Varactor-based frequency-tuned microwave open-circuited stub resonator

A tuning circuit based on a varactor-diode is proposed to tune the resonant frequency of a microwave open-circuited stub resonator. The circuit is placed at the end point of the stub to give a shunt capacitor effect. As the biased voltage applied to the varactor-diode to control the value of the resultant capacitor, the overall electrical length of the stub can be varied according to the desired resonant frequency. The complete tuning circuit is simulated and presented in this paper along with its tuned frequency-response using the integration of microstrip technology and lumped elements on FR4 substrate. The final tunable stub resonator achieved 66% tuning range.