Radiation Pattern Synthesis of a Lens Horn Antenna

In order to achieve a transponder antenna for intersection collision avoidance systems in Intelligent Transport Systems, a lens hom antenna that generates a cosecant squared beam is developed. This paper clarifies the method for designing the antenna to achieve accurate radiation pattern synthesis. A H-plane sectral horn is selected. The ray tracing method is employed in the design of the lens shape. The aperture of the hom is determined to be seven wavelengths based on a comparison of calculated radiation patterns and the desired cosecant squared beam shape. Accurate electrical performance, such as radiation patterns and electrical fields in the horn, is calculated using Finite Difference Time Domain software. Electrical field disturbances caused by reflected waves at the lens surfaces expanded widely inside the small horn. As a result, sidelobe levels of the radiation patterns are increased. In order to eliminate these disturbances, matching layers are attached to the shaped lens surface. Then, electrical field distributions in the horn are recovered and disturbances disappear. Measured radiation patterns become almost the same as that designed using the ray tracing method. The results show that application of the ray tracing method to radiation pattern synthesis of a small lens horn antenna is effective. We clarify the electrical field disturbances caused by reflections at the lens surfaces and show that eliminating the reflection at the lens surface by attaching matching layers is very important to achieving radiation pattern synthesis.