Thank you for the suggestion, yet SDR radios are both too expensive (can range up to hundreds of USD) and too good for the desired effect. For the intent the radio mustn't have good reception, the more interference the better, as long as it isn't just white noise, I don't actually want to be listening to whatever is said over radio waves.
So one of those inexpensive soap-shaped AM receivers that old men used for listen to football matches when I was a kid seams more appropriate. Turn the varicap around can be achieved either by a small stepper or servo.
I'll start drawing and testing around this week, to see what I can achieve with that setup.