2.2 The Cavity Model

The transmission line model is conceptually simple, but has a number of drawbacks. The transmission line model is often inaccurate when used to predict the impedance bandwidth of a rectangular microstrip antenna for thin substrates. The transmission line model also doesn't take into consideration the possible excitation of modes which are not along the linear transmission line. The transmission line model assumes the currents flow in only one direction along the transmission line. In reality, currents transverse to these assumed currents can exist in a rectangular microstrip antenna. The development of the cavity model addressed these difficulties.

The cavity model, originated in the late 1970s by Lo et al.,, views the rectangular microstrip antenna as an electromagnetic cavity with electric walls at the groundplane and the patch, and magnetic walls at each edge. ^{[19] [20]} The fields under the patch are the superposition of the resonant modes of this two dimensional radiator. Equation (2.10) expresses the ( ) electric field under the patch at a location (x,y) in terms of these modes. This model has undergone a considerable number of refinements since its introduction. ^{[21] [22]}

The fields in the lossy cavity are assumed to be the same as those which will exist in a short cavity of this type. It is assumed that in this configuration where ( h << ? ₀) only a vertical electric field will exist ( ) which is assumed to be constant along , and only...