Bandwidth Enhancement of Platform-Mounted HF Antennas Using the Characteristic Mode Theory

Many high-frequency (HF) antennas have significantly smaller dimensions than the wavelength at which they operate and thus suffer from narrow bandwidths. In many military applications, such HF antennas are mounted on relatively large metallic platforms. In this paper, we examine how a platform-mounted antenna can be used to excite the characteristic modes (CMs) of the platform itself to increase the overall bandwidth of the system. In this case, the platform will act as the main radiator and the mounted antennas act primarily as the coupling mechanism between the antenna and the external circuit. We use the theory of CMs to identify the appropriate platform modes and determine the practical means of exciting them. This allows for significantly increasing the bandwidth of the antenna system compared to the bandwidth of the antenna system in isolation. This approach is employed to enhance the bandwidth of a horizontally polarized HF loop antenna system by as much as 10 times compared to that of a stand-alone full-loop antenna. Scaled models of the proposed antennas were fabricated and experimentally characterized. Measurement results are in good agreement with the theoretically predicted results and demonstrate the feasibility of using the proposed approach in designing bandwidth-enhanced platform-mounted HF antennas.