In this patent documentation, we propose a novel balanced bandpass filter (BPF) using the folded substrate integrated waveguide (FSIW) cavity. Two rectangular-shape FSIW cavities are used to compose the proposed balanced bandpass filter with each cavity operated in a dual-mode (or term as degenerate mode) resonance. The filter has a central frequency of 13.7 GHz with 0.88 GHz bandwidth and the maximum in-band insertion loss of 2.54 dB. In this proposed patent, the balanced BPF has its common-mode signal’s rejection levels greater than 40 dB over a wide frequency range. The microstrip feed together with the coupling slot enable the SIW cavity balanced BPF to have excellent common-mode signal suppression without affecting the differential-mode passband.

The proposed novel balanced bandpass filter (BPF) is composed of two folded substrate integrated waveguide (SIW) cavities. They are operated in a dual-mode (or term as degenerate mode) resonance. The filter has a central frequency of 13.7 GHz with 0.88 GHz bandwidth and the maximum in-band insertion loss of 2.6 dB. In this proposed patent, the balanced BPF has its common-mode signal’s rejection levels greater than 40 dB over a wide frequency range and the differential-mode signal’s insertion losses around 2.5 dB in the passband. The microstrip feed together with the coupling slot enable the SIW cavity balanced BPF to have excellent common-mode signal suppression without affecting the differential-mode passband.

To achieve better environmental noise immunity and to prevent electromagnetic interference (EMI), balanced circuits have been considered indispensable components in microwave systems. Naturally, a well-performed balanced BPF should pass differential-mode (DM) signals and simultaneously reject common-mode (CM) ones over a given frequency range. Hence, the quality of a balanced BPF mainly relies on how well CM signals are rejected without affecting the DM response. The performance of the SIW (cavity) was superior to that of the microstrip because of the SIW’s higher Q value and higher power-handling capability. In particular, SIW cavity exhibiting two degenerated modes can be easily applied to design a dual-mode filter that offers the advantage of reduced size and a high-Q value.