Design of microstrip antenna feeds. Part 2: Design and performance limitations of triplate corporate feeds

A microstrip antenna array ideally requires a corporate feed that does not radiate and has a designable, reproducible performance. To simplify manufacture it is also required that one half of the triplate feed is a continuation of the microstrip substrate, and typical thicknesses associated with the antenna substrate constrain the triplate feed to operate closer to the condition where higher-order modes can be generated and dispersion effects may then be important. A mode-matching technique is employed based on the triplate equivalent-waveguide model to calculate the dispersive behaviour of each junction and taper section within the triplate corporate feed structure; the resulting phase corrections are then used in computer graphic production of the entire feed. Measurements of the characteristics of a corporate feed at 17 GHz establish that useful feds can be designed using the mode-matching technique and it is found that dispersion effects can be negligible for some discontinuities, in which case static calculations would be sufficient. Comparisons are also made with the results of an integral-equation method. It is concluded that the feeds can be manufactured within certain limits dictated by material tolerances, extraneous radiation loss at the triplate/microstrip transitions and unwanted mode generation in the triplate. Practical methods of reducing the latter two effects are described.