Theoretical and Experimental Study to Improve Antenna Performance Using a Resonant Choke Structure
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Every antenna requires a feed network to supply its RF energy. In the case of a simple dipole antenna, this could be a coaxial cable with a tuning element and matching balun. For mostly omnidirectional antennas, currents can easily couple to metallic surfaces inside an antenna's near field that includes the outer conductor of the coaxial feed line. These outer conductor currents can radiate into the far field to skew overall antenna radiation patterns. Other parameters such as VSWR may also be significantly affected. Electromagnetic field absorbers placed on the coaxial waveguide pose other problems where multiple RF carriers exist and non-linear dielectric materials can cause issues. Coil structures can also lead to radiation problems. This leads towards a metallic resonating choke solution, which will allow the antenna to radiate without affecting performance. The primary goal of this research is to integrate a metallic resonant choke structure that will prevent currents from travelling down the feed line outer conductor. In this work, an in-depth analysis is performed on each antenna component. This includes the feed network elements (waveguide coaxial line, tuning element, matching balun) and the radiator (dipole arms, resonant choke, outer feed). Each element is analyzed and designed to allow the manufactured antenna to have similar performance to its ideal center-fed counterpart for a tuned frequency band. To predict the performance of the manufactured antenna, several simulation models are constructed. To model the radiator and resonant choke structure, a Method of Moments code is written with Matlab. These results are compared with HFSS and measurements with good correlation. Specifically, the axisymmetric MoM code uses a KVL approach to integrate the internal choke structure that works well to reduce simulation time to a fraction of that taken by FEM solvers. To design the feed components, a combination of circuit models and HFSS allows for quick design with accurate results when compared with measured values. This systems design approach has the flexibility to add complexity to improve accuracy where needed.