ANALYSIS OF STAND-ALONE RECTANGULAR PATCH ANTENNAS BY THE TRANSMISSION LINE MODEL Andrea Vallecchi Dept. of Electronics and Telecommunications, University of Florence. Via C. Lombroso 6/17, I-50134 Florence, Italy. E-mail: andrea@lam.det.unifi.it. The rectangular patch is by far the most widely used microstrip antenna configuration. It is very easy to analyze by using both the transmission line and cavity models, which are most accurate for thin substrates [1]. A rectangular microstrip radiating element excited in the lowest resonant mode is schematically shown in Figure 1. The element is fed by a transmission line in the plane of the patch, or from the back by a coaxial cable whose inner conductor extends through the dielectric and is soldered to the radiating patch, to give a field distribution which is usually uniform along the width w. In this case the structure can be treated as a microstrip line resonator which is open circuited at both ends and supports quasi-TEM modes. Furthermore, radiation essentially takes place at the ends of the line resonators, while radiation from the strip is negligible for wide strips. Accordingly, the simplest analytical description of a rectangular microstrip patch exploits transmission-line theory and models the patch as two uniformly illuminated radiating slots, taking into account the fringing of the fields at the patch edges, separated by a transmission line of very low characteristic impedance Z c [3], as shown in Figure 2. The length of this line ` is about half a wavelength to reverse the field in the slots. Indeed, the fields at the edges can be resolved into both normal and tangential components with respect to the ground plane. However, the normal components of the electric field are in opposite directions, and their contributions tend to cancel out each other in the broadside direction. Contrarily, the components of the fields parallel to the ground plane adds in phase to give a maximum radiated field normal to the surface of the structure. Input impedance Each radiating slot can be represented by a parallel equivalent admittance Y s with conductance G s and susceptance B s (Figure 2). The slots are denoted by #1 and #2. The Figure 1. Rectangular microstrip radiating element.