Shorting-pin loaded annular ring microstrip antenna

Microstrip antennas have been used for a wide variety of applications. Some applications require an omnidirectional radiation pattern with the maximum gain at the horizon. While the maximum gain of the lowest mode for the microstrip antenna is at the zenith direction, a higher order mode can be used to obtain the maximum gain at the horizon. But, in a 450 MHz frequency band, the size of the typical microstrip antenna with the higher order mode is too large to be useful. This paper presents a new design for the annular ring microstrip antenna that utilizes two shorting pins to reduce the antenna to 25% of its original size by decreasing the resonant frequency. Impedance matching and frequency tuning are easily accomplished by adjusting the pin diameters and locations. With the shorting pins to reduce the size, the radiation pattern is vertically polarized and omnidirectional with the maximum gain at the horizon.