A Printed Unidirectional Antenna With Improved Upper Band-Edge Selectivity Using a Parasitic Loop

In this communication, a printed unidirectional antenna with improved upper band-edge selectivity is proposed to realize bandpass filtering gain performance. This antenna consists of a printed rectangular loop with two gaps: 1) a parasitic strip and 2) a parasitic loop. The rectangular loop provides good lower band-edge selectivity and the parasitic strip decides the location of upper band-edge selectivity. The parasitic loop, which acts as a director, is used to improve the capacity of upper band-edge selectivity. In addition, the passband bandwidth controllability of the antenna has also been explored. To demonstrate the effectiveness of this design, an antenna prototype is fabricated and tested. Experimental results verified the effectiveness of the proposed design. Measured results show that the fabricated antenna provides a 56.6% passband bandwidth, ranging from 2.18 to 3.9 GHz. Good bandpass filtering gain performance with a flat passband gain better than 4.4 dBi is achieved. Stable unidirectional radiation patterns with nearly identical $E$- and $H$-plane patterns in the operating bandwidth are also achieved. The measured front-to-back ratio is better than 10 dB.