Going way back to the original question...
I used to fly a Socata Tobago TB10 that had a restricted rpm range for instrument approaches. Apparently this was to prevent electromagnetic interference with the glideslope antenna when flying ILS's.
The antenna was under the fuselage behind the propeller disc, needless to say.
Now is this because the two-bladed propeller at 2250rpm has a blade passing in front of the antenna 75 times a second, and one part of the glide slope signal is modulated at 2x75 = 150Hz? Wouldn't have a clue - perhaps someone else can elucidate.
Merry Christmas to all,
O8