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Abroadcast range (alsolistening range orlistening area forradio, orviewing range orviewing area fortelevision) is the service area that abroadcast station or othertransmission covers viaradio waves (or possiblyinfraredlight, which is closely related). It is generally the area in which a station'ssignal strength is sufficient for mostreceivers to decode it. However, this also depends oninterference from other stations.
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 meter of signal strength) or 3.16mV/m (millivolts per meter) for FM stations in theUnited States, according toFederal Communications Commission (FCC) regulations. This is also significant inbroadcast law, in that a station must cover itscity of license within this area, except fornon-commercial educational andlow-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 inNorth America, though forclass B1 stations it is 0.7mV/m, and as low as 0.5mV/m for full class B stations (the maximum allowed in denselypopulated areas of bothCanada and the U.S.).
In reality,radio propagation changes along with theweather andtropospheric ducting, and occasionally along with other upper-atmospheric phenomena likesunspots and evenmeteor showers. Thus, while abroadcasting authority might fix the range to an area with exactboundaries (defined as a series ofvectors), this is rarely if ever true. When a broadcast reaches well outside of its intended range due tounusual conditions,DXing is possible.
The local terrain can also play a major role in limiting broadcast range.Mountain ranges blockFM broadcasts,AM broadcasts, and TV broadcasts, and other signals in theVHF and especiallyUHF ranges, respectively. Thisterrain shielding occurs when the line of sight is blocked by something through which theradio waves cannot pass, particularly stone. At times this may bemoot due to weather, such as when the tallcumulonimbus clouds of asquall line ofthunderstorms reflect the signal over the top, like an extremely tallradio tower. Conversely, heavy rain mayattenuate the range of even local stations.ATSCdigital television is affected by wind and trees (even if not surrounding the transmitter or receiver locations), apparently related to its use of8VSBmodulation instead ofCOFDM.
AM broadcasting stations have different issues, due to using themediumwave band. Broadcast range in these stations is determined byground conductivity, and the proper use andmaintenance ofgrounding radials which act as aground plane for themast radiators used.Skywaves reflect off theionosphere at a much greater distance aboveEarth's surface at night. This in turn causes mediumwave, mostshortwave, and evenlongwave stations to travel much further at night, which is the side of the Earth where thesolar wind pulls the ionosphere (andmagnetosphere) away from theplanet, 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 stillgrandfathered onclear channels.Border blaster stations in northernMexico also used this effect, along with very high-powertransmitters, to extend their nighttime broadcast ranges well over theUS/Mexico border and across most of the United States.
Variousbroadcast relay stations can help to extend a station's area by retransmitting them on the same or anotherchannel. What is usually called arepeater inamateur radio is called abroadcast translator (different channel) or booster (same channel) inAmerican broadcasting, or the much broader category orrebroadcasters inCanadian broadcasting (which includes more than just thelow-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 nationalsingle-frequency networks do inEurope.Distributed transmission has also undergone tests in the U.S., but to preserve stations'market share in their homemedia 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.
Those at the edge of a station's broadcast range will typically noticestatic in ananalog broadcast, whileerror correction will keep adigital signal clear until it hits thecliff effect and suddenly disappears completely.FM stations may flip back and forth (sometimesannoyingly rapidly when moving) due to thecapture effect, whileAM stations (including TV video) may overlay or fade with each other.
FM stereo will tend to get static more quickly than themonophonic sound due to its use ofsubcarriers, so stations may choose to extend the usable part of their range by disabling thestereo generator. Listeners can also choose to disable stereo decoding on the receiver, though loss of the stereopilot 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 themanufacturer'sproduct design, manually disabling this when at the edge of the broadcast range prevents the annoying noisy-stereo/quiet-mono switching.
The same is true ofanalog TV stereo andsecond 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 sameidentifier, 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 afallback 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'sbroadcast engineer has synchronized the two.
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, andforward error correction. In each of these three factors, thecaveat is that a higherdata signaling rate means atradeoff with reduced broadcast range. Thehierarchical modulation used onDVB 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 analogwattage, in part because it is anin-band on-channel method, which usessidebands that must prevent interference toadjacent channels, especially for older or cheaper receivers which have insufficientsensitivity and/orselectivity.