14-30 MHz Magnetic Loop Antenna

Construction of a Compact and Efficient Portable High Frequency Antenna

by Dr. Carol F. Milazzo, KP4MD (posted 03 September 2012)

E-mail: [email protected]

The small magnetic loop antenna is a compact efficient antenna that is ideal for portable deployment or for limited spaces and that can be improvised inexpensively. The antenna is essentially a tuned circuit with an inductor formed by a loop of wire measuring less than 1/4 wavelength and resonated to the operating frequency with a capacitor. Due to its low radiation resistance and large circulating current, the loop must be constructed of a large outer diameter conductor of low resistance for best efficiency. Typically these loops are built from coaxial cable, hardline, or copper or aluminum tubing. These loops have a very narrow bandwidth and require a variable capacitor (and preferably a reduction drive) to be resonated at the operating frequency. Air variable capacitors or vacuum variable capacitors are used due to the voltage on the order of several thousand volts that is developed across the capacitor. In order to maintain the lowest possible series resistance, soldered connections and a "butterfly" or split-stator capacitor are preferred. The addition of a fixed capacitor in parallel with the variable capacitor will allow operation of this antenna on 7 or 10 MHz at reduced efficiency.

