One-way voice link
A numbers station, also known as a one-way voice link (OWVL),
is a special type of unusual radio broadcast, generally on the Short Wave (SW)
radio bands, reading out incomprehensible lists of (spoken) numbers
or morse coded
messages. The most common type features a female voice,
reading long strings of numbers, generally in groups of five, often preceeded
by a preamble and/or a series of musical notes.
In most cases, such stations carry
OTP encrypted messages.
During the Cold War number stations were often operated from Eastern Block
countries, such as Czechslovakia,
East-Germany (DDR) and Russia (USSR),
with their broadcasts aimed at spies and secret agents operating undercover
in West-European countries like Belgium, Germany, France and The Netherlands.
For this reason, many of the broadcasts were in German, as this language
was understood in most countries. Radio Amateurs will certainly remember
the artificial female voice that listed endless sequences of seemingly
random numbers on the short wave (SW) bands.
The agents were instructed to listen to specific SW frequencies
at fixed days and times, often using a
commercially available SW-receiver, from brands like
Sony, Panasonic and Grundig.
The image on the right shows a Sony ICF-2001D,
a commonly available receiver that became really popular amongst
the spies during the 1980s.
In most cases, the messages hidden behind the numbers,
were encrypted by means of the truely unbreakable
One-Time Pad (OTP) cipher
and the spies were trained to decode such messages and
destroy the keys immediately after reception.
Returning messages was much more difficult. In some cases, the agent delivered
a hand-written message is a so-called
dead letter box,
after which it was handled by someone else.
In such cases the messages were sent by courier or radio,
often with help from an East-European embassy.
There were also situations in which a spy used his own transmitter
to send the message.
Such transmitters are commonly known as 'spy radio sets',
many of which are covered on this site.
Sending a message from a spy radio station was extremely dangerous as
the authorities of the guest country were well aware of them and
were constantly monitoring the waves
for clandestine transmissions.
Once a suspected signal was intercepted, they
would immediately try to locate the station by means of fixed or mobile
Radio Direction Finding (RDF)
During the Cold War, a number of clandestine radio stations,
operated by foreign secret agents, were intercepted
and captured by Western intelligence agencies. One documented example is
the capture of a Dutch citizen, who acted as an East-German agent in
The Netherlands, in 1969. When he was exposed, the intelligence agency
found a completely operational
Russian R-353 spy radio set
(shown above) in his home, along with a partly used
Number stations are not exclusive to the former Warsaw Pact countries.
Like the Russians, the Americans and their Allies also had spies operating
undercover behind the Iron Curtain, and number stations were used for passing
coded messages to them. In the same vein, countries like Cuba and China
have operated such number stations, some of which are still active today.
In fact, even Russian and European Number Stations can still be found
on the SW-bands today.
Over the years there has been much speculation as to the real purpose of
number stations. It has been suggested that the stations were used for
drugs smuggling, but this seems highly unlikely. Illegal radio stations
operating at high output power are easily located by means of
radio direction finding (RDF),
and would have been dismantled by the authorities after some time.
It is commonly accepted today, that number stations operate at high output
power and are operated by governments as a simple and fool-proof method
to send messages to spies and agents working undercover in a foreign country.
The reason for using the
SW bands for this, is that, under the right conditions, SW signals can span
the entire earth without using satellites or common civil communications
networks such as telephone lines and the internet. Unlike modern
communications methods like the internet,
the recipient of a SW broadcast can not be traced.
In 2014, the Czech Government officially confirmed the existence of
at least two number stations in Czechoslovakia during the Cold War.
In reply to a request by Priyom.org, they released some official documents .
In 2015, this was confirmed by the Swedisch Intelligence Agency SÄPO .
We currently recognise the following types of Number Stations:
- Voice stations (reading numbers)
- Morse code stations (CW)
- Multi-tone stations (RTTY, FSK, MFSK)
- Digital stations
- A combination of the above
Although in some cases dedicated receivers were supplied to an agent,
standard off-the-shelf commercial receivers were often used for the reception
of the Number Stations. The reason for this is that such receivers
could be bought in nearly every Western country without attracting any attention of the authorities. Furthermore, it gave Eastern Block countries
access to the latest technology that was available commercially in the West.
One of the first domestic receivers used for this purpose was the
Zenith Royal 1000;
the first all-transistor SW-receiver from the USA.
It was eventually surpassed by the
Grundig Satellit 2000 and finally the
digital Sony ICF-2001D.
It may surprise you, but Number Stations are still active today (2015)
as countries are still spying on each other and the short-wave band is
one of the safest ways of sending messages without leaving traces.
Below is an example of a recently picked up message .
More examples are here.
If you want to know more about the current Number Stations,
please check out the links below.
G06 · Russia
5422 kHz (German)
06132 75514 79681 94217 21443 31441 81797 17512 62689 33103
48930 93432 25709 93628 48683 18809 85052 49870 63962 04884
532 20 00000
Sender ID is in bold (947).
Recipient is in red (532) followed by the number of groups in the message (20).
Recipient and number of groups are repeated at the end of the message (532 20).
All number groups are repeated (except for the terminator).
00000 is the message terminator.
Sample kindly supplied by Karsten Hansky .
As the Number Stations that broadcast clandestine messages are illegal,
they are not officially identified by a name or number.
In the past, Number Stations were often given nicknames, such as
Lincoln Poacher and Cherry Ripe, often based on certain
characteristics of the transmission, such as the opening tune.
For this reason, a group of enthusiastic listeners, known as ENIGMA 1 ,
has assigned unique ID number to each station, along with prefixes,
suffixes and family IDs.
In the example above 06 is the station ID.
The prefix G indicates that the (voice) transmission is
in German. The suffix 'a'
indicates that it is a variant of the regular station G06.
In some cases, if a stations belongs to a family of stations,
the Family ID is given in Roman numerals. In this case the family ID
is IA, which means it is related to family I
For a full list of Station IDs, prefixes, suffixes and familiy IDs,
please refer to the current Enigma Control List .
S17e Bulgarian Betty
In some cases, the (nick)name of the station is written behind the
station ID, such as in the example of the S17e above. In this case,
the station was operated by the Czechoslovakian
StB, just like
S05 OLX (with the OLX callsign officially being assigned to a Czech news
In this context, ENIGMA does not refer to the
German Enigma Cipher machine,
but is the abbreviation of European
Numbers Information Gathering and
Monitoring Association. Although the original ENIGMA
group still exists, the list of station IDs, known as the Enigma
Control List, is now maintained by ENIGMA 2000 .
More about Number Stations
If you want to know more about Number Stations, their locations, frequencies
and the time at which they are expected to broadcast, here are a couple of
useful links to websites with the latest information.
Enigma 2000 are the
current maintainers of the Enigma Control List, whilst website
offers a real-time schedule of expected transmissions. In addition,
The Conet Project
has compiled an impressive collection
of sound samples, past and present, onto a stack of CDs.
Any links shown in red are currently unavailable.
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© Crypto Museum. Created: Sunday 15 March 2015. Last changed: Tuesday, 28 March 2017 - 14:33 CET.