Tunable, Electrically Small, Inductively Coupled Antenna for Transportable Ionospheric Heating

Abstract

An electrically small antenna is evaluated for use as the principle radiating element in a mobile ionospheric heating array. Consisting of a small loop antenna inductively coupled to a capacitively loaded loop, the electrically small antenna provides high efficiency with the capability of being tuned within the range of ionospheric heating. At a factor 60 smaller in area than a High‐Frequency Active Auroral Research Program element, this antenna provides a compact, efficient radiating element for mobile ionospheric heating. A prototype antenna at 10 MHz was built to study large‐scale feasibility and possible use with photoconductive semiconductor switch‐based drivers. Based on the experimental study, the design has been extrapolated to a small 6 × 4 array of antennas. At a total power input of 16.1 MW this array is predicted to provide 3.6‐GW effective radiated power typically required for ionospheric heating. Array cross talk is addressed, including effects upon individual antenna port parameters. Tuning within the range of ionospheric heating, 3–10 MHz, is made possible without the use of lossy dielectrics through a large capacitive area suited to tune the antenna. Considerations for high power operation across the band are provided including a method of driving the antenna with a simple switcher requiring no radio frequency cabling. Source matching may be improved via adjustment of the coupling between small loop antenna and capacitively loaded loop improving |S11| from −1 to −21 dB at 3 MHz.

Geometry Tuning of an Electrically Small Antenna for Ionospheric Heating

“…A simulated tuning band of 30–105 MHz was achieved for 0° to 16° included angle of the CLL gap, Figure . Simulations did not include coupling tuning to improve input port reflection as it has previously been shown to be effective (Esser et al, ); however, the prototype does include this feature as will be elucidated later in this document.…”

“…The petals effectively form a large parallel plate capacitor that is tuned for the necessary capacitance for the lowfrequency range, similar to the horizontal design. As examined previously (Esser et al, 2018) (Nguyen et al, 2017), to avoid breakdown at high power levels (500 kW), a gap distance of approximately 10 cm is needed at the low-frequency end (~3 MHz) with a gap area of 21 m 2 . This paper will focus on a design with dimensions similar to the original design (3 × 2.5 × 1.2 m, L × W × H, height not including parallel plates) and its tenth scale prototype, with a gap size of 30 × 25 × 0.3 cm.…”

“…Previously, electrically small antenna (ESA) designs capable of sufficiently high power and tuning range to enable transportable ionospheric heating were described (Esser et al, ), (Esser et al, ), (Esser et al, ), based on the 3‐D EZ magnetically (inductively) coupled ESA design introduced in Erentock and Ziolkowski () with a 100‐MHz design also presented (Lin et al, ). These designs are inherently matched to a 50 Ω feed line, requiring no additional matching networks, such as the networks used by the High‐Frequency Active Auroral Research Program (HAARP) (Jacobson et al, ); thereby, power capability is not limited by such external networks.…”

“…These designs are inherently matched to a 50 Ω feed line, requiring no additional matching networks, such as the networks used by the High‐Frequency Active Auroral Research Program (HAARP) (Jacobson et al, ); thereby, power capability is not limited by such external networks. However, the designs had limitations of either power loss in dielectrics used to tune the resonant frequency (Esser et al, ) or difficulty in manufacturing a functional antenna (Esser et al, ). In principle, the design is capable of high‐power operation as compared to wire (Lin et al, ) or filtennas (Tang et al, ) where power is limited due to small conductor cross sections or limits inherent to filter elements.…”

“…This design is difficult to manufacture while keeping the parallel gap geometry consistent. (Esser et al, ). (b) Vertical petal design with two hinged CLL halves shown opened such that the resonant frequency is approximately 100 MHz, 16° included angle, in 1/10th scale.…”