- Broadcast range
A broadcast range (also listening range for
radio , or viewing range forTV ) is the service area that abroadcast station or other transmission covers viaradio wave s (or possiblyinfrared light , which is closely related). It is generally the area in which a station'ssignal 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 itsheight above average terrain (HAAT), with the latter actually being much more important in the higherradio band s.For a station to double its effective
radius , and therefore quadruple its servicearea , it must quadruple itssignal strength as measured in millivolts per square meter (mV/m²). To do this, it must increase itseffective 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 thetransmitter power output , which is usually several times less.) This applies even for stations with adirectional 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 orUHF signal can reach, an effect which continues to increase further up into themicrowave bands used both terrestrially and incommunications satellite s. Part of this is due to the curvature of the Earth, where the height increases the distance to thehorizon , 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 signal s like this also causeground clutter on older non-Dopplerradar s, 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 entirecontinent , though this is also largely becausesatellite dish sfocus 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 theantenna aperture to what is called a footprint, which can be larger than a continent or as small as aspot 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 (
decibel s relative to one microvolt per square meter of signal strength) or 3.16mV/m² (millivolts per square meter) for FM stations in theUnited States , according toFCC regulation s. This is also significant inbroadcast law , in that a station must cover itscity of license within this area, except fornon-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 bothCanada and the U.S.).Practical application
In reality,
radio propagation changes along with theweather andtropospheric ducting , and occasionally along with other upper-atmospheric phenomena likesunspot s and evenmeteor shower s. Thus, while abroadcasting authority might fix the range to an area with exactboundaries (defined as a series ofvector s), 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 range s and even single largemountain s blockFM broadcast s 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 wave s cannot pass, particularlystone . At times this may be due to weather, such as when the tallcumulonimbus cloud s of asquall line ofthunderstorm s reflect the signal over the top, like an extremely tallradio tower . Conversely, heavyrain mayattenuate the range of even local stations. For unclear reasons,ATSC digital television is affected bywind andtree s (even if not surrounding the transmitter or receiver locations), apparently related to its use of8VSB modulation 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 radial s which act as aground plane for themast radiator s used. While thegroundwave 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, whenskywave reflect s off theionosphere at a much greater distance aboveEarth's surface . This in turn causes mediumwave, mostshortwave , and evenlongwave stations to travel much further at night, which is the side of theEarth 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 channel s.Border blaster stations in northernMexico also used this effect, along with very high-powertransmitter s, to extend their nighttine broadcast ranges well over theUS/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 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 orrebroadcaster s 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 network s do inEurope .Distributed transmission has also undergone tests in the U.S., but to preserve stations'market share in their homemedia 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 theirhigh-rise buildings cause to the many current and potentialcustomer s that are concentrated there.Edge-of-range issues
Those at the edge of a station's broadcast range will typically notice
static in an analogbroadcast , whileerror correction will keep adigital signal clear until it hits thecliff effect and suddenly disappears completely.FM stations may flip back and forth (sometimes annoyingly rapidly when moving) due to thecapture effect , while AM 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 ofsubcarrier s, 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 thethreshold 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 of
analog TV stereo andsecond 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 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's engineer hassynchronize d 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 , andforward error correction . In each of these three factors, thecaveat is that a higherdata rate means atradeoff with reduced broadcast range. Thehierarchical 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 analogwatt age, in part because it is anin-band on-channel method, which usessideband s that must prevent interference toadjacent channel s, especially for older or cheaper receivers which have insufficient sensitivity and/orselectivity .
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