MiniRadio Anti-Theremin Effect Mod

All versions of the SI4732-based MiniRadios particularly the current Version 4 produce a distinctive sound when receiving a signal in SSB mode. 

This “Theremin” effect occurs when the base of the telescopic whip antenna or the pin of the SMA socket is touched. The frequency of any received signal gets detuned creating a very disturbing audible effect In SSB mode . 

This originates from an internal 3.4 GHz PLL-controlled VCO leaking to the SI4732’s pins.

This effect can be removed with either an internal modification or an external adapter. Both introduce inductance into the antenna path to dampen the 3.4GHz signal.

The external adapter also includes two diodes to protect the receiver’s input from static electricity (ESD), a feature necessary in early MiniRadio versions V1 to V3S.

External adapter with 3.4GHz damping coil and ESD diodes suitable for all versions of the MiniRadio

External adapter with 3.4GHz damping coil and ESD diodes 

Leaking internal SI4732 VCO resulting in  - 35dBm@3.4GHz at the antenna input

The leaking VCO signal dampened by the adapter resuling in -56dBm@3.4GHz

The input stage of a V4 MiniRadio  with the VCO damping coil 

Internal mod of a V4 with divided SMA pin

Internal mod of a V4 with a  cut in the PCB trace

More about this modification for the MiniRadios at https://peterneufeld.wordpress.com/2025/12/06/anti-theremin-antenna-adapter-for-miniradios/