New developments in the design of the switchable microwave filters in some cognitive radio system are essential to meet the ever increasing demands to discriminate between wanted and unwanted signals.There also has a demand for miniaturization of microwave communications systems. A compact designcan be achieved through the implementation of planar microstrip technology. Conventional electronically tunable bandstop filters suffer performance degradation due to the finite unloaded Q of the resonators and also the loss associated with the switching elements. In 2005, a new filter topology using lossy resonators has been introduced where the topology can be used to partially compensate for the loss. The lossy Allpass network demonstrates the concept and design of absorptive bandstop response with a perfectly matched at all frequencies. The filter is based on the half-wavelength λ/2 resonator with gap coupling, parallel with an Allpass nominally-90°-phase-shift element, which can be optimized to achieve a high Q-factor. A frequency agile bandstop filter based on this topology has been developed, but such filters as well as conventional switchable bandstop filters encounter performance degradation in terms of tuning bandwidth and stopband bandwidth due to the frequency dependant losses and couplings. Through this thesis the switchable microwave filter is investigated and developed, where this filter is able to switch from absorptive bandstop (ON state) to bandpass (OFF state) response. This switchable filter is designed using parallel coupled resonator. The parallel coupled resonator consisted of two low-Q lossy resonator connected with 90° wavelength and with correct k-inverter factor to produce an absorptive bandstop response. The filters are integrated with PIN diode to change the filter response. The absorptive bandstop filter operates at 2.4 GHz where S11 is below than 15 dB and S21 has high selectivity with the narrow bandstop response. For a bandpass response, it depends on the filter structure itself.

Badrul Hisham Ahmad graduated with honours degree in Electronic and Communication Engineering from the University of Leeds, UK in 1998. He received the M.Sc. degree from University Kebangsaan Malaysia in 2003 and Ph.D. degree from University of Leeds, UK for a thesis on the design and development of SIW filters in 2008. His area of interest is Advance Microwave Filters and Antenna Propagation. He is also an active researcher with a good track record with a number of research projects in RF and Microwave Engineering. He has published more than 200 journals and conference proceedings. He is a Senior Member of IEEE and a member of International Steering Committee of Asia Pacific Microwave Conferences. He is the General Chair of Asia Pacific Microwave Conference 2017. Currently he is a Professor of Microwave Engineering in Universiti Teknikal Malaysia Melaka since 2018.