a microwave frequency tunable filtering balun is provided . the microwave frequency tunable filtering balun comprises a first microwave split ring transmission line resonator and a second microwave split ring transmission line resonator arranged in a bilaterally symmetrical manner , a fourth variable capacitor and a fifth variable capacitor of same parameters . it combines two functions of balun and tunable bandpass filter into one circuit , resulting in a compact design . the balun characteristic and frequency - tuning mechanism are investigated , and the design equations are derived .