Electrically Small Antenna Design for Transportable Ionospheric Heating

Abstract

Ionospheric Heating, traditionally performed utilizing large fixed location arrays, while effective, limits research efforts to those fixed locations. As one may envision, a more compact transportable array will provide additional research opportunities at latitudes previously unexplored. For instance, closer to the equator where the Earth's magnetic field is primarily parallel to the ground. An electrically small inductively coupled antenna is considered for its suitability in such an array. A Small or Semi Loop Antenna (SLA) inductively couples to a Capacitively Loaded Loop (CLL) providing high efficiency and natural 50 $\Omega$ port matching. The CLL, of hinged petal design, consists of a large parallel plate capacitor, and half cylinder inductive sections which are hinged at the base to allow for tuning in the range of ionospheric heating (3 - 10 MHz) with a plate included angle of 0 - 16°. A prototype antenna was designed and evaluated at 1/10th scale - for ease of research efforts - with tuning range of 30 - 100 MHz including capability of tuning the coupling between SLA and CLL to achieve good source matching across the tuning range, particularly at the low and high ends. This tuning method may also be used to compensate for mutual impedance effects in array operation, improving array performance. A gain of approximately 5 dBi is observed when placed upon a sufficiently large ground plane. Tuning of the antenna prototype is achieved via stepper motor driven system to remotely adjust both the CLL and SLA angles continuously.