A faulty “donut” antenna shipped with an ESP32-based ATS-Mini receiver was wired to cancel its own signal. Rewiring the loop fixed it and doubled signal strength in testing.

Investigating the Loop Antenna

My new ATS-Mini radio, an all-band radio receiver based on the ESP32 and Si4732, came with both a telescopic antenna and a “donut WB antenna (10 kHz – 180 MHz). While the receiver works well with the telescopic antenna, the loop antenna failed. But there was a solution.

I traced the circuit board layout and found that (as shown in detail in Figure 1):
 

  • The printed coil track runs from the ground of the SMA connector to an exactly centered through-hole plating at the top
  • This coil winds its way back to the SMA center contact on the opposite side of the circuit board.
 
Donut antenna PCB tracks
Figure 1: The two coils are connected in opposite directions (top circuit), but it should be like in the circuit diagram at the bottom.
After turning the circuit board over at least...