Question T3A06
From subelement T3 - T3A
What is the meaning of the term “picket fencing”?
Why is this correct?
Picket fencing describes the rapid flutter or fluctuation in mobile signal strength caused by multipath propagation. When driving, your signal reaches the receiver via multiple paths - some direct, others reflected off buildings or terrain. These signals arrive slightly out of phase and alternately cancel or reinforce each other as you move, creating rapid strength variations that sound like looking through a picket fence. Options A, C, and D describe unrelated amateur radio concepts.
Memory tip
Look for terms that describe signal behavior patterns - 'picket fencing,' 'flutter,' and 'fading' typically relate to propagation effects, not equipment or operations. The visual analogy in the term itself (picket fence = alternating pattern) hints at the rapid on-off nature of the phenomenon.
Learn more
Picket fencing demonstrates why mobile digital modes often use error correction. As signals bounce off buildings and terrain, they create multiple propagation paths with slightly different delays. Your receiver gets the same signal multiple times, microseconds apart. When these delayed copies arrive in phase, signal strength peaks. When out of phase, they cancel and signal drops. This multipath environment is why packet radio and other digital emission types include forward error correction protocols.
Think about it
Why do you think picket fencing is more noticeable on VHF/UHF frequencies in urban areas compared to HF frequencies in rural areas?