Microwave multiplexing networks are widely used in satellite communication systems. Their performances, such as the insertion loss and rejection between channels, are extremely critical. This research aims at effective design of multiplexing networks for applications in communication satellites exploring different design methodologies. First, the enhanced type of multiplexer has been proven to be able to significantly improve its performances by forming an extra pole in the pass-band of each channel using the existing manifold. For completeness of the design methodology, the design of the enhanced type of multiplexers with H-plane T-junctions is investigated in this thesis. Design equations are given. By changing the distance between the channel filters and the distance between manifold waveguide and channel filter, the resonances and the magnitude of coupling can be readily controlled to satisfy the design requirement. Examples are used to demonstrate the feasibility and effectiveness of the new design using H-plane T-junctions. Secondly, Artificial Neural Network (ANN) is applied in the spurious mode compensation in the multiplexer design. Channel filter tuning, circuit model simplification, and neural network training processes are detailed in the thesis. Space-Mapping optimization technology has been applied in the tuning of a channel filter. The artificial neural network is applied as the method to compensate disadvantages of both electromagnetic (EM) simulation, which is accurate but extremely time consuming, and circuit model, which is fast but with limited accuracy. The neural network model of a simplified circuit can rapidly and accurately simulate the spurious mode of a channel filter in its out-of-band. Design examples are used to demonstrate the modeling steps. Good agreements are observed between EM simulation and the results from the developed model. / UOIT