Design of Stable High-Gain Multiband Circular Microstrip Antenna

Some recent investigations on multiband microstrip antennas have been focused upon obtaining the frequencies of interest, however, much less attention is paid to obtain a relatively stable high-gain [1], [2]. A single-layer circular microstrip antenna with 2-D defected ground structure (DGS) was reported to obtain the stable gain [3]. However, it might be difficult to increase the gain of this antenna because the gain is much dependent upon the material constants such as a dielectric of the substrate and a conductivity of the metal surfaces. It is also quite difficult to control the material constants effects [4]. Otherwise, one can design a high radiation efficiency antenna. Once it can be realized, the afterward problem is that the reflection loss should be minimized. This method is less difficult than the previous method [3] since there are some available methods that have been reported to obtain a good impedance matching. The stacked microstrip antenna configuration has been recognized as an effective method to enhance the antenna bandwidth with a relatively high-gain. A stacked rectangular microstrip antenna excited by coplanar waveguide (CPW) was reported to realize the bandwidth of approximately 23% (VSWR<2) [5]. The antenna gains reach around 8.0 dBi with a stable performance over the bandwidth.