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Portable HF Transmitting Loop Antenna

Portable HF Transmitting Loop Antenna

This tunable magnetic loop antenna disassembles into small but rugged pieces and can be tuned from 14 MHz to 30 MHz.
For those unfamiliar with magnetic loop antennas, the principle behind them is simple. A small conducting loop (often on the order of 1/10 wavelength) is broken and a capacitor bridges the gap. The loop’s inductance, combined with the added capacitance, forms a high-Q tank circuit. A coupling loop or other matching network delivers power from a transmitter, exciting the tank circuit at its resonant frequency. By changing the capacitance, the antenna can be tuned over a fairly broad range of frequencies.

For any given tuning, bandwidth is very limited, due to the high Q of the circuit. Efficiency is limited by the resistance of the conductor. While the resistance of copper pipe is vanishingly small, the radiation resistance of such a tiny radiator is often even smaller. My loop, for example, has a radiation resistance of 20 milliohms at 14 MHz, but its ohmic resistance is more than 40 milliohms! The lost power (two thirds of it in this case) does little for global communication, though it may contribute ever so slightly to global warming.



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