Novel broadband designs of patch antennas with an air substrate have been proposed in this dissertation. In the dual-frequency design, the study of single-fed dual-frequency microstrip antenna with a V-shaped slot has been first presented. As for the broadband patch antenna design, antennas with a U-shaped slot or a pair of wide slits can have an operating bandwidth more than 2.0 times that of a conventional one at a given operating frequency. The antenna bandwidth of then can be enhanced about 25%. In the designs of reducing cross-polarization, by using dual capacitive feeds with equal input powers, but with a 180¢X-phase shift, The cross-polarization radiation can be reduce about 10 to 15dB in the H-plane. Finally, A broadband shorted patch antenna fed by an aperture-coupled feed with an H-shaped coupling slot is proposed. By using an air substrate of thickness about 0.07 free-space wavelength of the center operating frequency, the proposed antenna can have an impedance bandwidth of about 26.2%. And the antenna design reduced the antenna size to be 40%~50%of the simple case.

The novel designs of circular microstrip antennas have been investigated in this dissertation. In the linearly polarized designs, the study of single-feed dual-frequency circular microstrip antenna with an open-ring slot has been firstly presented. As for the broadband circular microstrip antenna designs, antennas with two open-ring slots, embedded reactive loading by probe feed and microstrip-line feed have been presented. The antenna bandwidth of them can be enhanced about 4% to 6%. In the circularly polarized designs, we have proposed a circularly polarized microstrip antenna with a spur line. By choosing a suitable length of the spur line, CP operation can be obtained. And then, such a design can be applied to a compact circular patch antenna with bent slots. These reduced the antenna size to be 40%~50% of that of the simple case. Finally, we use the ideas of CP operation with elliptic patch and dual-frequency operation with stacked patch to propose a dual-band circularly polarized stacked elliptic microstrip antenna design. The frequency ratio of this design is about 1.39.