Shortwave radio is old technology. In the late 1920s it became a popular way to communicate over long distances, thanks in large part due to its propagation traits.
Shortwave radio energy is determined by the power of the transmitter. The bigger and more powerful the transmitter, the farther the signals travel. The physics of shortwave energy help, too, because it bounces off of the Earth's ionosphere.
The ionosphere is a region of space about 50 to 370 miles (80 to 595 kilometers) above the Earth's surface. It's an area of the upper atmosphere (or thermosphere) in which solar ionization takes place, causing an electrical charge. This charge reflects shortwave radio signals and bounces them back down to the terra firma, and in doing so, greatly increases the distance that the signal travels. This is called skip, or more fancily, skywave.
With a quality handheld radio, a person can tune into a strong shortwave signal from the other side of the planet. That's a useful characteristic for all sorts of communications purposes, and as it turns out, it's also great for spying.
If you're the head of an intelligence agency in the United States and you need to send a secret message all the way to an embedded spy in an area of conflict like Iraq, shortwave messages might be the best method. Power up an extra-powerful transmitter, broadcast your coded message at a predetermined time and frequency, and your agent abroad will immediately know whether to keep pretending to be a businessman or to assassinate a political foe.
And because shortwave radios are so common, the technology doesn't draw suspicion. An agent who is apprehended with a radio has plausible deniability on his side, whereas carrying specialized computer equipment and cryptography software is a red flag that could result in imprisonment ... or worse.
Shortwave is also immune to oppression. In times of strife, authorities can literally switch off a country's Internet access or kill the power to communications satellite. They can't stop every shortwave transmission.
What's more, computerized messages, even those created with supposedly crack-proof encryption, can eventually be traced and broken. In stark contrast, no one can trace who is receiving shortwave radio messages. Often, it's hard work even finding the powerful transmitters.
Because shortwave broadcasts are so easy to tune into, anyone with a cheap radio can eavesdrop by finding the right frequencies. That includes you. Many frequencies are listed at various web sites like SpyNumbers.
What you'll hear varies by station. Some stations broadcast continually. Others spring to life at scheduled times. Most keep strict schedules so that agents know when to tune in to the right frequency. On the next page you'll see more about what, exactly, numbers stations are sending through the airwaves.