Ham radio typically refers to the amateur use of various radio frequencies, including bands within the high frequency (HF), very high frequency (VHF) and ultra high frequency (UHF) ranges. An HF ham radio can specifically receive and transmit within the high frequency range, which is typically designated as being between three and 30 MHz. VHF and UHF are usually used for local communication, while an HF ham radio may be used at extremely long distances. Ham radio operators are typically regulated by their local governments as to how powerful their equipment can be, though power levels are often sufficient to allow for HF ham radio transmissions to propagate across the world.
High frequency radio transmissions are also referred to as shortwave, due to their wavelengths being shorter than those of previously used low and very low frequency bands. The utility of shortwave transmissions in facilitating long distance communication was first proven by ham radio operators in the early 1920s, when transmissions were sent and received between the North American and European continents. Later, HF ham radio transmissions were sent and received between New Zealand and California, and amateur use of various high frequency radio bands continues to the present day.
The way that a HF ham radio can be capable of transmitting to the other side of the world often involves the Earth's ionosphere and a concept referred to as skywave propagation. When certain radio wavelengths hit the ionosphere, which is composed of ionized air, they are reflected back to the earth. This can allow the radio signals to travel much further than they could if they were limited to direct transmission. Higher frequencies, such as VHF and UHF, are capable of piercing the ionosphere and are only reflected back under very specific circumstances.
Electronic noise pollution has become a concern to many HF ham radio operators. A number of electronic devices can create interference in the HF bands, and one particular concern is broadband over power lines (BPL). Not only can some BPL systems use high frequency bands, the transmission lines themselves may leak interference, creating noise in amateur radio frequencies and interfering with the operation of HF ham radios. Certain BPL systems are designed to avoid these situations, while others can be modified to reduce interference with HF ham radios. Some governments have regulations in place to require these sorts of measures be taken, while others have not yet settled on regulations to reconcile the two competing technologies.