A mode-based design method for dual-band and self-deplexing antennas is presented. By analyzing the E-field distributions and cut-off frequencies of the individual modes in a T-stub loaded aperture, we selected two proper modes for the antenna designed for dual-frequency operations. The desired feature is that at the two different frequencies, the antenna may perform the same or similar radiation characteristics including the polarization, the pattern shape, and the gain level. A rectangular aperture loaded with double T-stubs was employed for the illustration basis. The proposed method was demonstrated to the dual-band antenna and self-diplexing antenna design with promising performances. The presented dual-band antenna performed a wide range of the frequency ratio from 1.3 to 3.0 under a fixed antenna size. The proposed self-diplexing antenna achieved good port isolation better than 26 dB in both operating bands. Both illustrated antenna designs were successfully verified with built prototypes of which the simulation and measurement results were quite consistent.