A broadband aperture-coupled U-slotted microstrip patch antenna with a Π-shaped coupling slot fed by stripline is developed by means of discovering the different reactive natures between a concentrated and a distributed excitation to the U-slotted microstrip patch. At first a probe-fed U-slotted patch antenna covered by a thin dielectric superstrate as protective layer is designed and tested at Ku-band. The bandwidth for VSWR ≤ 2:1 is 39.5 % (11.6 - 17.3 GHz) in simulation and 36.6 % (11.6 ~ 16.8 GHz) in experiment. Then keep the structure of the patch with substrate and superstrate, but replace the probe-fed structure by using a Π-shaped coupling slot as the aperture fed by stripline. As a verification it is tested at both S-band and Ku-band. The bandwidth for VSWR ≤ 2:1 is 48.4 % (10.6 ~ 17.4 GHz) and 46.6 %(2.8 - 4.5 GHz) in simulation and 41.3 % (10.5 ~ 16.0 GHz) and 38.8 % (2.7 ~ 4.0 GHz) in experiment. The pattern bandwidth is narrower than impedance bandwidth. Cross-polarization-level is lower than a probe-fed U-slotted patch antenna.