Coupling Loop Design for Magnetic Loop Antennas
A magnetic loop's main radiating loop usually isn't fed directly — a small, separate coupling loop mounted near it transfers power magnetically, transformer-style, with no galvanic connection to the tuned loop at all. This is a genuinely different part of the antenna from the tuning capacitor covered elsewhere in this series: the capacitor resonates the main loop, the coupling loop is how you get power into it in the first place. This guide covers coupling loop sizing, the two common construction styles, and the experimental placement process every build requires.
Why loops are fed this way instead of directly
A magnetic loop's main loop is a high-Q resonant circuit carrying substantial circulating current at its operating impedance — often just a fraction of an ohm to a few ohms at the point directly opposite the tuning capacitor, far below the 50Ω a coax feedline expects. Rather than tapping the loop directly at some point (which is possible but finicky to get right, and disturbs the loop's own resonant behavior), most builds couple power in magnetically: a small secondary loop, positioned near the main loop with no electrical connection to it, picks up (or induces) current purely through the two loops' shared magnetic field — exactly the same principle a transformer uses to couple its primary and secondary windings.
Sizing the coupling loop
A widely cited starting point in magnetic loop literature is sizing the coupling loop's circumference (or diameter) to roughly one-fifth of the main loop's own circumference — this isn't a precise formula so much as a reasonable first guess that gets most builds into a workable range without excessive trial and error. The coupling loop's exact size interacts with its distance from the main loop and its orientation, all of which jointly determine how tightly the two loops are coupled — treat the 1/5 ratio as where you start, not where you necessarily finish.
Wire loop vs. gapped-coax (Faraday) loop
- Simple wire loop: a small loop of ordinary wire, fed at its base by coax, positioned near the main loop. Easy to build, but as an unshielded loop it's also somewhat more prone to picking up local electrical noise and unwanted direct (non-magnetic) coupling from nearby objects.
- Gapped coax (Faraday) loop: a small loop made from a length of coax itself, with the shield deliberately broken (gapped) at the point diametrically opposite where the coax enters — the shield still provides electrostatic shielding around most of the loop's circumference, but the gap forces coupling to happen only magnetically, not capacitively. This is the more commonly recommended style in ham loop literature because it reduces unwanted noise pickup and stray capacitive coupling.
Why coupling isn't a plug-in formula
Like the Z-match tuner's link coupling covered elsewhere in this series, a coupling loop's actual coupling coefficient depends on the combined effect of loop size, spacing, and relative orientation in a way that isn't practical to solve from a single equation for a builder's specific geometry. This is normal for magnetically coupled circuits generally, and is exactly why every published magnetic loop coupling loop design describes starting from a size guideline and then adjusting position experimentally while watching SWR — there's no shortcut around that step, only a reasonable starting point to shorten it.
| Aspect | Wire Loop | Gapped Coax (Faraday) Loop |
|---|---|---|
| Construction | Single loop of wire, fed at its base | Loop of coax with shield gapped opposite the feed point |
| Noise/interference rejection | Lower — no shielding against local electrical noise | Better — shield blocks capacitive/electrostatic coupling, leaving only magnetic coupling |
| Build complexity | Simple — bend wire, connect coax | Slightly more involved — requires carefully placing and insulating the shield gap |
| Typical recommendation | Fine for quick experiments or receive-only use | Preferred for most transmitting loop builds |
Materials for a gapped-coax (Faraday) coupling loop
A gapped-coax coupling loop mounted near a magnetic loop antenna's main loop, feeding it magnetically with no direct connection.
Building and Placing a Coupling Loop
Budget real time for the placement step — nearly every published magnetic loop build describes it as trial and error around a starting guideline, not a one-shot calculation.
Measure your main loop's circumference
Measure the actual circumference of your main loop's conductor, and use roughly one-fifth of that figure as your coupling loop's starting circumference.
