DHO38: the German Navy's submarine transmitter on 23.4 kHz

What you hear

Tune a receiver to 23.4 kHz, far below the lowest broadcast band, below even the long-wave broadcasters, and a strong, steady signal is there at any hour: on the waterfall a bright band a few hundred hertz wide, in the headphones a rough, rushing tone that never pauses and never resolves into anything a human can read. That is DHO38, the German Navy's very-low-frequency transmitter near Rhauderfehn in East Frisia, and in Europe it is the loudest signal in this part of the spectrum by a wide margin.

The rough texture is the data. The transmitter sends a continuous stream at 200 baud in MSK, minimum-shift keying, a form of frequency shift that keeps the signal narrow and clean. The stream is encrypted, so there is nothing to decode: what a listener gets is the sound and the sight of it. The reward is different from a voice station's. It is the sense of reaching a part of the radio spectrum most people never knew existed, and finding that the biggest thing in it is a navy talking to boats under the sea.

Who transmits and why

Submarines spend their patrols submerged, and ordinary radio does not reach them there: sea water swallows shortwave and VHF within centimetres. Very long waves are the exception. A signal at 23.4 kHz penetrates a few metres into the water, enough for a boat at shallow depth, or one trailing a wire antenna, to receive without coming up. Navies with submarines therefore keep VLF transmitters running day and night, because a boat that cannot be reached is a boat that cannot be given orders. DHO38 is Germany's, and its antenna is a field of tall masts over the flat land of East Frisia, built for a wavelength of nearly thirteen kilometres.

The traffic itself is a military secret, and the content of the stream will stay unknown. Its existence, its frequency and its callsign are not secret: the station is listed, it transmits continuously, and listening to it is simply receiving a signal that fills half of Europe.

When and where to listen

VLF propagates in the gap between the ground and the lower edge of the ionosphere, like a wave in a pipe, and is steady by day and stronger but more variable at night, with a wobble at sunrise and sunset when the ionosphere changes height. DHO38 is strong enough that none of this matters much: it is heard across Europe at any hour. What does matter is the receiver. Not every public receiver has an antenna that works this low, and the band is full of mains hum harmonics and switching-supply noise near cities. A receiver in the countryside, ideally in Germany, the Netherlands, Denmark or Poland, shows the signal best.

The transmitter goes off the air now and then for maintenance; when it does, the band looks empty for a few hours, which is itself a lesson in how much of the VLF waterfall is this one station. The program tunes it in CW with a passband of about a kilohertz centred on the signal, so the data stream lands as a tone in the middle of the audio; it is a listening mode, not a decoding one, and no decoder would help.

In ShackDeck

Station catalogue → Mystery stations → DHO38 (23.4 kHz). The program picks a public receiver whose range reaches 23.4 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. If the waterfall looks noisy rather than signal-filled, try another receiver: the station is always there, the question is only whether the receiver can hear that low.