Compact Reconfigurable Band-Reject/All-Pass Microstrip Filter Using U-Shaped Slot

This paper proposes a compact reconfigurable filter that uses a resonant element in the form of a folded slot. The U-shaped slot is embedded into the 50 Ohm microstrip line. Due to folding, the slot length is reduced to 1/4 the effective wavelength leading to considerable miniaturization compared with the filters that use resonant elements like rings or coupled open-circuited or short-circuited stubs. The presented design example at the WLAN frequency of 2.45 GHz can be configured between a band-reject and all-pass states, by placing a PIN diode at the centre of the slot. The proposed filter was investigated using the CST Studio Suite Software - 3D Electromagnetic Simulation. The obtained results showed low insertion loss in the band-pass state and high rejection in the stopband state. The tests of the fabricated prototype showed comparable results to the simulation one thus verifying the design.