Editing Broadcast range

{{Short description|Service area for radio transmission}}
{{More citations needed|date=February 2023}}

A '''broadcast range''' (also '''listening range''' or '''listening area''' for [[radio]], or '''viewing range''' or '''viewing area''' for [[television]]) is the service area that a [[broadcasting|broadcast station]] or other [[transmission (radio)|transmission]] covers via [[radio wave]]s (or possibly [[infrared]] [[light]], which is closely related).  It is generally the area in which a station's [[signal strength]] is sufficient for most [[receiver (radio)|receiver]]s to decode it. However, this also depends on [[electromagnetic interference|interference]] from other stations.

==Legal definitions==
The "primary service area" is the area served by a station's strongest signal.  The "city-grade contour" is 70 dBμ ([[decibel]]s relative to one microvolt per meter of signal strength) or 3.16mV/m (millivolts per meter) for FM stations in the [[United States]], according to [[Federal Communications Commission]] (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 broadcasting|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/m for most stations in [[North America]], though for [[List of North American broadcast station classes#AM|class B1]] stations it is 0.7mV/m, and as low as 0.5mV/m for full class B stations (the maximum allowed in densely [[population|populate]]d 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 [[sunspot]]s and even [[meteor shower]]s.  Thus, while a [[broadcasting authority]] might fix the range to an area with exact [[Boundary (topology)|boundaries]] (defined as a series of [[Euclidean vector|vector]]s), this is rarely if ever true.  When a broadcast reaches well outside of its intended range due to [[anomalous propagation|unusual conditions]], [[DXing]] is possible.

The local terrain can also play a major role in limiting broadcast range.  [[Mountain range]]s block [[FM broadcast]]s, [[AM broadcast]]s, 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 wave]]s cannot pass, particularly stone.  At times this may be [[wikt:Moot|moot]] due to weather, such as when the tall [[cumulonimbus]] clouds of a [[squall line]] of [[thunderstorm]]s 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 [[Preventive maintenance|maintenance]] of [[grounding radial]]s which act as a [[ground plane]] for the [[mast radiator]]s used.  [[Skywaves]] reflect 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-channel station|clear channel]]s.  [[Border blaster]] stations in northern [[Mexico]] also used this effect, along with very high-[[electrical power|power]] [[transmitter]]s, to extend their nighttime broadcast ranges well over the [[US/Mexico border]] and across most of the United States.

Various [[broadcast relay station]]s can help to extend a station's area by retransmitting them on the same or another [[channel (broadcasting)|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 [[rebroadcaster]]s 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 network]]s do in [[Europe]]. [[Distributed transmission]] has also undergone tests in the U.S., but to preserve stations' [[market share]] in their home [[media market]]s, 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 [[White noise|static]] in an [[analog signal|analog]] broadcast, while [[error correction]] will keep a [[Digital signal (electronics)|digital signal]] clear until it hits the [[cliff effect]] and suddenly disappears completely.  [[FM broadcasting|FM]] stations may flip back and forth (sometimes [[Flutter (electronics and communication)|annoyingly rapidly]] when moving) due to the [[capture effect]], while [[amplitude modulation|AM]] stations (including TV video) may overlay or fade with each other.

[[FM stereo]] will tend to get static more quickly than the [[monaural|monophonic]] sound due to its use of [[subcarrier]]s, 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 program]]s, and even for color TV, all of which use subcarriers.  [[Radio reading service]]s 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 [[HD Radio|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 [[broadcast 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 emptor|caveat]] is that a higher [[data signaling rate]] means a [[tradeoff]] with reduced broadcast range.  The [[hierarchical modulation]] used on [[Digital Video Broadcasting|DVB]] is a unique case, which reduces the range of the full-[[screen resolution|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 [[watt]]age, in part because it is an [[in-band on-channel]] method, which uses [[sideband]]s that must prevent interference to [[adjacent channel]]s, especially for older or cheaper receivers which have insufficient [[sensitivity (radio receiver)|sensitivity]] and/or [[selectivity (radio)|selectivity]].

== References ==
{{Reflist}}

{{Federal Communications Commission}}

{{DEFAULTSORT:Broadcast Range}}
[[Category:Broadcast engineering]]
[[Category:Broadcast law]]