Method of Stopband Expansion for Pseudocombline Bandpass Filters

This report proposes a new method for expanding the stopband of pseudocombline bandpass filters (BPF). This BPF contains resonators from segments of half-wave transmission lines, both ends of which are open. Half-wave (λ/2) resonators are parallel to each other without mutual offset. This BPF design is compact enough, and the absence of short-circuited ends provides some advantage. The disadvantage of such BPF is a lot of spurious passbands and not wide enough stopband located between the main bandpass and first spurious bandpass. The proposed method is based on the features of the input functions of half-wave resonators for various coordinates of connection to these resonators. To obtain a wide stopband, spurious passbands associated with higher (parasitic) resonant frequencies 2f
₀
, 3f
₀
, 4f
₀
, etc. are consistently eliminated. To do this, the resonators are connected to each other at special points located at a certain distance from the open ends of the resonators. Using the suggested method it is possible to suppress a considerable number of parasitic passbands in a pseudocombline BPF with half-wave resonators, and to make the stopband wide enough. In the report the pseudocombline filter with capacitive couplings between resonators, and also between end resonators and load is designed. However, the proposed method allows using other couplings between resonators. Computer simulation of frequency response from the Microwave Office is provided. It is established that the ratio R of the resonant frequency of the first unsuppressed (parasitic) passband to the resonant frequency of the main passband increases with an increase in the order n of symmetric BPF. The method is quite simply implemented in microstrip filters.