Well-Known Member
I was reading the AIM and I stumble upon this statemt-
Certain propeller revolution per minute settings or helicopter rotor speeds can cause the VOR course deviation indicator to fluctuate as much as plus or minus six degrees.

Anybody Know why this happens?


New Member
Anybody Know why this happens?
Not specifically, but any changing flow of electrical current will produce radio waves. You have that in both the spark plugs and the magnetos, as well as accessories such as the alternator.


New Member
The propeller (or rotor) is a conductor. It can cause interference in two ways:

If the antenna is very close to the moving element it can see a variation in impedance.

Second, the conductor re-radiates a portion of the radio signal and the motion changes this pattern. It looks like an amplitude modulation superimposed on the VOR signal. You can think of it literally that the propeller "chops" the radio signal but I believe its really that the blades re-radiate as tiny antennas themselves and you get a multi-path interference at the VOR antenna that varies over the course of each revolution.

Since the VOR derives its bearing from the phase difference between an AM signal and a reference FM signal the superimposed chop of the rotor messes it up.