Learning
What can you hear on a shortwave radio? A listener's map of 3 to 30 MHz
Shortwave carries voices and signals around the world by bouncing them off the upper atmosphere. Here's what's out there today, where on the dial it lives, and when to listen.
Why shortwave reaches so far
Signals between about 3 and 30 MHz bounce off the ionosphere, layers of charged gas 50 to 300 miles up, and come back down hundreds or thousands of miles away. The sun charges those layers, so what you hear depends on the time of day, the season and the 11-year sunspot cycle. A simple rule: higher frequencies work in daylight, lower ones at night.
What you'll hear
Broadcast stations
International and religious broadcasters, plus some national services, still use shortwave, clustered in bands named by their wavelength: 49 meters (about 5.9-6.2 MHz), 41 (7.2-7.45), 31 (9.4-9.9), 25 (11.6-12.1), 19 (15.1-15.8), 16 (17.5-17.9) and 13 (21.45-21.85). A few also send DRM, a digital broadcast that sounds like a flat hiss on an ordinary radio.
Ham radio operators
Licensed amateurs talk to each other on their own bands, such as 80, 40, 20, 15 and 10 meters. Voice is usually single sideband, which sounds like Donald Duck until you tune it in on SSB mode. Near the bottom of each band you'll find Morse code and digital modes such as FT8, RTTY and PSK31.
Time stations
WWV in Colorado ticks every second and announces the time every minute on 2.5, 5, 10, 15 and 20 MHz; WWVH in Hawaii uses 2.5, 5, 10 and 15 MHz. They're the easiest way to check whether shortwave is working at all. See time stations.
Weather maps for ships
Coast stations send weather charts as sound: weather fax, a steady scratchy rhythm that draws a map line by line. Ships also get text warnings by NAVTEX just below the AM broadcast band.
Aircraft and ships
Planes over oceans use HF voice and a data system called HFDL; ships use DSC calling bursts. Military and government stations link up with ALE chirps and fast data modes such as STANAG 4285.
The mysteries
Numbers stations read digits aloud; UVB-76, "the Buzzer", has buzzed on 4625 kHz for decades; over-the-horizon radars sweep across whole bands with a rattling or swishing sound.
Noise
Much of what you hear is not a transmission at all: buzzing, hissing and ticking from electrical devices nearby. Why is my radio buzzing? explains how to tell them apart.
What you need
- A portable shortwave radio. Models with an SSB setting can tune ham voice and most digital signals properly.
- An SDR (software-defined radio) plugged into a computer. Some cheap SDRs need an add-on or a "direct sampling" mode to reach shortwave.
- An antenna. A few meters of wire strung outdoors, away from the house's electrical noise, beats any built-in telescoping antenna.
- Or none of it: many hobbyists share their receivers online as web SDRs you can tune from a browser.
A first evening's listening plan
- Tune 10 MHz and find WWV's ticks. If you hear it, conditions toward Colorado are open.
- Scan 5.9-6.2 and 7.2-7.45 MHz for broadcasters.
- Switch to SSB and try 7.074 MHz: the chirping is FT8. Just below, 7.000-7.060, is Morse.
- If you hear something you can't name, write down the frequency, time and what it sounded like, and use the field method for identifying signals.
Listening is legal in the United States and most countries; only transmitting needs a license.