Height above average terrain

{{short description|Height based on large area surrounding object; often used in U.S. for antenna towers}}
{{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).

==FM==
===Zones I and I-A===
*Maximum HAAT: {{convert|150|m|ftin}}
*Maximum ERP: 50 kilowatts (47[[dBW]])
*Minimum co-channel separation: {{convert|241|km|mi|abbr=on}}

===Zones II and III===
*Maximum HAAT: {{convert|600|m|ftin}}
*Maximum ERP: 100 kilowatts (50dBW)
*Minimum co-channel separation: {{convert|290|km|mi|abbr=on}}.

==TV==
In all zones, maximum ERP for analog TV transmitters is as follows:

*VHF 2-6: 100 kilowatts (50dBW) (analog); 45 kilowatts (46.5dBW) (digital)
*VHF 7-13: 316 kilowatts (55dBW) (analog); 160 kilowatts (52dBW) (digital)
*UHF: 5,000 kilowatts (67dBW) (analog); 1,000 kilowatts (60dBW) (digital)

===Maximum HAAT===
*Zone I: {{convert|305|m|ftin}}
*Zones II and III: {{convert|610|m|ftin}}

===Minimum co-channel separation===
{|
|-
! Band
! Zone I
! Zone II
! Zone III
|-
| VHF
| 272.7&nbsp;km<br/>(169.5 miles)
| 304.9&nbsp;km<br/>(189.5 miles)
| 353.2&nbsp;km<br/>(219.5 miles)
|-
| UHF
| 248.6&nbsp;km<br/>(154.5 miles)
| 280.8&nbsp;km<br/>(174.5 miles)
| 329.0&nbsp;km<br/>(204.5 miles)
|}

==Zone layouts==
Zone I (the most densely populated zone) consists of the entire land masses of the following states: [[Connecticut]], [[Delaware]], [[Illinois]], [[Indiana]], [[Maryland]], [[Massachusetts]], [[New Jersey]], [[Ohio]], [[Pennsylvania]], [[Rhode Island]], and [[West Virginia]]; in addition to the northern and eastern portions of [[Virginia]]; the areas of [[Michigan]] and southeastern [[Wisconsin]] south of 43° 30' north latitude; the coastal strip of [[Maine]]; the areas of [[New Hampshire]] and [[Vermont]] south of 45° north latitude; and the areas of western [[New York (state)|New York]] south of 43° 30' north latitude and eastern New York south of 45° north latitude. In addition, Zone I-A (FM only) consists of all of [[California]] south of 40° north latitude, [[Puerto Rico]] and the [[United States Virgin Islands|U.S. Virgin Islands]] (If the dividing line between Zones I and II runs through a city, that city is considered to be in Zone I.).

Zones I and I-A have the most [[Grandfather clause|"grandfathered"]] overpowered stations, which are allowed the same extended coverage areas that they had before the zones were established. One of the most powerful of these stations is [[WBCT]] in [[Grand Rapids, Michigan]], which operates at 320,000 watts and 238 meters (781&nbsp;ft) HAAT.

Zone III (the zone with the flattest terrain) consists of all of [[Florida]] and the areas of [[Alabama]], [[Georgia (U.S. state)|Georgia]], [[Louisiana]], [[Mississippi]], and [[Texas]] within approximately {{convert|241.4|km|mi|sp=us}} of the [[Gulf of Mexico]].

Zone II is all the rest of the Continental United States, [[Alaska]] and [[Hawaii]].

==See also==
*[[Above mean sea level]] (AMSL)
*[[Above ground level]] (AGL)
*[[Canadian Radio-television and Telecommunications Commission]] (CRTC)
*[[List of broadcast station classes]]
*[[Topographic prominence]] – a similar measurement for mountains

==External links==
*[http://www.access.gpo.gov/nara/cfr/waisidx_05/47cfr73_05.html  47 CFR Part 73 Index (2005)]
*[https://web.archive.org/web/20050117155245/http://ftp.fcc.gov/oet/info/maps/mmb/ FCC: Mass Media Calculated Contours]
*[https://www.fcc.gov/media/radio/haat-calculator FCC: HAAT Calculator]
*[http://www.w9wi.com/articles/superfm.html "Superpower" Grandfathered FM stations]

[[Category:Antennas]]
[[Category:Broadcasting]]
[[Category:Height]]
[[Category:Vertical extent]]