An annular-ring reduced surface wave microstrip antenna

Previously, a class of microstrip antennas called reduced surface wave (RSW) antennas was introduced, which consists of antenna geometries that are variations of a circular patch design (Jackson et al. 1993). Although these RSW designs demonstrated good reduction in the surface-wave and lateral-wave excitation, one disadvantage of these designs is that the antenna radius is significantly larger than that of a conventional circular patch. In Jackson et al. (1998) a new general method for constructing RSW antennas was introduced, which applies to planar microstrip antennas of arbitrary shape. In this method the substrate inside the patch cavity (below the patch metalization) is replaced with another material that has an optimum relative permittivity, given by the square of the normalized wavenumber of the TM/sub 0/ surface wave (the TM/sub 0/ wavenumber divided by k/sub 0/). This design principle is used here to design and study a RSW antenna in the shape of an annular ring. The annular-ring shape is chosen because an annular ring antenna is smaller than a circular patch antenna for a given permittivity inside the patch cavity. This leads to a RSW antenna that is smaller in size than the previous RSW designs discussed in Jackson et al. (1993). The trade-offs between antenna size and impedance characteristics, including bandwidth and input resistance, are discussed in detail.