Build the coupling loop
For a gapped-coax loop, form the coax into a loop of the target size, cut and insulate the shield at the point diametrically opposite where the coax's core and shield connect to the feedline, and confirm the gap doesn't touch or re-bridge under mechanical stress. Leave the coax's center conductor and dielectric intact all the way around — only the outer shield gets broken.
Mount the coupling loop concentrically with the main loop
Position the coupling loop so it shares the same general plane and center point as the main loop, typically positioned near the bottom of the main loop opposite the tuning capacitor/gap, supported by an insulated bracket that holds its position and orientation securely.
Sweep and adjust for the best match
With the main loop tuned to resonance at your target frequency, sweep SWR with a NanoVNA and adjust the coupling loop's distance from the main loop (and, if needed, its size) until you reach the best available match. Move the coupling loop closer or enlarge it slightly if the match is too loose (a shallow, broad dip); move it farther away or shrink it if too tight (a very narrow dip, or evidence the main loop's Q is being noticeably loaded down).
Secure the final position
Once you've found a satisfactory match, lock the coupling loop's bracket firmly in place — vibration or wind flex that shifts the coupling loop's position after tuning will detune the match just as surely as moving the main loop's own capacitor would.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| No signal transfer at all — the loop appears completely dead | On a gapped-coax loop, the shield gap is missing or was never actually cut, shorting the loop electrically instead of coupling it magnetically | Check continuity across the intended gap location — it should read open, not continuous | Cut the gap if it was missed, or re-insulate it if it has re-bridged |
| Match is too loose — SWR dip is shallow and broad, or won't come down enough | Coupling loop too small or positioned too far from the main loop | Compare the current spacing/size against the starting-point guideline | Move the coupling loop closer to the main loop, or enlarge it slightly, and re-sweep |
| Match is too tight — very narrow dip, or the main loop's Q seems noticeably reduced | Coupling loop too large or positioned too close to the main loop, over-loading it | Compare tuning bandwidth against what's typical for a loop of your size and Q | Move the coupling loop farther away or shrink it slightly, and re-sweep |
| Coupling loop position that worked in the shop shifts once mounted outdoors | Bracket not rigid enough to hold position under wind loading or vibration | Check for visible movement of the coupling loop relative to the main loop under light physical pressure | Rebuild the mounting bracket with a more rigid material or additional support points |
| More noise on receive than expected, especially with a wire-style coupling loop | Unshielded wire loop picking up local electrical noise directly, in addition to the intended magnetic coupling | Compare receive noise floor against a nearby antenna, if available, to confirm the loop itself is the source | Rebuild as a gapped-coax (Faraday) loop, which shields against this specific noise-pickup pathway |
How is this different from the loop's tuning capacitor?
They're two separate parts of the antenna doing different jobs. The tuning capacitor resonates the main loop at your operating frequency. The coupling loop is how power actually gets into (or out of) the main loop in the first place, via magnetic coupling rather than a direct connection. Both are needed; neither substitutes for the other.
How big should the coupling loop be?
Start with roughly one-fifth of the main loop's circumference as a first guess, then adjust size and/or spacing experimentally while watching SWR — the exact right size depends on your specific loop's geometry and isn't practical to calculate precisely in advance.
Wire loop or gapped-coax loop — which should I build?
A gapped-coax (Faraday) loop is generally the better choice for a transmitting loop, since its shielding reduces unwanted noise pickup and stray capacitive coupling. A simple wire loop is easier to build and reasonable for quick experiments or receive-only use.
Where exactly does the gap go in a coax coupling loop?
At the point on the loop diametrically opposite where the coax's center conductor and shield connect to the feedline — this placement is what forces the loop's coupling to happen magnetically around its circumference rather than through a continuous shield.
Can I skip the experimental tuning step and just build to the formula?
Not reliably — the 1/5 circumference ratio is a starting point, not a precise design value, since actual coupling depends on spacing and orientation too. Every practical magnetic loop build budgets time for adjusting the coupling loop's position while watching SWR, the same way Z-match tuner link coupling is set experimentally rather than calculated exactly.