Bandwidth enhancement of platform-mounted HF antennas using the characteristic modes theory

In many HF communication applications, the antennas are mounted on vehicular platforms. Due to the large wavelengths, the platform in these applications is part of the antenna. However, most HF antennas are not designed to take advantage of the presence of the platform. In this paper, we report the results of a study in which we investigate how the performance of a platform-mounted antenna can be improved by treating the platform as part of the antenna. Specifically, we examine how the mounted antenna can be used to excite the natural resonant modes of the structure. In this case, the platform will act as the main radiator and the mounted antenna as the coupling mechanism to the external circuit. In doing so, the characteristic mode theory is used to identify the efficient means of excitation of the platform current. We demonstrate that this technique can be used to enhance the bandwidth of HF antennas operating at the lower end of the HF band by a factor of 13.2. Scaled model measurements are used to confirm the theoretical predictions of this paper.