Definition 1 of 2
Plain English Meaning
Spinning blades chop up the radio signal slightly as it reaches the antenna. That can make the navigation needle wobble or read a little off until the propeller or rotor RPM is changed.
Full Definition
A navigation signal error caused when a VHF radio wave passes through the spinning blades of a propeller or helicopter rotor, which periodically interrupt and reflect the signal. This produces small, rapid fluctuations in the received signal that can cause unsteady or inaccurate indications on VOR or other VHF-based navigation receivers.
Context Anchor
Seen in instrument navigation discussions of space-wave radio signals, especially when explaining why VHF and UHF navigation signals can be affected by the itself.