In this paper, a new dual-frequency, compact antenna, which uses an H-shaped microstrip patch with a shorting pin, is described. Compared with the conventional rectangular patch antenna, this antenna can achieve both a significant reduction of antenna size and a dual-frequency operation with a single feed. More freedoms for tuning the resonant frequencies, the frequency ratio, and the input impedance are available because of more design parameters. A detailed parameter study is performed, and the theoretical analysis is based on the finite difference time domain (FDTD) method. The FDTD programs are developed and validated by measurement results. The effects of several antenna parameters on two resonant frequencies, frequency ratio, and radiation pattern characteristics of the antenna are analyzed and compared. It is shown that various frequency ratios (1.91–4.23) can be obtained by varying the design parameters of this antenna. Several design curves are presented.