Characteristic mode based impedance bandwidth analysis for optimal feed location

Abstract

Optimizing the impedance or return loss bandwidth is important in antenna design. It ensures that the major portion of energy inputted into the antenna is not reflected back to the power source that could waste energy or cause damage to the circuit. The reflection depends on the antenna design, while the feed location is one of the important factor. Therefore, the feed location should be properly selected to optimize the impedance bandwidth. In this work, the impedance bandwidth analysis with respect to different feed locations using characteristic mode (CM) analysis will be presented. This technique can be applied to the structure with a metallic plane on top of a metallic ground separated by a thin air gap. In this approach, the CMs of the structure over a frequency range will be calculated first. The impedance of the feed port over the frequency range will then be calculated from superposition of contributions from individual CM. Finally, the impedance bandwidth could be evaluated by the input impedance. The CMs of a structure are only needed to be calculated once, while the modes can be used to synthesis the impedance bandwidth when the antenna is fed from any possible locations of the structure. Two bandwidth criterions will be focused: the largest single band bandwidth and the largest overall multiband band. The first criterion evaluate the largest continuous impedance bandwidth over a frequency range, while the second criterion evaluates the number of frequency points with good impedance matching condition over a wide frequency range. To demonstrate that the approach can be working on various shapes, the CM impedance bandwidth analysis has been tested on an aircraft wing shape as well as a ship platform shape. The topic is important to the antenna design community because it helps the antenna design to select the optimal feed point which can maximize the impedance bandwidth or to utilize more operational bands.