Fixing the ATS Mini Donut WB Loop Antenna (Anti-Series Wiring Defect)
The "Donut WB Antenna (10 kHz – 180 MHz)" bundled with many ATS Mini units is a printed-PCB magnetic loop. On a large number of units it ships with a wiring defect that makes it almost non-functional out of the box — and it's a simple, well-documented fix.
The Defect
Tracing the PCB shows two printed coils: one running from the SMA plug's ground pin, clockwise, to a central via; the other coiling from that via back out to the SMA center contact. On defective units, these two coils are wired in anti-series — meaning their magnetically-induced voltages cancel each other out instead of adding. The antenna picks up signal in both halves but delivers close to nothing at the connector.
This was confirmed with a 77.5kHz DCF77 signal generator and a magnetic loop as the primary antenna: an oscilloscope showed strong signal at both half-coils individually, but zero voltage at the SMA output when measured together.
How to Tell If Yours Is Affected
- Compare reception on a strong AM/LW station: whip vs. donut loop. If the loop performs dramatically worse than expected on LW/AM (where loops should normally excel), suspect the defect.
- Visually trace the PCB under bright light or backlight. If both coils spiral in the same rotational direction when traced from their respective connection points (rather than mirrored/opposing), they're wired anti-series.
Tools Needed
- Sharp hobby scalpel or X-Acto knife
- Fine-tip soldering iron and thin solder
- Two short lengths of thin wire (30AWG wire-wrap or magnet wire works well)
- Magnifier or loupe — the traces are small
- Multimeter with continuity mode
The Fix
- Expose the traces. Locate the via where the two coil halves currently meet. Using the scalpel, carefully scrape away the protective solder mask/varnish at two points: at the via, and further along the second coil where the new connection will be made. Go slowly — you're removing lacquer, not cutting copper yet.
- Cut the bad connection. Cut the trace at the via to fully isolate the two coil halves from each other. Confirm with a multimeter in continuity mode that the two coil ends no longer read continuous to each other.
- Re-wire in proper series. Solder a short jumper wire between the two coil ends, but reversed relative to the original via connection, so current flows through both coils in the same rotational sense (aiding rather than bucking). This typically takes two solder joints on each side (four joints total).
- Verify. Reconnect via SMA and tune to a known strong LW/MW station. Compare signal strength before and after. A correctly-fixed loop roughly doubles in amplitude on loop-favorable bands (longwave through lower shortwave).
Expected Results After the Fix
- The loop can now rival the whip antenna for HF reception, and is notably better than the whip for longwave and indoor/noisy-RF-environment use, since it rejects local electrical noise (Wi-Fi routers, smart meters, chargers) that the whip's capacitive-probe design picks up.
- It still performs poorly on FM broadcast — that's a design limitation, not something the fix addresses.
- Expect a couple of mild parasitic resonance peaks on a VNA sweep even after the fix; this is normal for a compact printed loop and doesn't meaningfully hurt real-world use.