Design of vehicle-mounted, compact VHF antennas using characteristic mode theory

Compact and low-profile VHF antennas with enhanced bandwidth have a wide spectrum of applications in both commercial and military communication systems. Having both of these criteria in a single antenna, however, is very challenging. In this paper, we demonstrate a method for bandwidth enhancement of vehicle-mounted electrically-small antennas operating in the VHF band. In this method, the vehicle platform is considered to be the major part of the radiating structure, while a meandered monopole antenna is used as a capacitive coupling element to excite a desired set of characteristic modes of the platform. The size of the coupling element is 0.06λ × 0.06λ × 0.06λ, where λ is the free-space wavelength at the frequency of operation, 60 MHz (ka ≈ 0.47). The proposed approach was employed to successfully enhance the bandwidth of a vehicle-mounted VHF antennas by at least 400%.