Challenging issues arising in the broadband matching of small antennas and how we might solve them

In this paper we discuss some of the challenging issues arising in the process of designing broadband matching systems for small antennas that are inherently high-Q devices. One of the novel approaches to realizing broadband performance from a small antenna is to consider the possibility of loading it with metamaterials that have negative epsilon characteristics. It has been conjectured that it may be possible to balance the capacitive reactance of a small antenna by using such a negative epsilon material, say in the form of a spherical shell, which encloses the antenna. Though this concept is intriguing, its practical implementation is far from straightforward. Typically, the negative epsilon materials are physically realized by following the concept introduced by Rotman, in a now-classical paper, where he used thin wires to realize plasma-type characteristics. However, its performance does not meet our expectations when such an artificial slab is morphed into a thin, spherical, shell-type geometry, illuminated by a spectrum of plane waves emanating from a small antenna located at the center of the sphere. To-date, attempts to realizing ideal negative epsilon shells that circumvent the problems mentioned above have not met with noticeable success, though work in this area is still in progress.