  A varactor-based tunable bandstop filter has been proposed in this article.
The proposed filter is based on a dualmode circuit developed by introducing
inductive and capacitive couplings into a notch filter. The frequency
tunability is achieved by using varactor diodes instead of the lumped
capacitors in the circuit. Next, the equivalent circuit model has been
implemented in planar microstrip technology using thin inductive traces and
varactor diodes. The fabricated filter prototype shows a continuous center
frequency tuning range of 0.66 - 0.99 GHz with a compact size of 0.12lg*0:16lg,
where lg is the guided wavelength at the middle frequency of the tuning range.
