Application of Kramers-Kronig transformations to increase the bandwidth of small antennas

Abstract

The internally stored electric energy (Q-energy) of a disk monopole antenna increases as compared to a monopole antenna without a top disk. Recently it was shown that the Q-energy can be significantly reduced and the bandwidth increased by shielding the disk monopole antenna using a thin magnetic material. In the present paper we consider the same structure to explain another method to increase the bandwidth by using a shield made of dispersive magnetic material. We apply the Kramers-Kronig transforms to derive physically correct real and imaginary parts of the dispersive magnetic material. We do not aim at a reduction of the internal energy but at a compensation of the electric by a magnetic stored energy for a wide frequency range. Disk monopole antennas with shells consisting of such dispersive permeability are finally numerically evaluated by means of a commercial frequency-domain field simulator.