Many years ago when I was in college and more active in shortwave listening than I am now, I would on occasion come across a transmission of numbers. These numbers were being transmitted in groups of five, with a female voice, and were on frequencies outside of the international broadcast bands as well as the amateur radio bands. Their spots on the dial weren't really meant to be easily found by the casual listener. For the most part, the operation took care never to operate on frequencies used by the Coast Guard or aircraft (there is one known exception to that). If a casual listener found the transmission, that listener was left with a mystery. What was it? Loggings of this nature, when they found their way into shortwave club bulletins, were written of as a "spy numbers" transmission and that was that.
One Wednesday evening in March of 1979 I came across, again, that lady reading the numbers. I had seen mention of a "mystery numbers" transmissions in a magazine and these were credited to Cuba. That made sense, as that the propagation pattern closely matched that of Radio Habana Cuba. The article mentioned that in 1975 some listeners heard audio from Radio Habana Cuba, and some months later I stumbled upon a major on-air slipup.......Radio Habana Cuba on some frequency well outside the broadcast bands, signing off their English program, and then the start of a numbers transmission.
Anyway, the following Wednesday evening I was again tuning around the bands, and guess what? I came across that numbers broadcast again! I noted that it was on the same frequency as the Wednesday before, same time slot, and it was then that it occurred to me that this operation followed a pattern. If I could spend enough time chasing that station around the dial, I could figure out their schedule, and then be listening in on the same time that whoever that spy was. The signals were very strong, indicating that they were beamed to North America, and that somewhere over here we've got an intended recipient who is getting some instructions of one kind or another. All of that figured out, there was still a lot that I didn't know but would end up learning. But here is what I learned in that timeframe.
One, the transmission would always start at the top of the hour. I synced by watch to WWV (time station) and learned that they were precise in that. Before transmitting, there would be a strong carrier, sometimes ten minutes before show time, but the curtain would always rise at the top of the hour.
Two, each transmission would be repeated. Some repeats were at the bottom of the hour, as in 30 minutes past the top. Others would repeat five minutes or so after the end of the first, and almost always on a different frequency. I quickly figured out which frequency ranges to tune in to, and I got them several more times.
Three, the format was easily figured out. For example, if the transmission started out with "Atención! Cinco ocho tres: seis dos" (repeated several times before going into the message), it meant that 62 groups of five digit numbers were in the message. In this example, I am considering the 583 to be the header and 62 the group count. I noticed in my monitoring that group counts were usually a number divisible by 5, but that wasn't always the case.
There was more that I was going to learn. My interest in this station fell off for a few years, but was rekindled in 1984 when I came across an article in the August 1984 Popular Communications magazine. The link is below, and you'll have to scroll down to page 16 of the magazine.
https://worldradiohistory.com/Archive-Popular-Communications/80s/Popular-Communications-1984-08.pdf
I'll come back to this topic again in my next post.
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