Compact wideband balun bandpass filter based on half-mode corrugated slot

A design method of compact wideband balun bandpass filter (BPF) with high selectivity based on corrugated slot is proposed here. To realize wideband balun response in compact size, metal strips are deployed parallel to the half-mode corrugated slot (HMCS). Besides, the coupling of corrugated slot to the metal stripline provides the conversion of slot to microstrip structure at the balanced ports. By placing a pair of resonators at the balanced ports, a wideband balun BPF response with high selectivity can be realized. In addition, a U-shaped resonator is also loaded at the end of the corrugated slot to suppress harmonics. Afterwards, a compact wideband balun BPF centred at 2.42 GHz with 3-dB FBW of 45% is designed, fabricated, and measured. The effective size is 0.13 lambda(g) x 0.39 lambda(g) (lambda(g) is the guided wavelength of the centre frequency) and the measured results indicate that the magnitude imbalance is less than 0.45 dB and the phase imbalance is less than 4.8 degrees within the operation frequency band.