Summary
International Symposium on Antennas and Propagation
2009
Session Number:1B2
Session:
Number:1B2-6
Minimum Q Electrically Small Spherical Magnetic Dipole Antenna ? Practice
Oleksiy S. Kim, Olav Breinbjerg,
pp.89-92
Publication Date:2009/10/21
Online ISSN:2188-5079
Summary:
Since the first works on a fundamental limit QLB for the radiation quality factor Q of an electrically small antenna were published by Wheeler [1] and Chu [2], it has been a question how close a realistic antenna can get to this fundamental limit. Recently, the Q factor was derived for an electric current distribution on a spherical surface in free space radiating electric or magnetic dipole fields [3]. It is shown that if the stored energy inside the sphere is taken into account, the quality factor rises to Q = 1.5QLB and Q = 3.0QLB for an electric TM10 and magnetic TE10 spherical mode, respectively. Until now, the result closest to the fundamental limit among practical antennas has been demonstrated by the spherical helix antenna designed by Best [4]. This antenna radiates the TM10 spherical mode and indeed yields the quality factor of Q ? 1.5QLB. In a resent paper [5], three TE10-mode antenna configurations approaching the limit Q = 3.0QLB were presented. These results seem to show that a magnetic dipole antenna offers a Q factor inferior to that of its electric dipole counterpart. In fact, as suggested by Wheeler [6], the magnetic dipole antenna can theoretically be brought arbitrary close to the lower bound by filling it with magnetic material having relative permeability μ → ∞: Q = QLB(1 + 2/μ). (1) In practice, however, application of Wheeler’s suggestion is not so straightforward as it might look at first glance. Especially, if one take into account core resonances and parasitic higher-order modes, which in a realistic antenna cannot be completely eliminated. In this paper, we discuss practical aspects of using a magnetic core to reduce the radiation Q factor of spherical magnetic dipole antennas. Taking outset in a TE10-mode multiarm spherical helix antenna ? one of the three spherical magnetic dipole antennas presented in [5] ? we numerically investigate the influence of the core permeability on electric characteristics of the antenna and show how its Q factor approaches the lower bound. Numerical simulations are performed using a code based on the surface integral equation technique and higher-order method of moments [7].