Numbers Stations, The Woodpecker etc.

The so-called “Numbers Stations“ are a favorite topic on the Interwebs. Some obvious open source information is available but wild speculation about them is widespread. I won’t try to duplicate any of that here but below are some of my observations. Although they are not as common as in the past, they are still in operation and can be monitored.

I copied a lot of 5 number/letter Morse Code cypher groups from them over the years. They were excellent code practice as I studied for my various ham license tests.

Below: Putting the CIA’s RS-6 Spy Radio to work listening to various numbers stations. Spying on spies; and keeping your coffee warm while on a late-night Op at the Safe House.

Mission Essential Equipment

Encrypted short wave “numbers” transmission is a favorite cheap, simple and effective method to communicate. It is known to be used for example by the Cuban Dirección de Inteligencia service to communicate with their spies in the U.S.* (Reference 92.)

*During the prosecutions of numerous Cuban spies in the U.S. the FBI was required by court proceedings to reveal seized evidence of the Cuban spies use of Short Wave radio and associated encryption systems.  Sufficient to convince a lay Jury.  Convictions followed, but as a result Castro learned of his own spy communications vulnerabilities.

When a message is encrypted with a One Time Pad (OTP) and used correctly, the cypher is unbreakable: the contents of these “numbers” transmissions are unrecoverable.

This system is presumably used by many other countries and organizations. As an example, the U.S. used the DIANA offline encryption system during combat Ops in Vietnam.

Below are 2 recordings I made late at night of a well known, reportedly Cuban transmitter, dubbed “M08A”. The first 2 using Morse Code cut-numbers of a non-standard (Cuban, or false-flag) variety. 1=A, 2=N, 3=D, 4=U, 5=W, 6=R, 7=I, 8=G, 9=M, 0=T (Reference 93).

The first one sounds like a call up to several “out stations”. Starting with call signs; (note the triple DDD and GGG. Statistically very highly unlikely in an encrypted message, especially in adjacent Groups.) This preceded a transmission of many cypher groups. 5800 kc around 0600Z. Copied with my CIA RS-6 radio set (above). Click the Arrow buttons:
Numbers Station callup

The above MCW (A2A) transmission continued with 5 cut-numbers cypher groups. Someone’s One Time Pad would be getting a workout. The “Cuban 5” spy ring perhaps?

An advantage of using cut-numbers is that the agent does not need to be “Morse Code qualified”. With minimal communications training they just need to know those 10 simple, 2 or 3 element letters. Those are also much faster to send than the actual 5 element numbers in the standard code. That reduces transmitter “on air” and agent exposure time.

A related OTP system transmits letters versus numbers. This system requires higher level Morse Code skills typical of professional communicators; all letters are utilized. As with the “cut-numbers” system the conversion of letters into different letters when mixed with the OTP key is via an UNCLAS “Trigraph” table during encryption/decryption.

There are many posts online demonstrating the construction and use of one-time-pads. Fundamentally, you need simple number-letter-number UNCLAS conversion tables AND a set of truly random 5 digit numbers. Not computer generated as those are deterministic.

A tried, proven, totally random and therefore secure (but slow) technique uses 10-sided dice to generate an OTP key. Make only 2 copies, one for the transmitter, the other for the receiver. Do not store on, or forward via a computer. Destroy keys immediately after use.

Random Number Generation

Although there are some stations transmitting spoken alpha/numeric codes, the classic way is via Morse Code. For a given transmitter power, distance, interference level etc, Morse is much more effective than voice in getting through to the receiving station. Also the transmitter requirements are much simpler. It works. I taught it to our Cub Scouts.

The Soviets taught their children in organizations like ДОСААФ basic radio technology and Morse Code with kits like this: Радио (Radio) Magazine, Vol 4, 1980. “Komplects Radioamateur” (My dice added, but not a stretch.) What kid didn’t like playing with codes and cyphers! Note the blue morse code key.

