A novel feed mechanism for microstrip patch antennas is presented. The structure of the feed can be regarded as a coaxial balun, and it can be resonant in a one-wavelength loop mode. When the loop mode is properly coupled with the fundamental mode of the microstrip antenna, a broad impedance bandwidth and a symmetrical radiation pattern can be obtained. For the proposed feed, parametric analyses are first performed when it is applied into a square-ring patch antenna. Based on the simulation results, a dual-polarized ring patch antenna is then developed, and its prototype is constructed. Both measured and simulated results indicate that the antenna prototype has a gain of higher than 8 dBi and a cross polarization level of lower than -25 dB within an impedance bandwidth of about 12 %. In addition, the two orthogonal input ports has an isolation level of smaller than -30 dB across the whole operating frequency band.