Etherwave Std & ESPE01 - infinite pitch field

I have a used Etherwave Standard and DIY installed the ESPE01 board (at suggestion of C. Eyck on her website). I can tune the instument to zero beats at an appropriate distance. When I move further away from the pitch antenna the pitch begins to rise again. It does not go silent outside of the zero beats distance. 

Have I made a mistake installing the ESPE01 board or adjusting the internal pots? Please advise this rank beginner. And thank you in advance! 

nothing criminal, that is the usual behavior of all the analog theremins.

As far as I know, only Fred's Skywave H1 has the auto mute function on negative beat zone (thanks to the use of digital IC).

Thank you! This makes sense but I have no experience with the effect. Onward!