Soviet Radio Training Kit – Note the Morse Code key

Below is another station I copied on 11462 kc with a voice preamble in Spanish followed by a multi-tone alert and then digital data streams. Someone has gone “high tech” in this one. Can you say Sound Card? Easy to distribute OTP Keys on a thumb drive via a Dead Drop.

Another interesting HF signal that could be called a “numbers station” is the High Frequency Global Communications System, HFGCS. Unvetted observations and wild speculation on this system are available on the Interwebs so nothing further here. But it is interesting to monitor with the trusty R-390A, especially with the echoes.


Do not answer……….. (Shades of the AN/PRT-4 / AN/PRR-9 !)

Generating cyphertext for a live transmission by an actual “numbers station”:

The M-209 Converter is an interesting system used by the U.S. during WWII and Korea as an encryption device. This is a low level tactical encryption system that produced cyphertext as letters, to be transmitted either by voice or Morse Code. It used a daily-changing Key detailing the mechanical settings internal to the M-209 versus an OTP-based system.

The M-209 and KeyMat

An exercise by the West Coast Military Radio Collectors Group and the Maritime Historical Radio Society was conducted in 2013. A message encrypted with an M-209 was transmitted on HF via Morse Code at 15 WPM and received by an AN/GRC-109 at the MRCG event site 200 miles away.

The MHRS station KFS is licensed commercially and is authorized to transmit encrypted messages unlike Amateur Radio where that is prohibited. The M-209 Key to this message was published in advance so other M-209 owners could participate in the exercise.

The message sent was an actual message transmitted in 1944 by Merrill’s Marauders during combat operations in Burma during WWII. This is the plaintext:


It was encrypted as a 26 group message and transmitted by KSM on their assigned 8642 kc frequency and simultaneously on their 6 and 12 mc channels. It was received at the MRCG event site, as transcribed below. Note the first and second groups are also the last 2 groups.

26 Group M-209 Message as received

Upon decryption by the above M-209 at the receiver site, we “Rogered” for it on 7050 kc CW with the same GRC-109.

A second message also encrypted by an M-209 was then sent and partially received via RATT using a GRC-46 setup at our receiver site. (NOTE: CW wins the day. Again!)

Further details here: and on M-209 Guru Mark’s site here:

The “Secrets” of the Woodpecker: Another signal I couldn’t avoid hearing with my trusty S-120 and my HA-350 was the Soviet дуга Over The Horizon Radar that they had built near Chernobyl in Ukraine. Variously known as DUGA-1, DUGA-3…

“DUGA” (Arc in Russian). Its signal was plenty loud in the U.S. at the time with its massive effective radiated power. Arc probably indicating its over-the-horizon mission.

This gigantic phased array antenna with its broadband cage dipoles fed by open wire transmission lines and linear wire reflector screen is 500′ tall and nearly a half-mile wide. There are actually 2 arrays here, one “small” and the other much larger. Likely for high and low frequency operation respectively. I’d have to visit.

(Can you find the man in the photo below?)

DUGA Radar antenna array. Photo credit: Peter Franc

The Ignoraty and Conspiracy Whacko’s described its purpose as mind control, weather control, Soviet HAARP, death rays, an earthquake trigger, etc. Dude! Like it totally caused Chernobyl to freakin’ EXPLODE! and THEN Explain Hurricane Katrina to me. Huh! Huh!

CNN opined in a March 4th 2019 article that it has “a far more sinister and mysterious reputation.”   CNN continues: “its true purpose and the important details of its functioning are covered in mystery.”  (Umm no CNN, it’s purpose and function was EXACTLY understood – 40 years ago.) 

That CNN Travel article included a map with the locations of the radar and the Chernobyl nuclear power plant (reversed).   “News you can trust”…

The radar sounded like a staccato pulse train of about 10 pulses per second, spread over a very large bandwidth due to the fast pulse risetime. It jumped around in frequency as they adjusted it to deal with HF propagation realities, appearing nearly everywhere at times in the HF spectrum.

