ICV Tavolara: the NATO submarine transmitter off Sardinia on 20.27 kHz

What you hear

A steady band of data on 20.27 kHz, a few hundred hertz wide on the waterfall and a rough, unbroken tone in CW mode. The texture is MSK, minimum-shift keying, the same narrow data signalling every submarine transmitter in Europe uses, and the stream is encrypted, so there is nothing to decode and nothing to read. From a receiver in Italy or southern France it is one of the strongest things in the VLF band; from northern Europe it is a fainter line between the giants, DHO38 at 23.4 kHz and GQD at 19.58 kHz, and finding it between them is the small satisfaction the station offers.

Who transmits and why

ICV is the VLF transmitter on Tavolara, a limestone island a few kilometres off the north-east coast of Sardinia, and it serves the submarines of NATO in the Mediterranean. The reason for the frequency is the reason for every station in this group: sea water stops ordinary radio at the surface, but very long waves penetrate a few metres, enough for a boat at shallow depth or trailing a wire antenna to receive a message without surfacing. A navy that wants to reach its boats at all times therefore keeps a VLF transmitter running around the clock, and for the Mediterranean that transmitter is on Tavolara.

Beyond that, honest information is thin. The content of the stream is secret and will stay so; the schedule is continuous, with occasional breaks for maintenance that are not announced to the public; and the history of the site and the details of the antenna are not well documented in sources this guide would rely on. What is certain is the frequency, the location and the role. The station's existence is public, and listening is only receiving.

When and where to listen

The signal is on the air day and night. VLF propagation is stable, steady by day and somewhat stronger and more variable at night, so the main variable is the receiver rather than the hour. Receivers in Italy, Sardinia and Corsica, the south of France, Spain and Greece hear ICV best; from central Europe it is readable on a quiet rural receiver, and from Britain or Scandinavia it is weak but usually visible on the waterfall as a line between the louder stations.

Any public receiver used for VLF needs an antenna that works this low and a site away from mains harmonics and switching-supply noise; if the first receiver shows only a wall of noise lines, try another, preferably further south. The program tunes the station in CW with a passband of about a kilohertz, so the stream sits as a tone in the middle of the audio. With a wide waterfall, ICV, GQD, HWU and DHO38 can all be seen at once between 19 and 24 kHz, a row of navies keeping their channels warm.

In ShackDeck

Station catalogue → Mystery stations → ICV Tavolara (20.27 kHz). The program picks a public receiver whose range reaches 20.27 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. A receiver in Italy or southern France shows the station at its best.