Design of microstrip antenna feeds. II - Design and performance limitations of triplate corporate feeds

It is noted that a microstrip antenna array ideally requires a nonradiating corporate feed whose performance can be designed and reproduced. So that manufacture can be simplified, it is also required that one-half of the triplate feed be a continuation of the microstrip substrate; typical thicknesses associated with the antenna substrate must 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 used based on the triplate equivalent-waveguide model to calculate the dispersive behavior of each junction and taper section within the triplate corporate feed structure; the resulting phase corrections are then employed in computer graphic production of the entire feed. Measurements of the characteristics of a corporate feed at 17 GHz determine that useful feeds can be designed using the mode-matching technique and it is established that dispersion effects can be negligible for some discontinuities, in which case static calculations would be sufficient. It is concluded that the feeds can be fabricated within certain limits dictated by material tolerances, extraneous radiation loss at the triplate/microstrip transitions and unwanted mode generation in the triplate.