Dual-mode microstrip bandstop filters using square loop resonators

A new design of dual-mode bandstop filters with four poles employed by single dual-mode microstrip square loop resonators are proposed. Two fourth order filters with two pairs of reflection zeros at real or imaginary finite frequencies are demonstrated with simulations and measurements. It is shown that the location of reflection zeros and degenerated modes can be easily controlled by simply changing the perturbation element size. It is shown that one of fourth order filters has two finite-frequency reflection zeros on only left side of the stopband, while the other one has two pairs of finite-frequency reflection zeros in the lower and upper passbands. Good agreement between simulated and measured results is achieved.