This post is part of Building Your First Ham Radio Station.
I’ve written before about why magnetic loops are the HOA-proof HF antenna. What I haven’t covered is actually building one, which is a genuinely approachable weekend project if you understand the handful of parts that matter.
The Loop Itself
Copper tubing is the standard choice, typically 3/8 to 1/2 inch diameter, formed into a circle or octagon roughly 3 feet in diameter for 20 through 10 meters, larger for lower bands. Larger diameter tubing means lower loss, which is the whole game with mag loops since their inherently narrow bandwidth punishes any resistive loss in the loop conductor.
The Tuning Capacitor
This is the part that makes or breaks the project. You need a high-voltage variable capacitor, an air-variable or vacuum-variable unit rated for several kV, since RF voltage across the capacitor gets surprisingly high even at modest power. Vacuum variables (often salvaged from surplus or bought used) handle more power in a smaller package but cost more than air-variable units.
The Coupling Loop
A small secondary loop, roughly a fifth the diameter of the main loop, feeds the antenna without a direct electrical connection, coupling into it magnetically. Getting this ratio right takes some experimentation, too small and you won’t couple enough power in, too large and you’ll detune the main loop.
Tuning in Practice
Mag loop bandwidth is genuinely tiny, often just a few kHz at 20 meters, so you’ll be retuning the capacitor every time you move more than a little within the band. A remote-controlled motor drive on the capacitor, run from the operating position, turns this from a nuisance into a non-issue and is worth building in from the start if you can.