and may be small enough to remain undetected by enemy sonar / radar.
Due to the low frequency, a VLF broadcast aerial needs to be quite big. In fact, broadcasting sites are usually a few square kilometers (or miles). This of course prevents such aerials being installed on submarines. Submarines only carry a VLF reception aerial, and do not respond on such low frequencies. So a ground-to-submarine VLF broadcast is always a one way broadcast, originating on the ground and received aboard the ship. If two-way communication is needed, the ship must surface and communicate on other, higher, frequencies (such as UHF or VHF).
Electromagnetic waves in the ELF frequency range (3–3000 Hz) (see also SLF) can travel through the oceans and reach submarines anywhere. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system operates at 76 hertz the Soviet/Russian system (called ZEVS) at 82 hertz. The latter corresponds to a wavelength of 3658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be constructed, as it would spread across a large country.
A VLF Antenna site in US. A typical VLF Transmitter site occupies a huge surface area and stretches for kms mostly . An example is a Grid laid out in the form of a Six pointed star (Star of David), with the Main Transmitter Station (MTS) at the center and transmission towers at the points. There are 13+ Transmission towers totally. The grid is composed of a central mast and two concentric circles of six masts each.
A surfaced submarine can use ordinary radio communications. Submarines may use naval HF, VHF and UHF voice and teleprinter circuits. Where available, dedicated military communications satellite systems are preferred for long distance communications, as HF may betray the location of the submarine. The US Navy's system is called Submarine Satellite Information Exchange Sub-System (SSIXS), a component of the Navy Ultra High Frequency Satellite Communications System (UHF SATCOM).
A Radio room inside a submarine.
No wonder the Indian Navy attached high priority to submarine communications even decades ago and subsequently anticipated the importance of Very Low Frequency (VLF) underwater transmissions instantaneously. As part of an ambitious naval modernisation program, during the mid-1980s the Indian Navy had constructed a VLF broadcasting station in Tamil Nadu.
INS Kattabomman, located at Tirunelveli, Tamil Nadu is home to a VLF (Very Low Frequency) station and communications network. It allows the Navy to communicate with its submarines, underwater, at long ranges. It can monitor surface vessels and can distinguish between merchant vessels and naval vessels over long distances in the Indian Ocean. It is the first of its kind in Asia and its technology was developed locally. VLF facilities exist at Chennai and Calcutta as well.
G.Vipin Shankar (SWL)