In horn and other directional antennas, the apparent phase center is used since radiation is only emitted at certain angles .
In antenna design theory, the phase center is the point from which the electromagnetic radiation spreads spherically outward, with the phase of the signal being equal at any point on the sphere. Apparent phase center is used to describe the phase center in a limited section of the radiation pattern.
J. D. Dyson, “Determination of the Phase Center and Phase Patterns of Antennas,” in Radio Antennas for Aircraft and Aerospace Vehicles, W. T. Blackband (ed.), AGARD Conference Proceedings, No. 15, Slough, England Technivision Services, 1967.
Y. Y. Hu, “A Method of Determining Phase Centers and Its Applications to Electromagnetic Horns,” Journal of the Franklin Institute, Vol. 271, pp. 31–39, January 1961.
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