The 2 main DUGA broadside antenna arrays (NATO variously Code named Steel Yard or Steel Work/s) were built along an axis of 053/233 degrees true (See Google Earth 51 degrees 18.30N 30 degrees 4.00E).  This is reported as the receiver site, the abandoned/dismantled transmitter site is about 50 km away to the northeast on that same 053 degree axis.

The system emits and then receives the reflected return signal at right angles to the dipoles/reflector array off to the northwest on an azimuth of 233+90=323 degrees true. If you plot that azimuth from the site, it goes over eastern Europe, Scandinavia, Greenland, Hudson Bay then directly over the ICBM missile fields in north-central U.S. Check.

No “secret” or surprise there, that was the threat axis they wanted to monitor.  How about range capability?

With its 10 pulses-per second, the interpulse period is 100 milliseconds.  As every radar operator knows, radar pulses (radio/light) travel 328 yards per microsecond.  That works out to a one-way distance of about 20,000 miles before the next pulse is transmitted.  Since radar is a round-trip system, the max detection range is half that, about 10,000 miles before the next pulse is transmitted, deafening the receiver.

That range has to include extended “side” trips up to the ionosphere and back, at least twice, likely 4 times.  Also, the Transmit/Receive “T-R” switch that protects the receiver from the massive transmitted pulse needs some recovery time, as does the receiver itself. So the effective detection range to a target would be more like the 5,000 ground miles to the center of the ICBM missile fields from DUGA. Check.

There CNN, solved if for ya.  “Important Details” revealed with a 4-tube Hallicrafters S-120 receiver, Google Earth (or then with CORONA satellite imagery) and a simple slide rule.

They located the radar in the southwest part of the then USSR (in Ukraine) to avoid HF radio propagation disturbances over the north polar areas, think “Northern Lights” etc.

As an aside, today there are the remains of a what appears to be a big Wullenweber or related CDAA direction finding receiver antenna. The site is exactly 1 mile SW of the receiver array along that 233 degree array axis, probably also in a DUGA transmitter antenna system pattern null to protect it from the transmitter pulse energy.

This site is known as “KRUG” (circle in Russian) and has been reported as a possible ionospheric chirp sounder. Makes sense – queing DUGA to the optimum frequencies for operation. The word азимут (azimuth) is seen on an interior control panel. Indicating to me that this detects favorable propagation paths both in frequency and direction.

I noticed when DUGA often shifted into the internationally allocated Amateur Radio ham bands (in violation of international law which the Soviets signed: nothing to see here, move along). When it showed up, that 10 PPS signal sounded just like a woodpecker, obliterating all communications on those frequencies. Hence its colloquial name among hams and other radio communicators: the Russian Woodpecker

FUN FACT: When THAT happened I found myself needing to test my transmitter and electronic CW keyer on frequencies allocated for ham radio use. Testing my keyer at 10 dots per second, or thereabouts, on the DUGA frequency, or thereabouts, I could often get them to move off frequency. My puny 40 watt CW signal was significantly stronger than any reflection they could detect off a missile or aircraft or maybe even North Dakota ground clutter.

Doing my part in the Cold War…LOTS of other Hams tested their equipment in a similar manner. “Woodpecker Hunting”…

Must have driven their operators crazy as it was apparently fairly easily “jammed”, accidentally of course. I don’t know what kinds of pulse-forming networks* or signal processing in the receivers they used at the time but it was probably adequate given the technology available. Transversal matched-filter gymnastics notwithstanding…

My chirpy Novice transmitter pulses probably looked like up-doppler shifted INCOMING!…

* Russian engineers were/are particularly good at designing pulse forming networks and pulse power machines. Apparently not so good at operating nuclear power plants. The DUGA site was abandoned in 1989 due in part to significant radiation contamination from the melt down of Chernobyl (and the melt down of the USSR and the Communist Party).

There are some You Tube videos of this antenna array and probably thousands of online photos. Sadly, NONE of them were taken by an Electronics Engineer!

Post under construction – more to follow.