Rectangular Microstrip Antenna

Microstrip or patch antennas are becoming increasingly useful because they can be printed directly onto a circuit board. They are becoming very widespread within the mobile phone market. They are low cost, have a low profile and are easily fabricated. Consider the microstrip antenna shown in Figure 1, fed by a microstrip transmission line. The patch, microstrip and ground plane are made of high conductivity metal. The patch is of length L, width W, and sitting on top of a substrate (some dielectric circuit board) of thickness h with permittivity . The thickness of the ground plane or of the microstrip is not critically important. Typically the height h is much smaller than the wavelength of operation. (a) Top View (b) Side View Figure 1. Geometry of Microstrip (Patch) Antenna. The frequency of operation of the patch antenna of Figure 1 is determined by the length L. The center frequency will be approximately given by: The above equation says that the patch antenna should have a length equal to one half of a wavelength within the dielectric (substrate) medium. The width W of the antenna controls the input impedance. For a square patch fed in the manner above, the input impedance will be on the order of 300 Ohms. By increasing the width, the impedance can be reduced. However, to decrease the input impedance to 50 Ohms often requires a very wide patch. The width further controls the radiation pattern. The normalized pattern is approximately given by: In the above, k is the free-space wavenumber, given by . The magnitude of the fields, given by: The fields are plotted in Figure 2 for W=L=0.5. Figure 2. Normalized Radiation Pattern for Microstrip (Patch) Antenna. The directivity of patch antennas is approximately 5-7 dB. The fields are linearly polarized. Next we'll consider more aspects involved in Patch (Microstrip) antennas.