Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
If you are designing complex networks of power dividers, combiners and couplers using standard structures like common transmission lines, coupled lines, etc. what you need is a circuital software. A good starting point is QUCS, which is simple and free. Qucs project: Quite Universal Circuit Simulator
If you are designing not networks, but specific elements where circuital models don't provide a good enough accuracy, you need a electromagnetic software. In this case, there are not many free simulators. May you can try Sonnet. Sonnet Lite
Are these microwave circuits are based on planar transmissionline implementation such as microstrip/stripline/cpw?
If so use Sonnet Lite 13.51 which is free electromagnetic simulator based on Method of Moments.... Free Download - Sonnet Lite