Development of an Antenna Coupler that Fully Compensates the Deficiencies of Electrically Short LF/MF Antennas

John R. Pinks

Abstract:	Advances in semiconductor technology have made today.s transmitters much more efficient than their predecessors. The subsequent reduction in their physical size has made them significantly more susceptible to the effects of mismatch of their terminating impedance that results in increased heat dissipation in their final amplifiers. They usually contain sensors that measure the reflected power and automatically reduce the output power to prevent it from exceeding a safe level. Transmitting antennas operating in the low and medium frequency bands are usually very short compared to the operating wavelength. This results in an antenna input impedance comprising a large capacitive reactance in series with a low value resistance. Antenna tuning units are used to convert this impedance to the 50 ohms terminating impedance required by the associated transmitter. These usually comprise a large loading coil which is tuned to series resonate the antenna capacitance and a matching device to convert the resulting resistive load to 50 ohms. This arrangement causes the antenna to operate as a narrow band-pass filter that requires continuous automatic tuning of the loading coil inductance. The more stable resistive term has historically been set for a matched condition using selectable taps during installation. A number of installations have experienced unacceptable variations of the radiated field strength caused by variations of the antenna equivalent resistance value. This paper describes the development of an Antenna Tuning Unit, which provides automatic adjustment of both the loading coil inductance and the resistive matching network so that an acceptable transmitter terminating impedance is maintained over wide variations of antenna characteristics. An additional feature measures the antenna current and adjusts the transmitter.s output power to maintain it and consequently the radiated power at a stable value. The use of microcontrollers within both the tuning unit and the associated transmitter with serial communication between them provides the ability to remotely monitor the operation of the antenna tuning unit from the location of the transmitter or from any location via a serial connection.
Published in:	Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004) June 7 - 9, 2004 Dayton Marriott Hotel Dayton, OH
Pages:	719 - 728
Cite this article:	Pinks, John R., "Development of an Antenna Coupler that Fully Compensates the Deficiencies of Electrically Short LF/MF Antennas," Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004), Dayton, OH, June 2004, pp. 719-728.
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