Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?

VHF (Very High Frequency) signals have relatively short wavelengths, typically around 2 meters. Because these signals travel primarily in a line-of-sight manner, they easily bounce off buildings, hills, vehicles, and other conductive objects. When a signal takes multiple bounced paths to reach a receiving antenna, the different waves arrive at slightly different times. This is called multipath propagation. Depending on their phase when they meet, these arriving waves can either add together (reinforcing the signal) or cancel each other out (creating a dead spot). Because the wavelength is so short, moving the antenna just a few feet can take it out of a cancellation 'null' and put it into a reinforced 'peak.' This rapid spatial variation has nothing to do with water vapor or ionospheric changes.