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

• A. Atmospheric pressure changes rapidly
• B. Multipath propagation cancels or reinforces signals
• C. Interference from electronic devices increases
• D. VHF signals are less stable than other frequencies

Answer: (B)

VHF signal strengths can fluctuate significantly with even slight movements of the antenna due to the phenomenon known as multipath propagation. This occurs when radio signals bounce off various surfaces such as buildings, hills, or other structures before reaching the receiving antenna. When the antenna is moved, the path that the signals take to reach it can change, leading to variations in the signal strength.

When multiple signals arrive at the receiver from different paths, they can interfere with each other. Depending on their phase relationship, this interference can result in either reinforcement (where the signals combine to create a stronger signal) or cancellation (where they partially or completely cancel each other out). The constructive interference may enhance the received signal, while destructive interference may weaken it, causing the observed fluctuations in signal strength.

Other factors like atmospheric conditions and electronic device interference may have some impact on signal quality but do not account for the rapid changes associated with small antenna movements as effectively as multipath propagation does. Similarly, while VHF signals can be less stable compared to other bands under certain conditions, this does not directly explain the immediate variations caused by changes in antenna position related to multipath effects.