On the Incident Power on a Receiving Slender Antenna and the Optical Theorem in the Near Field

Incident power upon a slender scatterer or a receiving antenna is not well defined, since physical apertures of such structures have little meaning. However, an area over which the incident power is admitted can nonetheless be defined. For a definition that relies solely on the receive mode, we develop a near-field version of the optical theorem. When equated with the conventional far-field version, an equation for the admitting area appears. This equation is solved via both a full-wave simulation and a quasi-analytical way. The result is also supported by two transmit-mode assessments. It is shown that under this definition, the half-wavelength dipole can be replaced by an equivalent aperture antenna facing the direction of the incident wave.