An antenna that has an elevation pattern defined and no downtilt typically has the major lobe (of the elevation pattern) in the horizontal plane. In this case, the choice between horizontal plane or major lobe power is unimportant, since they are the same. However, if the antenna has downtilt (either electrical or mechanical) then the two values are different, with the horizontal plane power less than the down tilted major lobe.

In other TAP propagation modules (Bullington, Okumura, Longley-Rice) the actual elevation angle and corresponding gain in the vertical plane are used to determine the power to be used for each field strength calculation to a given point on a radial. However, the Carey model uses only the radial HAAT for all calculations, rather than computing individual points along a radial. Therefore, one power (ERP) value must be specified for the distance to contour calculation on each radial. In the TAP Carey program, this value can be either the power in the major lobe or the power in the horizontal plane.

In most applications, the major lobe power is used with the Carey curves for computing distance to a contour on each radial. In this case you should select "Major Lobe" for the power to use. If you application requires use of the horizontal plane power, you should select "Horizontal Plane" for the power to use. Since many uses of the Carey programs are for submissions to the FCC, you must be sure of which power value is accepted for your type of study.