Editing Height above average terrain (section)

{{short description|Height based on large area surrounding object; often used in U.S. for antenna towers}}
{{No footnotes|date=November 2024}}
{{Redirects|EHAAT|the air ambulance service|Essex & Herts Air Ambulance}}

'''Height above average terrain''' ('''HAAT'''), or (less popularly) '''effective height above average terrain''' ('''EHAAT'''), is the [[vertical position]] of an [[antenna (radio)|antenna]] site above the surrounding landscape. HAAT is used extensively in [[FM radio]] and [[television]], as it is more important than [[effective radiated power]] (ERP) in determining the range of broadcasts ([[very high frequency|VHF]] and [[ultra high frequency|UHF]] in particular, as they are [[Line-of-sight propagation|line of sight]] transmissions). For international coordination, it is officially measured in meters, even by the [[Federal Communications Commission]] in the [[United States]], as [[Canada]] and [[Mexico]]  have extensive border zones where stations can be received on either side of the international boundaries. Stations that want to increase above a certain HAAT must reduce their power accordingly, based on the maximum distance their station class is allowed to cover (see [[List of North American broadcast station classes]] for more information on this).

The [[Federal Communications Commission|FCC]] procedure to calculate HAAT is: from the proposed or actual antenna site, either 12 or 16 [[Radial (radio)|radials]] were drawn, and points at 2, 4, 6, 8, and {{convert|10|mi|km}} radius along each radial were used. The entire radial graph could be rotated to achieve the best effect for the station. The altitude of the antenna site, minus the average altitude of all the specified points, is the HAAT. This can create some unusual cases, particularly in [[mountain]]ous regions&mdash;it is possible to have a negative number for HAAT (the transmitter would not be located underground, but rather in a [[valley]], with hills on both sides taller than the transmitter itself, for example).

The FCC has divided the [[Contiguous United States]] into three zones for the determination of spacing between FM and TV stations using the same frequencies. FM and TV stations are assigned maximum ERP and HAAT values, depending on their assigned zones, to prevent [[co-channel interference]].

[[Image:US FM broadcast zones.png|thumb|right|300px|FM broadcast zones in the U.S.]]
The FCC regulations for ERP and HAAT are listed under Title 47, Part 73 of the [[Code of Federal Regulations]] (CFR).