Effective radiated power

== FM example ==
[[Image:FM broadcasting antenna Willans Hill.jpg|thumb|upright=0.5|Four-bay crossed-dipole antenna of an FM broadcasting station]]
For example, an [[frequency modulation|FM]] [[radio station]] which advertises that it has 100,000 [[watt]]s of power actually has 100,000 watts ERP, and ''not'' an actual 100,000-watt transmitter.  The [[transmitter power output]] (TPO) of such a station typically may be 10,000 to 20,000 watts, with a gain factor of 5 to 10 (5× to 10×, or 7 to 10 [[Decibel|dB]]).  In most antenna designs, gain is realized primarily by concentrating power toward the [[horizontal plane]] and suppressing it at upward and downward angles, through the use of [[phased array]]s of antenna elements.  The distribution of power versus [[elevation angle]] is known as the ''vertical pattern''.  When an antenna is also directional horizontally, gain and ERP will vary with [[azimuth]] ([[compass]] direction).  Rather than the average power over all directions, it is the apparent power in the direction of the peak of the antenna's main lobe that is quoted as a station's ERP (this statement is just another way of stating the definition of ERP).  This is particularly applicable to the huge ERPs reported for [[shortwave]] broadcasting stations, which use very narrow [[beam width]]s to get their signals across continents and oceans.

=== United States regulatory usage ===
ERP for FM radio in the United States is always relative to a theoretical [[Dipole antenna#Dipole as a reference standard|reference half-wave dipole]] antenna. (That is, when calculating ERP, the most direct approach is to work with antenna gain in dBd). To deal with antenna polarization, the [[Federal Communications Commission]] (FCC) lists ERP in both the horizontal and vertical [[measurement]]s for FM and TV.  Horizontal is the standard for both, but if the vertical ERP is larger it will be used instead.

The maximum ERP for US FM broadcasting is usually 100,000 watts (FM Zone II) or 50,000 watts (in the generally more densely populated Zones I and I-A), though exact restrictions vary depending on the class of license and the antenna [[height above average terrain]] (HAAT).<ref>47 [[Code of Federal Regulations|CFR]] 73.211</ref> Some stations have been [[grandfather clause|grandfathered]] in or, very infrequently, been given a [[waiver]], and can exceed normal restrictions.