Height above average terrain

From Academic Kids

Height above average terrain (HAAT) is used extensively in radio and television, as it is actually much more important than effective radiated power (ERP) in determining the range of broadcasts (VHF and UHF in particular, as they are line of sight transmissions). For international coordination, it is officially measured in meters, even by the Federal Communications Commission in the United States. 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 broadcast station classes).

Before metrification at the FCC, the procedure to figure HAAT was relatively simple: from the proposed or actual antenna site, either 12 or 16 radials were drawn, and points at 2, 4, 6, 8, and 10 miles radius along each radial were used. The entire radial graph could be rotated to achieve the best effect for the station. The average altitude of all the specified points, minus the altitude of the antenna site, was the HAAT. This can create some unusual cases, particularly in mountainous regions—it is possible to have a negative number for HAAT.

Contour data with 120 radials spaced 3 degrees apart can be downloaded from the FCC.

Related terms are height above mean sea level (AMSL), and height above ground level (AGL).

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