A microwave interference cancellation system is presented in this thesis. The technique achieves high Tx/Rx isolation with relatively low degree filters. A four-port diplexer consists of two back-to-back three-port diplexers combined with a 180° phase shift in one branch. High signal isolation between Tx and Rx module is achievable by only using second-order filter topology and the design technique is based on amplitude and phase cancellation between two diplexer branches of the four-port diplexer. Three and four-port networks are intensively analysed and synthesised for solving S-parameter equations.
The four-port diplexer exploits the microstrip open-loop structure. A four-port microstrip diplexer for RF interference rejection is presented in IMT-2000 applications whereas device miniaturisation and low infrastructure cost are required. The microstrip-open loop structure with coupled-feed and tapped-feed are designed for alternative techniques and cost reduction. A 180° phase shift in one branch can be achieved by delayed transmission line. The simulated microstrip four-port network is designed at the centre frequency of Tx/Rx at 1.95 GHz and 2.14 GHz, respectively.
An alternative technology to reduce overall signal losses and increase power handling with the same or better isolation compared to the four-port microstrip technology is four-port combline coaxial resonator structures. To achieve filter design with a 180° different phase shift, the positive (90° inverter) and negative (-90° inverter) coupled filters are required. The design frequencies of the four-port combline diplexer are 1.73 GHz and 2.13 GHz for Rx and Tx modules, respectively. Two different designs of four-port diplexer prototypes, based on filter designs with similar and dissimilar Q-factors, are fabricated and measured to verify the new design technique. Finally, microwave interference cancellation techniques can be used in wireless communication systems where small size, low losses and low complexity are required
