isn't the cone of silence a term most properly used with the aural range? or Maxwell smart?
zone of confusion or ambiguity might be the better term.
In the AIM of the USA there is quite a nice page or two on the subject. I can't imagine that you haven't read the aim or similar of your own country.
station passage of the VOR is noted by the first positive change of the ambiguity detector from to to from or vice versa.