A Low-Profile Differentially Fed Unidirectional Tri-Band Filtering Antenna Under Penta-Mode Resonance

A differentially fed tri-band microstrip patch antenna under penta-mode resonance with unidirectional radiation patterns is proposed. The proposed antenna consists of three patches, two metal strips, and folded ground walls. Folded ground walls are used to achieve a low profile, improve the impedance matching, and enhance the antenna gains. The first operating frequency is generated by the TM10 mode of the central patch, and the TM02 mode of the outer patches is excited for the second resonant frequency. The first and second bands are 2.4–2.44 GHz (0.04 GHz, 1.8%) and 3.49–3.53 GHz (0.04 GHz, 1.1%), respectively. In the third band, the strip not only is used to feed the three patches but also functions as a radiator. By subtracting six slits from the middle patch, an array of small patches is produced with a high gain of 13.86 dBi. Thus, hybrid modes of big outer patches, a pair of quarter-wavelength monopoles generated by the strip, and a number of small patches are combined for a wide band of 4.86–6.37 GHz (1.51 GHz, 26.9%) with a higher gain than 10 dBi. Four out-of-band radiation nulls are also generated by loading the strips and six slits to have good selectivity, and ultralow cross-polarization within 360° across the triple bands is achieved.