In this thesis the design of a diplexer system used in terrestrial communication systems is presented. As a part of the diplexer, the transmitter and receiver filters, the combiner section and a low noise amplifier is designed. The main goal of the design is the cheap mass production, so that no tuning after manufacturing is possible. At the working frequency of 15GHz, this requirement is unusual, given the precision of production. Usually these products are equipped with tuning screws. My new finline filter with transmission zeros is an improvement over the traditional finline filters, with steeper cutoff, thus providing better isolation between the two channels. The combiner is also realized on the inserts of the filters. The advantages of the PCB inserts can be utilised by placing active components in the diplexer. A low noise amplifier is designed in planar configuration as a possibility in the waveguide. The advantage of this configuration is the smaller noise figure. A further possibility is an antenna, which has the same PCB insert as the other components. With this insert different structures can be drawn, modifying the gain and radiation pattern. With these components, a compact diplexer is presented, which includes the preamplifier and the antenna too, and no tuning of components is needed. In the first part, the basic concepts of designing microwave components are presented. The next part is dedicated to the theory of the coupling matrix, which is used in my own MATLAB application. After the design process, 3D EM simulation is carried out for optimization of the structures. At the end the measurement results of the manufactured prototypes are presented.