A Wideband Circularly Polarized 2 × 2 Filtenna Array with Multiple Radiation Nulls

This letter presents a wideband sequential-rotated (SR) circularly polarized (CP) 2 × 2 filtenna array with multiple radiation nulls for high out-of-band rejection level. A pair of arc-shaped via walls is introduced to split the degenerate modes TE120/TE210 of the substrate integrated waveguide (SIW) cavity for achieving CP characteristics. SIW cavity modes TE110 and TE220 are generated at the lower and upper band edges, respectively, to form a bandpass filtering response. A CP patch is embedded to enhance the AR bandwidth and create an additional radiation null near the upper band edge. Four filtering slots are further introduced around the radiation aperture to create another radiation null in the upper stopband. This compact filtenna element can achieve high stopband rejection without external filtering circuits. A SR feed network is designed for 2 × 2 array implementation with enhanced gain and AR bandwidth. By integrating a simple filtering structure in the SR feed network, the out-of-band rejection level in the lower stopband can be significantly improved. For demonstration, a 2 × 2 SR filtenna array was fabricated and tested. Good agreements between the measured and simulated results are observed, with a wide AR bandwidth of 18% (centered at 10 GHz), a peak gain of 13.2 dBic, and out-of-band rejection levels of 33 dB and 23 dB in the lower and upper stopbands, respectively.