Super low frequency

Lore & Background

Because of the extreme difficulty of building transmitters that can generate such long waves, frequencies in this range have been used in very few artificial communication systems. However, SLF waves can penetrate seawater to a depth of hundreds of meters, making them valuable for military submarine communications. The U.S. naval service Seafarer operated at 76 hertz, becoming operational in 1989 but discontinued in 2004 due to advances in VLF communication systems. The Russian service ZEVS operates at 82 hertz, and the Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines.

The requirements for receivers at SLF frequencies are less stringent than for transmitters, because the signal strength (set by atmospheric noise) is far above the noise floor of the receiver, so small, inefficient antennas can be used. Radio amateurs have received signals in this range using simple receivers built around personal computers, with coil or loop antennas connected to the PC's sound card. Signals are analyzed by a software fast Fourier transform algorithm and converted into audible sound.

Reader's Guide

The significance of super low frequency lies in its unique ability to penetrate seawater to depths of hundreds of meters, a property exploited by the U.S., Russian, and Indian militaries for submarine communication. Despite the extreme difficulty of building transmitters for such long waves, these nations constructed huge radio transmitters operating at SLF frequencies—the U.S. Seafarer at 76 hertz, the Russian ZEVS at 82 hertz, and the Indian facility at INS Kattabomman. The U.S. service became operational in 1989 but was discontinued in 2004 as VLF communication systems advanced, reflecting the niche and evolving nature of SLF use. The legacy of SLF includes its role in enabling a critical military capability, while also demonstrating that receiver technology for these frequencies is accessible to radio amateurs, who can capture signals using simple PC-based setups with coil or loop antennas and software analysis. The conflicting designation with ELF (which in some contexts includes all frequencies up to 300 hertz) adds a layer of terminological nuance to the band's history.

Did You Know?

• SLF frequencies include the AC power grid frequencies of 50 hertz and 60 hertz.
• The U.S. naval SLF service Seafarer operated at 76 hertz from 1989 to 2004.
• The Russian ZEVS service operates at 82 hertz.
• Radio amateurs can receive SLF signals using a PC sound card and a coil or loop antenna.