Why submarines listen at 24 kHz
2026-04-02 · EA5IYR
There is a radio station in Cutler, Maine with the callsign NAA. It transmits on 24 kHz at up to 1.8 megawatts, which makes it one of the most powerful transmitters on the planet, and its antenna is not a mast but two of them — twin umbrella arrays, each a central tower ringed by twelve more, holding up a web of horizontal cable.
All of that exists to talk to submarines that do not want to come to the surface. Saltwater is a conductor, and conductors eat radio
The governing number is skin depth: how far a field gets into a conducting medium before it drops to about a third of its surface strength. In seawater it depends on frequency, and it depends on it harshly.
At 24 kHz you get roughly 1.6 metres. Push up to the lower shortwave bands around 3 MHz and it collapses to about 15 centimetres. At the top of HF you are down to a few centimetres, which is to say the signal does not get into the water at all.
Skin depth is not the same as usable range — with enough transmitter power a signal remains readable considerably deeper than one skin depth. But the shape of the curve is what matters, and it points in exactly one direction: go lower. The price is everything else
Choosing 24 kHz solves the depth problem and creates every other problem.
The bandwidth available at that frequency is tiny, so the data rate is measured in a handful of characters per second. The signal is a continuously encrypted minimum-shift keyed carrier, running whether or not there is anything to say, because a transmitter that only switches on when it matters announces that something matters.
And the antenna has to be enormous, because at 24 kHz a wavelength is over twelve kilometres and nothing you can build is electrically large. What it can actually say
Not much, and that is by design. A station like Cutler is closer to a doorbell than a telephone. It can carry a short instruction, and most usefully it can tell a submerged boat that there is traffic waiting — at which point the boat can come shallow, deploy a buoy or a trailing wire, and receive properly on a faster link.
Cutler is not alone. Jim Creek in Washington State covers the Pacific, DHO38 at Rhauderfehn serves the German navy, and the pattern repeats in every navy that operates ballistic missile submarines. They are among the few radio facilities where you can stand at the fence and see, physically, how much power a government is willing to spend on a very small number of bits. Can you hear it?
Yes, and it is one of the more rewarding things a receiver can do. VLF sits far below the broadcast bands and needs either an SDR that reaches down there or a simple long-wire and a sound card. NAA on 24 kHz is a steady presence over much of the North Atlantic.
You will not decode anything. You will just hear a carrier that has not stopped since 1961.
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Sources: Wikipedia — VLF Transmitter Cutler, Wikipedia — Jim Creek Naval Radio Station, Wikipedia — VLF transmitter DHO38.