Broadcast range


Broadcast range

A broadcast range (also listening range for radio, or viewing range for TV) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which is closely related). It is generally the area in which a station's signal strength is sufficient for most receivers to decode it, however this also depends on interference from other stations.

Contributing factors

The range of a station is primarily determined by its effective radiated power (ERP), and its height above average terrain (HAAT), with the latter actually being much more important in the higher radio bands.

For a station to double its effective radius, and therefore quadruple its service area, it must quadruple its signal strength as measured in millivolts per square meter (mV/m²). To do this, it must increase its effective radiated power (ERP) by another square, for a total of 16 times ( [2²] ², or 24). This explains why low-power stations (100 watts or less) can still go several kilometers, despite having a tiny fraction of the power of large stations that can have up to 100,000 watts ERP in some countries. (It should be noted that this is not the transmitter power output, which is usually several times less.) This applies even for stations with a directional antenna, which extends the broadcast range out more in some directions than others.

While power is an easier concept to understand from a simple number, a station's height is often much more important to its broadcast range. In contrast to power, a modest increase in height can have a dramatic effect on the distance a VHF or UHF signal can reach, an effect which continues to increase further up into the microwave bands used both terrestrially and in communications satellites. Part of this is due to the curvature of the Earth, where the height increases the distance to the horizon, and therefore the line of sight.The other part is due to the same obstructions (buildings and trees) that cause attenuation or scattering of the signal. Reflected signals like this also cause ground clutter on older non-Doppler radars, when the scattering causes the signal to return to the point from which it came (retroreflection).

TV satellites are so high (often over 20,000 miles or well over 30,000 kilometers) that each transponder needs less than 100 watts to cover an entire continent, though this is also largely because satellite dishs focus the weak signals and eliminate most interference from adjacent satellites. Radio satellites do not use a dish, but also have no other uses on the same frequencies, allowing the low-power signal to cover a continent more like the reception of terrestrial stations. In the case of satellites, the broadcast range is deliberately restricted by the antenna aperture to what is called a footprint, which can be larger than a continent or as small as a spot beam will allow.

Legal definitions

The primary service area is the area served by a station's strongest signal. The city-grade contour is 70 dBµ (decibels relative to one microvolt per square meter of signal strength) or 3.16mV/m² (millivolts per square meter) for FM stations in the United States, according to FCC regulations. This is also significant in broadcast law, in that a station must cover its city of license within this area, except for non-commercial educational and low-power stations.

The legally-protected range of a station extends beyond this range, out to the point where signal strength is expected to be 1mV/m² for most stations in North America, though for class B1 stations it is 0.7mV/m², and as low as 0.5mV/m² for full class B stations (the maximum allowed in densely-populated areas of both Canada and the U.S.).

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 and even single large mountains block FM 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 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. For unclear reasons, 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. While the groundwave signals at these low frequencies is not nearly as affected by terrain or other obstructions in the line of sight, a completely different issue occurs at night, when skywave reflects off the ionosphere at a much greater distance above Earth's surface. 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.

Edge-of-range issues

Those at the edge of a station's broadcast range will typically notice static in an analog broadcast, while error correction will keep a digital signal clear until it hits the cliff effect and suddenly disappears completely. FM stations may flip back and forth (sometimes annoyingly rapidly when moving) due to the capture effect, while AM stations (including TV video) may overlay or fade with each other.

FM stereo will tend to get static more quickly than the monophonic sound due to its use of subcarriers, so stations may choose to extend the usable part of their range by disabling the stereo generator. Listeners can also choose to disable stereo decoding on the receiver, though loss of the stereo pilot tone causes this to happen automatically. Because this tends to turn on and off when at the threshold of reception, and the threshold is often set too low by the manufacturer's product design, manually disabling this when at the edge of the broadcast range prevents the annoying noisy-stereo/quiet-mono switching.

The same is true of analog TV stereo and second audio programs, and even for color TV, all of which use subcarriers. Radio reading services and other subcarrier services will also tend to suffer from dropouts sooner than the main station.

Technologies are available that allow for switching to a different signal carrying the same radio program when leaving the broadcast range of a station. Radio Data System allows for switching to a different FM or station with the same identifier, or even to (but not necessarily from) an AM station. Satellite radio also is designed to switch seamlessly between repeaters and/or satellite when moving outside the range of one or the other. HD Radio switches back to the analog signal as a fallback when the edge of the digital range is encountered, but the success of this from the listener's perspective depends on how well the station's engineer has synchronized the two.

Digital versus analog

Digital transmissions require less power to be received clearly than analog ones. The exact figure for various modes depends on how robust the signal is made to begin with, such as modulation, guard interval, and forward error correction. In each of these three factors, the caveat is that a higher data rate means a tradeoff with reduced broadcast range. The hierarchical modulation used on DVB is a unique case, which reduces the range of the full-definition signal, in exchange for an increase in the usable range of the lower-definition part of the video.

Digital stations in North America usually are operated by the same groups as the analog side, and thus operate their own independent facilities. Because of this, the FCC requires U.S. TV stations to replicate their analog coverage with their digital signal as well. However, ATSC digital TV only requires about one-fifth the amount of power to reach the same area on the same channel as analog does. For HD Radio, the figure is only one percent of the station's analog wattage, in part because it is an in-band on-channel method, which uses sidebands that must prevent interference to adjacent channels, especially for older or cheaper receivers which have insufficient sensitivity and/or selectivity.


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