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Antenna Q and Stored Energy Expressed in the Fields, Currents, and Input Impedance
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.ORCID iD: 0000-0001-7269-5241
2015 (English)In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 0096-1973, Vol. 63, no 1, 240-249 p.Article in journal (Refereed) Published
Abstract [en]

Although the stored energy of an antenna is instrumental in the evaluation of antenna Q and the associated physical bounds, it is difficult to strictly define stored energy. Classically, the stored energy is either determined from the input impedance of the antenna or the electromagnetic fields around the antenna. The new energy expressions proposed by Vandenbosch express the stored energy in the current densities in the antenna structure. These expressions are equal to the stored energy defined from the difference between the energy density and the far field energy for many but not all cases. Here, the different approaches to determine the stored energy are compared for dipole, loop, inverted L-antennas, and bow-tie antennas. We use Brune synthesized circuit models to determine the stored energy from the input impedance. We also compare the results with differentiation of the input impedance and the obtained bandwidth. The results indicate that the stored energy in the fields, currents, and circuit models agree well for small antennas. For higher frequencies, the stored energy expressed in the currents agrees with the stored energy determined from Brune synthesized circuit models whereas the stored energy approximated by differentiation of input impedance gives a lower value for some cases. The corresponding results for the bandwidth suggest that the inverse proportionality between the fractional bandwidth and Q-factor depends on the threshold level of the reflection coefficient.

Place, publisher, year, edition, pages
2015. Vol. 63, no 1, 240-249 p.
Keyword [en]
Antenna Q, antenna theory, Brune synthesis, Stored energy
National Category
Telecommunications Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-159619DOI: 10.1109/TAP.2014.2368111ISI: 000347383500025OAI: diva2:787163
Swedish Research CouncilSwedish Foundation for Strategic Research VINNOVA

QC 20150209

Available from: 2015-02-09 Created: 2015-02-05 Last updated: 2015-02-09Bibliographically approved

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Jonsson, B. Lars G.
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Electromagnetic Engineering
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