1. Calibrate — OSL (Open/Short/Load) at 6–30 MHz. Use 50Ω SMA cal kit or your own cal standards. Save to a memory slot.

2. Connect — Replace the ATU controller with the NanoVNA at the feedpoint. Connect directly to the loop's gamma match (bypass the ATU).

3. Sweep — Set start 6.5 MHz, stop 30 MHz, 201 points. You should see a sharp dip in |S11| at the resonant frequency. The dip should be below -10 dB (1.6:1) minimum, ideally below -20 dB (1.2:1).

4. Interpret
• Deep dip at target freq → Loop itself is fine. ATU controller is the problem.
• No dip anywhere → Capacitor shorted or open, or the flexible ground strap is corroded. Disassemble and inspect.
• Dip present but wrong freq → Capacitor range issue or loop has been mechanically altered.
• Broad shallow dip → Excessive losses (corrosion, poor connections, or loop too close to metal).

5. Motor position check — Manually move the actuator to fully closed, open halfway, then fully open. A working loop should show the resonance dip moving from low (~7 MHz) to high (~29 MHz) as the capacitor opens.