This paper presents two novel substrate integrated waveguide (SIW) diplexers with transmission zeros placed below and above the passband. Diplexer I is based on two bandpass filters (BPFs) using eighth mode SIW (EMSIW) cavities with Rx and Tx frequencies at 3.68 GHz and 6.09 GHz. The second one is operated at 2.37 GHz and 6.04 GHz using EMSIW and thirty-second SIW (TMSIW) cavity. The diplexers are all combined through a T-junction by carefully choosing the length and width of two branches to allow each filter to match the antenna, while maintaining an open circuit at the middle band of the other. The proposed diplexers possess compact size, because of the EMSIW and TMSIW cavity. The diplexers are fabricated in SIW technology. The minimum insertion losses including SMA connectors are measured to be 1.39/1.61 dB and 0.38/0.85 dB. Meanwhile, the diplexers exhibit 37 dB and 42 dB isolations between the channels, respectively. Good agreement is achieved between simulated and measured results.
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