Compact Absorptive Filtering Patch Antenna

This article presents a compact absorptive filtering patch antenna. It consists of a filtering patch antenna and a bandstop filter (BSF), with their transfer functions being complementary to each other. A slot is fabricated in each of the patch and ground, giving a total of two radiation nulls for the lower bandedge. By using a dual-stub feed, two radiation nulls are also obtained for the upper bandedge. For the BSF, a λ_g/2 defected ground structure (DGS) and a λ_g/4 defected microstrip structure (DMS) are used in the design. It is terminated by a chip resistor. Since the filtering patch antenna and BSF have complementary transfer functions, the incident energy can be radiated effectively in the passband but largely absorbed by the resistor in the stopbands. As a result, only little energy will be reflected over a wide frequency range, giving a reflectionless characteristic. To demonstrate this idea, an absorptive filtering antenna operating at 5.8 GHz was designed, fabricated, and tested. Its impedance is matched from 5 to 6.5 GHz, with the measured out-of-band suppression being higher than 17 and 20 dB for the lower and upper stopbands, respectively. The measured peak realized gain is 7.28 dBi.