Transmission-line based metamaterials in antenna engineering

Abstract

In this chapter, transmission-line-based metamaterials are presented, and their application to the design of passive and active antennas is outlined. Transmission-line metamaterials, also termed negative-refractive-index transmission-line (NRI-TL) metamaterials, are formed by periodically loading a transmission line with lumped-element series capacitors and shunt inductors, and it is shown that they can support both forward and backward waves, as well as standing waves with a zero propagation constant. These rich propagation characteristics form the underlying basis for their use in many antenna applications, including leaky-wave antennas, compact resonant antennas, and multiband antennas. The resonant characteristics of the NRI-TL metamaterial structures reveal how these structures can be designed to offer multiband responses whose resonant frequencies are not harmonically related while offering large degrees of miniaturization. Design equations for rapid prototyping are presented, enabling the simple design of metamaterial antennas to a given specification using standard microwave substrates and loading elements in either fully printed form or surface-mount chip components. A number of passive metamaterial antenna applications are presented, including examples of zeroth-order resonant antennas, negative-order resonant antennas, epsilon-negative antennas, mu-negative antennas, metamaterial dipole antennas, and metamaterial-inspired antennas. Active non-Foster matching networks for small antennas are also presented using negative impedance converters (NICs) and negative impedance inverters (NIIs), and it is demonstrated how these can be applied to metamaterialinspired antennas. Finally, a new method of implementing reactive non-Foster elements using loss-compensated negative-group-delay (NGD) networks is presented that exhibits improved stability, dispersion, and achievable bandwidth.