STORED ELECTROMAGNETIC ENERGY AND ANTENNA Q (Invited Paper)

Abstract

Abstract-Decomposition of the electromagnetic energy into its stored and radiated parts is instrumental in the evaluation of antenna Q and the corresponding fundamental limitations on antennas. This decomposition is not unique and there are several proposals in the literature. Here, it is shown that stored energy defined from the difference between the energy density and the far field energy equals the energy expressions proposed by Vandenbosch for many but not all cases. This also explains the observed cases with negative stored energy and suggests a possible remedy to them. The results are compared with the classical explicit expressions for spherical regions where the results only differ by the electrical size ka that is interpreted as the far-field energy in the interior of the sphere.

Antenna Bandwidth Optimization With Single Frequency Simulation

“…This resonance was compared with the corresponding resonance of the input impedance obtained from the commercial solver Eeld. The optimization-predicted and impedance self-resonance agree to a large extent, conrming the validity of the expressions in [4,10,11,24].…”

“…Here we consider, for simplicity, surface currents. The stored electric and magnetic energies are respectively W e = µ 0 w (e) /(16πk 2 ) and W m = µ 0 w (m) /(16πk 2 ), see also [11]. Correspondingly the total radiated power is P rad = η 0 p (rad) /(8πk).…”

“…Using the results by Vandenbosch [24] and Geyi [4] we compute the electric and magnetic energies stored in the elds excited by an antenna and the radiated power. We assume the studied antennas are electrically small, i.e., Q 1, such that the error in the Q computation is negligible (equal to ka 1 [11]). The previously introduced computation is performed following the procedure in [8,10], at a single frequency, usually the center of the intended operating band.…”

Physical Bounds of Antennas

“…One of the challenges of this method is to express the antenna quantities involved in defining the parameters of interest, in terms of the current density on the antenna. For example, here the energy expressions introduced by Vandenbosch in [113] for current densities in free space, see also [26,38,51,64], are used to express the Q-factor of antennas. These energy expressions are further used in minimization of the stored energy to determine current densities optimum in the sense of maximum gain Qfactor ratio for a fixed radiated field.…”

“…for the electric stored energy, where the asterisk * denotes complex conjugation and R 3 r = {r : lim r 0 →∞ |r| ≤ r 0 } denotes an infinitely large spherical volume, see also [24,27,38,77,85,119]. The magnetic energy is expressed in terms of a magnetic energy density analogous to (3.8).…”