Broadcast Range - Practical Application

Practical Application

In reality, radio propagation changes along with the weather and tropospheric ducting, and occasionally along with other upper-atmospheric phenomena like sunspots and even meteor showers. Thus, while a broadcasting authority might fix the range to an area with exact boundaries (defined as a series of vectors), this is rarely if ever true. When a broadcast reaches well outside of its intended range due to unusual conditions, DXing is possible.

The local terrain can also play a major role in limiting broadcast range. Mountain ranges block FM broadcasts, AM broadcasts, and TV broadcasts, and other signals in the VHF and especially UHF ranges, respectively. This terrain shielding occurs when the line of sight is blocked by something through which the radio waves cannot pass, particularly stone. At times this may be moot due to weather, such as when the tall cumulonimbus clouds of a squall line of thunderstorms reflect the signal over the top, like an extremely tall radio tower. Conversely, heavy rain may attenuate the range of even local stations. ATSC digital television is affected by wind and trees (even if not surrounding the transmitter or receiver locations), apparently related to its use of 8VSB modulation instead of COFDM.

AM broadcasting stations have different issues, due to using the mediumwave band. Broadcast range in these stations is determined by ground conductivity, and the proper use and maintenance of grounding radials which act as a ground plane for the mast radiators used. skywave reflects off the ionosphere at a much greater distance above Earth's surface at night. This in turn causes mediumwave, most shortwave, and even longwave stations to travel much further at night, which is the side of the Earth where the solar wind pulls the ionosphere (and magnetosphere) away from the planet, instead of pushing toward it as on the day side. Because of this, many AM stations must cut power or go off-air at night, except for the very earliest stations still grandfathered on clear channels. Border blaster stations in northern Mexico also used this effect, along with very high-power transmitters, to extend their nighttine broadcast ranges well over the US/Mexico border and across most of the United States.

Various broadcast relay stations can help to extend a station's area by retransmitting them on the same or another channel. What is usually called a repeater in amateur radio is called a broadcast translator (different channel) or booster (same channel) in American broadcasting, or the much broader category or rebroadcasters in Canadian broadcasting (which includes more than just the low-power broadcasting used in the U.S.) Boosters are used only within the broadcast range of the parent station, and serve the same function locally as regional and national single-frequency networks do in Europe. Distributed transmission has also undergone tests in the U.S., but to preserve stations' market share in their home media markets, these will be limited to the broadcast area of a single large station. Satellite radio, which is designed for use without a dish, also uses ground repeaters in large cities due to the many obstructions their high-rise buildings cause to the many current and potential customers that are concentrated there.