Design of a Small Array of Electrically Small Umbrella Monopoles for HF Communications

The feasibility of a small array for communications in the HF band is evaluated. Electrically small antennas comprise the array, and the array allows for pattern control. The use of impedance tuners on each element is assumed to achieve performance across the HF band. Performance of the array is examined using the theoretical maximum tuned instantaneous bandwidth (IBW). A total of 19 antennas are placed on a 1.67 m hexagonal grid achieving the widest extent of 8.35 meters (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&lt;1 \lambda$</tex> through 35.9 MHz). The array elements are 85.1 cm tall monopoles top-loaded with a 168 cm wide umbrella-like top-hat. The implemented loading not only allows for a low-cost, fastdeployable array, but it also lowers the resonant frequency and stabilizes the impedance variation. This leads to an increased tuned IBW, one of the proposed practically important metrics used herein. In the array environment, tight coupling further aids in counteracting the electrically small effects by further stabilizing the impedance variation, resulting in IBWs exceeding the latest wideband HF communications standard. Additionally, radiation resistance is increased through the parasitic coupling to nearby radiators, which indicates higher efficiency when practical losses are considered. Good pattern control and symmetry in azimuth are also demonstrated. The appearance of the negative input resistance is observed and recorded to bound the scanning and frequency ranges where this array can be used in its simplest form.