Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
The signal path encounters different concentrations of water vapor
VHF ionospheric propagation is very sensitive to path length
Multipath propagation cancels or reinforces signals✓Bonne réponse
The Doppler effect causes slight frequency shifts which result in changes in signal strength
Explication
At VHF a wavelength is only about one to two metres, so moving the antenna a few feet changes the relative path lengths of the direct and reflected signals by a large fraction of a wavelength. The arriving copies then swing between adding in phase and cancelling, which is multipath, not a Doppler or water-vapour effect.