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Abstract. In this paper, a branch-line coupler using closed-loop and open-loop resonators

In this paper, a lowpass filter with −3 dB cutoff frequency of 5.3 GHz using T-shaped and polygon resonators is presented. The applied resonators create a sharp transition band of 0.2 GHz from −3 dB to −40 dB. To obtain an ultra-wide stopband about 54 GHz (10.18f c) with a suppressing level of −21 dB, two different suppressing cells are employed. The overall circuit size is 59.16 mm 2 , which indicates a small occupied area. To clarify the performance of each resonator and describe the location of the transition zeros, exact equations based on the equivalent LC circuits have been calculated.

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Abstract-In this paper, a microstrip lowpass filter with −3 dB cutoff frequency of 1.8 GHz composed of two resonators with different polygon patches and six symmetric suppressing cells is presented. To design the proposed filter, the impact of each microstrip transmission line on the scattering parameters of the employed resonators is separately determined by extracting the equations of the insertion loss (S 21 ) and return loss (S 11 ) on the basis of their equivalent LC circuit. The designed filter is fabricated and measured, and a good agreement between the results of simulation and measurement is obtained. In the whole stopband region, a return loss better than −0.35 dB and an acceptable suppression level of −22 dB from 1.87 to 19.75 GH are achieved. Furthermore, a flat insertion loss in the passband and an acceptable return loss (−19.32 dB) in this band can verify desired in-band performance. The designed lowpass filter has a high figure of merit about 36969.34.