Platform-Based, Electrically-Small HF Antenna With Switchable Directional Radiation Patterns

A platform-based, electrically small antenna (ESA) operating in the high-frequency (HF) band with switchable directional radiation patterns in the azimuth plane is presented. The antenna is designed to exploit three orthogonal characteristic modes of a mid-size, ground-based platform. Two of the modes have orthogonal, figure-eight-shaped radiation patterns, and the third has an omnidirectional pattern along the azimuth plane. Electrically small coupling elements along with passive feeding and matching networks are designed to excite each mode purely with minimal coupling to the others. We demonstrate that the radiated fields of these antennas can be properly combined to generate cardioid-shaped radiation patterns in the azimuth plane whose directions of maximum radiations can be oriented toward the front, back, and the two sides of the vehicle. This antenna system is capable of generating such directional radiation patterns with front-to-back ratios better than 15 dB over the frequency range of 6-24 MHz. A 1:35 scaled-model prototype of this antenna was fabricated and experimentally characterized. Measurement results of the scaled model are consistent with the simulation results of the full-scale platform and demonstrate the efficacy of the proposed design approach.