In this paper, the finite difference time domain method is employed to analyze the T-probe-proximity-fed rectangular patch antennas. With a supporting foam substrate of thickness around 0.1 λ0, the patch antenna has shown a bandwidth (SWR ≤ 2) of 40% and an average gain of 7 dBi. Numerical results for the input impedance are presented and compared with the measurements. Good agreement between the computed and measured results is obtained. The effects of geometric parameters on the characteristics of the T-probe patch antenna are extensively studied. For design purposes, the variations of the input impedance at resonance with different geometric parameters are plotted on Smith charts, and the peak values for the resonant resistance are summarized in tables.