Microstrip Open-Loop Ring Bandpass Filter Using Open Stubs for Harmonic Suppression

Improved harmonic suppression is achieved by adding small shunt open stubs to a conventional open-loop bandpass filter. A design curve for the shunt open stub is derived using the ABCD matrix calculation. The insertion loss of the four-pole filter is 3 dB at 1.8385 GHz, and the return loss is better than 10 dB within 1.7935-1.8835 GHz. The suppression of the modified filter is better than 30 dB below 1.7 GHz and between 2-6 GHz, while the conventional filter shows a spurious passband at 3.7 GHz