Introduction: 

Number stations, a relic from the Cold War, continue to intrigue radio enthusiasts around the globe. These mysterious stations, broadcasting coded messages in Morse code or human voice, were used to transmit messages to spies working overseas without direct contact that could be traced. In this project, we explore the use of a tape measure antenna alongside a software-defined radio (SDR) to intercept and record these intriguing number stations.

List of number station operators by country


What Are Number Stations:

Numbers stations are cryptic shortwave radio broadcast stations that have intrigued radio enthusiasts for decades. The activity of these stations reached their peak during the Cold War, and only a fraction of these station still operate today. These stations jump around the shortwave spectrum, never transmitting at the same frequency twice in a row. They transmit seemingly random sequences of numbers, letters, or other codes for only a few minuets at a time. The true purpose of numbers stations remains shrouded in secrecy, it is all but confirmed that they are used by intelligence agencies to communicate with undercover agents in the field. While governments neither confirm nor deny their involvement, numbers stations have become a captivating puzzle for radio hobbyists who meticulously track and log their transmissions. These broadcasts, which anyone with a shortwave radio can tune into, are a fascinating reminder of the covert world of espionage where information is conveyed through cryptic broadcasts hidden in plain view on the airwaves.
Part of the broadcast schedule put together by radio enthusiasts at Priyom.org

The Setup: 

To pick up radio signals, we utilize a software-defined radio (SDR) device. This device can be plugged into a phone or computer, and by connecting a suitable-sized antenna, we can tune in to specific frequencies. While a basic antenna suffices for commercial radio stations operating around 100 MHz, number stations typically transmit in the shortwave region at around 10 MHz, requiring a much longer antenna. Here, we construct an antenna using two 10-meter tape measures, creating a tape measure antenna with an overall length of 20 meters.

A software defined radio, about the size of a usb stick

Constructing the Tape Measure Antenna: 

To build the shortwave antenna, we remove the tape measures from their cases and sand the ends to establish electrical connections between them. One end of the antenna is connected to the radio connector, and the rest of it is deployed outside through a window, ensuring a straight line for optimal reception. With the 20-meter tape measure antenna in place, we are ready to explore the intriguing world of number stations.

Connecting a tape measure to an antenna lead


Picking Up Number Stations:

 By researching the time and frequency of upcoming number station broadcasts, (https://priyom.org/number-stations) we can align our setup accordingly. In this example, we focus on the Polish number station E11, which typically starts its transmissions with a series of numbers, such as "333," followed by additional numerical sequences.

Picking up a number station on a phone connected to SDR

Result: 

Using the tape measure antenna in conjunction with the SDR, we can successfully intercept and decode these number station broadcasts. Although we are unable to decipher their encrypted content, this project enables enthusiasts to document and study these captivating relics of the Cold War era.

Records of number station transmissions

Conclusion:

 Through the use of readily available tools such as an SDR and a tape measure antenna, radio enthusiasts can engage in the hobby of intercepting and studying number stations. The combination of a 20-meter tape measure antenna and an SDR allows us to explore the intriguing world of these coded transmissions, offering a glimpse into the secret operations of the past. So grab your SDR, construct a tape measure antenna,...

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