Design of Bandwidth-Enhanced Platform-Mounted Electrically Small VHF Antennas Using the Characteristic-Mode Theory

We present the design of a vehicle-mounted electrically small antenna with enhanced bandwidth operating at the very high frequency (VHF) band. Due to the large wavelength of VHF electromagnetic waves, practical vehicle-mounted VHF antennas tend to have compact or electrically small dimensions. Therefore, they suffer from narrow bandwidth. To alleviate this problem, the platform body can be employed as the main radiator. In this case, the antennas mounted on the platform excite the desired platform mode(s). Efficient and pure excitation of a single mode, however, usually requires the use of multiple coupling elements distributed across the platform. In this work, we demonstrate that a single strategically located electrically small coupling element can be used to excite multiple significant characteristic modes of an electrically large platform. This allows for increasing the bandwidth of the antenna without increasing its occupied volume. This is demonstrated by using a capacitively coupling element with electrical dimensions of 0.06λ × 0.06λ × 0.06λ mounted on a military vehicle, where λ is the free-space wavelength. Our simulation and scaled-model experimental results demonstrate that this approach offers a bandwidth enhancement factor of 6× compared to the isolated antenna and 3.5× compared to an antenna system that excites a single platform mode.