Back in the 1960s, I used to spend hours tuning around the so-called tropical bands. I rarely heard any stations because my antenna wasn’t too good and signals reaching the UK, if any, were weak. However, there were occasions when conditions were good and I heard one or two stations in the 120 metre band, 2.3 to 2.595MHz. There were also times when I received a station in the 90 metre band, 3.2 to 3.4MHz. I believe there are only a few stations using these bands today but it’s still worth tuning around, especially at night. A few stations can be heard in the 75 metre band, 3.9 to 4MHz, although I don’t think they are transmitting from tropical regions. The same applies to the 60 metre band, 4.75 to 5.06MHz.
Why were so-called tropical bands introduced?
In tropical regions such as Africa, South America, and Southeast Asia, thunderstorms and high humidity create a lot of atmospheric noise. This is especially noticeable below 2 MHz where normal medium wave (AM) broadcast operates. This static made medium wave signals very noisy and unreliable for local or regional radio coverage. So, shortwave frequencies just above the AM band (2.3–5 MHz) were adopted. This was because they are less affected by static from tropical lightning and they propagate better over longer distances (hundreds of km), even at night.
Tropical band signals (120 m, 90 m, 60 m) can cover hundreds of kilometers so were reliable for domestic, regional coverage. This was ideal for services in countries with poor terrain such as jungle, mountains and rain forests. Shortwave signals at these frequencies refract off the ionosphere, even at low power, making it possible to reach isolated communities. Essentially, they filled the gap between medium wave (too noisy and short range), and the higher shortwave bands (too long range and variable for domestic use). The 2–5 MHz range is absorbed during the day but reflects well from the ionosphere (F layer) at night. That meant local and regional stations could broadcast effectively after sunset, when listeners were home. Perfect for evening news and cultural programs in rural areas.
The ITU (International Telecommunication Union) recognized this problem early on (in the 1930s–40s) and specifically reserved the so-call tropical bands for broadcasters in equatorial regions. These were not intended for international shortwave broadcasters such as the BBC World Service or Voice of America, but for domestic national services.
In many tropical countries, building a network of AM or FM transmitters to cover jungle or mountainous areas was impractical. A single shortwave transmitter on a tropical band could cover vast rural areas and was cheaper and simpler than dozens of local relays. In rural tropical regions, shortwave receivers were affordable and common. Even small battery-powered radios could pick up domestic shortwave signals. This helped governments and broadcasters reach remote populations with news, education, and health information.