Design of quad-band bandpass filter using nested dual-mode square loop resonators

In this letter, design of a quad-band microstrip bandpass filter is presented by using nested placed four dual-mode square loop resonators (DMSLR) having different electrical lengths. Arms of the square loop resonators (SLR) are formed with low/high impedance parts, in order to obtain a maximum miniaturization by nesting DMSLRs and perturbation effects by changing the lengths of low impedance parts for the excitation of degenerate modes. Thus, the proposed configuration allows the control of mode frequencies, bandwidths and transmission zeros for each passband individually. Resonators are coupled to input/output (I/O) ports by feedlines having open-circuited stubs which suppress the harmonics. A dual-mode quad-band bandpass filter has been designed with an electrical length of 0.25λ_g × 0.25λ_g and fabricated, where λ_g is the guided wavelength at 1.79 GHz (the lowest resonance frequency). Measured and simulated results are in a good agreement.