NAA Cutler: the most powerful submarine transmitter in the world on 24.0 kHz

What you hear

A continuous data stream on 24.0 kHz: on the waterfall a band a few hundred hertz wide, on a European receiver noticeably fainter than DHO38 just 600 hertz below it, in the headphones a steady tone with the rough texture of minimum-shift keying. There is nothing to read in it; the stream is encrypted naval traffic. What makes it worth tuning is the distance. The transmitter stands on the coast of Maine, in the north-east corner of the United States, and the signal that reaches a receiver in Europe has crossed the whole of the North Atlantic on a wavelength of twelve and a half kilometres.

On a receiver in New England or eastern Canada the picture is the reverse: NAA is the giant, and the European stations are the faint ones.

Who transmits and why

NAA is the United States Navy's VLF station at Cutler, Maine, built in the early 1960s to reach submarines of the Atlantic fleet while they stay submerged. Very long waves penetrate a few metres of sea water, which ordinary radio cannot, so a boat at shallow depth or trailing a wire antenna can take in a message without surfacing. For that to work over an ocean, the transmitter must be enormous, and Cutler is: a power of 1.8 megawatts, and an antenna of towers and wires spread over an entire peninsula, which makes it the most powerful submarine transmitter in the world. The callsign NAA is one of the oldest in American naval radio; it belonged to the Navy's station at Arlington, Virginia, in the early years of radio, and Cutler inherited it.

The content of the stream is secret and encrypted, and no listener will read it. The station's existence, frequency and callsign are public, it transmits continuously, and listening to it is simply receiving.

When and where to listen

Day or night, the stream is there; VLF signals are steady by day and a little stronger, with more variation, at night, and a transatlantic path like this one favours the hours when the whole path is dark, that is the European night and early morning. In North America the signal is strong everywhere on a receiver with a working VLF antenna. In Europe it is a question of a good receiver: a quiet rural site in Ireland, Britain, France, Portugal or Scandinavia hears it clearly, and from central Europe it is weaker but present.

The receiver matters more than the hour. Many public receivers were set up for shortwave and have antennas that are deaf this low, and near towns the VLF band is paved with mains harmonics. Try more than one. The program tunes the station in CW with a passband of about a kilohertz, which puts the stream as a tone in the middle of the audio; DHO38 sits close enough that both can be seen on one waterfall, and the contrast between them is the quickest way to grasp how far the American signal has travelled.

Like every big VLF transmitter, Cutler goes off for maintenance from time to time, and the frequency then stays empty for hours or days.

In ShackDeck

Station catalogue → Mystery stations → NAA Cutler (24.0 kHz). The program picks a public receiver whose range reaches 24.0 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. From Europe, choose a receiver on the Atlantic side if the first one shows only noise; from the Americas almost any receiver with a VLF antenna will do.