It’s Numbers All The Way Down With This Tape Measure Number Station Antenna

For all their talk of cooperation and shared interests, the nations of the world put an awful lot of effort into spying on each other. All this espionage is an open secret, of course, but some of their activities are so mysterious that no one will confirm or deny that they’re doing it. We’re talking about numbers stations, the super secret shortwave radio stations that broadcast seemingly random strings of numbers for the purpose of… well, your guess is as good as ours.

If you want to try to figure out what’s going on for yourself, all you need is a pair of tape measures and a software defined radio (SDR), as [Tom Farnell] demonstrates. Tape measure antennas have a long and proud history in amateur radio and shortwave listening, being a long strip of conductive material rolled up in a convenient package. In this case, [Tom] wanted to receive some well-known numbers stations in the 20- to 30-meter band, and decided that a single 15-meter conductor would do the job. Unlike other tape measure antennas we’ve seen, [Tom] just harvested the blades from two 7.5-meter tape measures, connected them end-to-end, and threw the whole thing out the window in sort of a “sloper” configuration. The other end is connected to an RTL-SDR dongle and a smartphone running what appears to be SDRTouch, which lets him tune directly into the numbers stations.

Copying the transmissions is pretty simple, since they transmit either in voice or Morse; the latter can be automatically decoded on a laptop with suitable software. As for what the long strings of numbers mean, that’ll remain a mystery. If they mean anything at all; we like to think this whole thing is an elaborate plan to get other countries to waste time and resources intercepting truly random numbers that encode nothing meaningful. It would serve them right.

12 thoughts on “It’s Numbers All The Way Down With This Tape Measure Number Station Antenna

  1. University CubeSats tend to use cut-off bits of tape measure for their antennae, since when squished into the launch vehicle’s fairing the tape will fold inside the sat’s rectangular body, but it can spring out automatically when released.

  2. Can you use a tape that has imperial units? Ha! Wavelength in meters caught on early, but outside of hams frequency stuck on this side of the Atlantic which seemed to solve our metric problem. Mark the other side of the tape in frequency calculated by 1/4 wavelength. If the tape measure case is hung up high enough and insulated the variable length should should be tuneable over some useful range with connection at the bottom end.

    These have been used in field SW listening with a portable radio for a long time. Teaching aid in physics worldwide.

  3. A fair argument can be made for tape measure antennas for portable use, but for a semi-permanent application they are pretty ridiculous. Wire is cheaper and won’t wildly flutter around in the wind.

    1. Wire is better for portable use as well. I think a 50-foot piece of hookup wire with a small weight on the end would have served him as well and would certainly have been easier. I guess it was all for the joke, tho…

  4. “If they mean anything at all”

    Sometimes they don’t. One thing you don’t want to reveal when you’re broadcasting secrets, is how many secrets you suddenly have. This information can be very helpful to your adversary if only a few people are privy to a big fat juicy tidbit. So it is my scientific wild-ass guess that most numbers stations are sending completely random numbers or letters most of the time, to be replaced by actual data when such are available.

    I operated a “numbers” station for a day during the Cold War, and I believe the only purpose of this particular exercise was to test what accuracy could be expected of an untrained GI. But it could also have been an exercise in misdirection; this happened on a day when our long-range RADAR site had a scheduled power outage. I reported to the operations officer, was sat down in front of a Collins HF SSB transceiver, shown the “talk” button and tuning dial, given a frequency and a few sheets of paper with five-letter sequences hand-written on them, and told to read these at a slow, steady pace, starting at a precise time. My only qualification for this was that I know the phonetic alphabet, which I used every day in my regular work, but there was also a chart on the wall in case I needed help. The officer came in at intervals with new sheets of sequences. There was probably an equally untrained GI a few hundred miles away, transcribing what I read.

    Years later, I learned how the 5-letter groups worked: books were produced in pairs, for encoding and decoding. The encoding book had a set of words and phrases in alphabetical order, each followed by a number of 5-letter codes, the number of which was proportional to how often that phrase or word is likely to be used. When encoding, the officer would look up a phrase or word, copy the first 5-letter code onto a piece of paper, and cross off that code in the book. Presumably, the decoding book was arranged in alphabetic order by 5-letter group, but I never saw that. Each book had only one copy, and could only used between two specific stations, guaranteeing that codes were never re-used within the lifespan of a given book. Exciting times.

    1. Then the question becomes “Why 5 letter groups in particular?” Is it tradition or is there a reason? Why not 7, or 11? Not arguing with you, loved the story but it sparked a thought.

      “And that was part of the National Wire Shortage of 2023 children.” What about going to the grocery store and getting a roll of aluminum foil? A spool of solder? Fences? Flagpoles? Drainpipes? Semi-trucks? Now I wonder about rivers.

      1. Just a guess, I think that aside from tradition, 5 letters is about all a typical person can reliably remember between hearing or seeing it and writing it down. Particularly important when you’re looking a code up in a decoding book. But for sure tradition is a big part of it.

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