Short Wave Listening or “SWLing” is monitoring domestic, and international broadcast stations usually between 1700Khz to 30Khz. These frequencies are commonly known as the HF or High Frequency band, consists of AM news and music broadcasts, HAM Radio, Marine, international aircraft traffic, The Atomic Clock, and military radio traffic (often encrypted).
Short wave receivers have gone down in price substantially over the years, although between reduced propagation from the solar minimums, and having to construct a sizable antenna to receive the distant stations, Web SDR’s (Software Defined Radio) have become very popular, as you only need an internet capable device for listening.
Web SDR’s are radio receivers that were setup often by ham radio operators for you to be able to control remotely over the internet. Depending on signal propagation and antenna characteristics, you may receive a stronger signal on one SDR over the other.
Radio Signal propagation
The mechanism that allows radio stations to be heard from around the world is called the ionosphere.
The ionosphere is the region above the earths atmosphere located between 100 to 1000Km above the surface of the earth, and contains atomic particles that become ionized when exposed to Xray and UV radiation from the sun. It has the ability to influence radio propagation around the world, by allowing signals to bounce off it and back to earth.
The ionosphere is separated into several layers, depending on the altitude above the earth. The most significant for shortwave listeners is the F2 layer, which occurs between 100 and 500Km above earth, and is the densest part of the ionosphere. It’s the part responsible for the most radio wave reflection. Reception via the ionosphere is called “Skywave”, in contrast to “Groundwave” propagation which follows the surface of the earth.
Long range reception varies between day and night due to the ionization caused by the sun. Skywave transmissions can be heard thousands of miles away from the broadcasting location.
The magnetic field of the sun flips during each solar cycle, with the flip occurring when the sunspot cycle is near is maximum. Levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation, from active to quiet to active again, with a period of 11 years.
Scanning the shortwave bands, you will find hundreds of stations from around the world. We’re going to only list off a few to get you started.
The National Institute of Standards and Technology (NIST) has two US broadcast stations: WWV based out of Fort Collins Colorado, and WWVH based at Kekaha Hawaii.
WWV and WWVH were established in 1945, and are two HF broadcast stations, broadcasting on 2.5Mhz, 5Mhz, 10Mhz, 20Mhz, and in experimental mode on 25Mhz. They are an automated system, that broadcast the current time in coordinated universal time. When listening, you will hear a series of beeps indicating the seconds, followed by “At the tone, “xx”hours “xx”seconds, coordinated universal time”. A sub audible 100Hz sub tone is also broadcasted with a digital time code interpretable by radio controlled clocks… This is the atomic clock!
Other useful information broadcasted by WWV and WWVH includes marine storm warnings from the National Weather Service, propagation information, geophysical alerts, and GPS satellite health reports from the US Coast Guard.
Voice of America– Established in 1942, VOA is the largest and oldest US funded international broadcaster, producing digital, TV, and radio content in 47 different languages which is distributes to affiliate stations around the globe. It is primarily viewed by foreign audiences, so VOA programming has an influence on public opinion abroad, regarding the United States and its people.