A Coupling Matrix Synthesis Design for Filtering Antenna With Good Out-of-Band Suppression

A new approach by using an
N
+3 coupling matrix for synthesizing the filtering antenna is proposed. Compared with the traditional one, the coupling matrix synthesis method mainly is applied to design filters. Due to the introduction of transistor parameters, the
N
+3 coupling matrix achieves the prediction of filtering performance and gain characteristics for the filtering antenna. This filtering antenna, as a multifunctional device with filtering and radiation functions, can therefore be well combined with the proposed matrix to achieve the integrated synthesis of the filtering antenna. Second, a novel multimode broadband filtering patch antenna is designed as an example to verify the effectiveness of this theory. Three modes and a patch mode are excited within the passband, resulting in a wide operating bandwidth. Multiple paths between the input/output stub lines are formed by three resonant modes and direct source-antenna coupling to produce four radiation nulls outside the desired band, achieving good out-of-band suppression. Finally, the measured results of the fabricated antenna prototype have demonstrated a good filtering response in the 3.44–3.72 GHz band. Specifically, this proposed antenna has achieved 6.9 dBi gain and out-of-band suppression above 30 dB, which is suitable for 5G communication.