This chapter presents the theory needed for analysing aperture antennas. It examines special cases of interest pertaining to the topic of radiation from an aperture. The parameters related to radiation that are used to describe the characteristics of radiation are defined. In the near‐field region, the field differs little from that in the aperture field itself although there are fluctuations due to diffraction from any nearby edges or rim and the phase will have become slightly non‐uniform. The exponential factor inside the integral contributes to a phase variation across the aperture, which is significant. The chapter talks about near‐field region, Fresnel zone, far‐field region, and far‐field radiation characteristics. The radiation pattern of an aperture antenna is sensitively dependent upon the phase distribution of the aperture field. Variation from the ideal uniform phase distribution causes phase aberrations.
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