A Differential Microstrip Antenna With Filtering Response

This letter presents a vertically integrated differential filtering antenna. This antenna consists of a differential-fed microstrip patch, a U-shaped differential resonator, and two pairs of quarter-wavelength J-inverters. All the elements are vertically integrated without increasing the dimensions. The traditional synthesis procedure of bandpass filters can be followed to design such a filtering antenna. One example with a second-order 0.2-dB equal-ripple response and the fractional bandwidth of 2.5% is designed and measured. The fabricated antenna exhibits a filter-like frequency response for both antenna gain and input return loss. It achieves an impedance bandwidth of 5% with the return loss better than 15 dB, and a gain of 6.85 dBi.