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Poynting's theorem, reactive energy, and radiated power

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2 Author(s)
Grimes, Dale M. ; Crale Inc., State College, PA, USA ; Grimes, Craig A.

An important parameter of any antenna is the ratio of standing reactive energy stored about it to the radiated power; by definition, quality factor Q is ω times that ratio. Extensive experience and electromagnetic theory arguments based upon Q seem to agree that an efficient antenna must be about a half wavelength long in at least one dimension. But is this a general property of radiation fields, or is it due to an inadequate understanding on our part? Can the size limitations be mitigated or even bypassed? The authors re-examine the fundamentals of radiating fields and conclude that there is a significant special case that has not been adequately examined previously. They examine it and conclude that errors of omission are made in previous analyses of compound antennas and significant improvements to the operation of small antennas may be possible. They start with reactive power in radiation fields, and from it solve for the reactive energy. That energy, in turn, is used to solve for radiation Q. With simple, small spherical antennas of radius “a” supporting either TE or TM fields the reactive power is an orthogonal function of the fields and the accepted restrictions on small ka=2πa/λ values hold: the gain is about 1.8 dB, Q varies as (1/ka)3, bandwidth as (ka)3, and the antenna input impedance as (ka)±3. Compound antennas with equal magnitude and properly phased TE+TM modes may be formed from combined linear and loop antennas. A conceptual embodiment is described, for which the reactive power is a non-orthogonal function of the fields and small (ka) restrictions are eased

Published in:

Electromagnetic Compatibility, 1994. Symposium Record. Compatibility in the Loop., IEEE International Symposium on

Date of Conference:

22-26 Aug 1994