Mobile HF Loop AntennasThis article has been adopted from information at the Stealth Telecom Ltd. website by approbation of the author.
R adio Amateurs most often use inductively-loaded vertical whip antennas in HF mobile applications, but is there a better alternative despite the long popularity of vertical whips? This article describes principles involved in design of the Stealth Telecom Ltd. Type ST940B NVIS (Near Vertical Incidence Skywave) loop antenna system. It also provides useful information about the nature of HF communications.
Theory of operation The ST940B antenna utilizes a Very Small Closed Tuned Loop with aperture dimensions less than 0.1λ compared to wavelength. That type of antenna often is referred to as a Magnetic Loop Antenna.
It differs from short vertical whip antennas in that the reactive component of its impedance has the opposite polarity, so that it can be compensated by a capacitor rather than an inductor. The loop radiator is effectively a single turn inductor that is continuously tunable throughout its useful frequency range by means of a remotely-adjustable variable capacitor.
The feed impedance and efficiency of such a loop mainly depends on its surface conductivity. The antenna has low, typically <10mΩ, radiation resistance, that drops with decreasing frequency and, at the low-end of antenna's frequency range, can be less than 1.0mΩ. The radiation efficiency is given by the ratio:
where Rr = radiation resistance and Rt = total resistance of the tuned circuit. It therefore is necessary to minimize the radiating element resistance by using a highly-conductive conductor. It also is necessary to use a low-loss capacitor to minimize losses in the reactance-compensation element. If these conditions are fulfilled, high RF current will flow in the loop and it will have high selectivity and a high quality factor (Q-factor) that is typically 10 to 20 times higher than the Q-factor of traditional antennas.
The RF currents and voltages in a loop antenna are multiplied by its Q-factor. If the ST940B antenna is fed by 100-watt radio transmitter, loop RF current can reach 30-50 Amperes and loop RF voltage can reach 5000-7000 Volts. That extraordinarily high voltage necessitates use of a high-voltage, low-loss vacuum variable tuning capacitor. It is remotely adjusted to resonance by means of a precision stepper motor, because tuning of the high-Q antenna circuit is very sharp.
Once the mechanical issues are overcome, the narrow bandwidth of the tuned antenna circuit turns to be an advantageous feature as it helps to minimize unwanted harmonics in the emitted signal and to increase the signal/noise ratio when receiving.
A small tuned-loop antenna, due to its low-impedance closed-loop circuit, tends to be less sensitive to capacitively-coupled local noise sources, such as capacitively-coupled interference from nearby high-voltage power lines. Because of that, it can provide excellent reception under conditions where reception with traditional whip antennas is noisy or completely impossible.
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