Rectangular Microstrip Antenna - Continued

Consider a square patch antenna fed at the end as before. Assume the substrate is air (or styrofoam, with a permittivity equal to 1), and that L=W=1.5 meters, so that the patch is to resonate at 100 MHz. The height h is taken to be 3 cm. Note that microstrips are usually made for higher frequencies, so that they are much smaller in practice. When matched to a 200 Ohm load, the magnitude of S11 is shown in Figure 1. Figure 1. Magnitude of S11 versus Frequency. Some noteworthy observations are apparent from Figure 1. First, the bandwidth of the patch antenna is very small. Rectangular patch antennas are notoriously narrowband; the bandwidth of rectangular patches are typically 3%. Secondly, the antenna was designed to operate at 100 MHz, but it is resonant at approximately 96 MHz. This shift is due to fringing fields around the antenna, which makes the patch seem longer. Hence, when designing a patch it is typically trimmed by 2-4% to achieve resonance at the desired frequency. The fringing fields around the antenna can help explain why the microstrip antenna radiates. Consider the side view of a patch antenna, shown in Figure 2. Note that since the current at the end of the patch is zero (open circuit end), the current is maximum at the center of the half-wave patch and (theoretically) zero at the beginning of the patch. This low current value at the feed explains in part why the impedance is high when fed at the end (we'll address this again later). Since the patch antenna can be viewed as an open circuited transmission line, the voltage reflection coefficient will be -1 (see the transmission line tutorial for more information). When this occurs, the voltage and current are out of phase. Hence, at the end of the patch the voltage is at a maximum (say +V volts). At the start of the patch (a half-wavelength away), the voltage must be at minimum (-V Volts). Hence, the fields underneath the patch will resemble that of Figure 2, which roughly displays the fringing of the fields around the edges. Figure 2. Side view of patch antenna with E-fields shown underneath. It is the fringing fields that are responsible for the radiation. Note that the fringing fields near the surface of the patch are both in the +y direction. Hence, these E-fields add up in phase and produce the radiation of the microstrip antenna. As a side note, the smaller is, the more "bowed" the fringing fields become; they extend farther away from the patch. Therefore, using a smaller permittivity for the substrate yields better radiation. In contrast, when making a microstrip transmission line (where no power is to be radiated), a high value of is desired, so that the fields are more tightly contained (less fringing), resulting in less radiation. This is one of the trade-offs in patch antenna design. There have been research papers written were distinct dielectrics (different permittivities) are used under the patch and transmission line sections, to circumvent this issue. Next, we'll look at alternative methods of feeding the antenna (connecting the antenna to the receiver or transmitter).