Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
The correct answer is C: Multipath propagation cancels or reinforces signals. VHF signal strengths can vary greatly when an antenna is moved only a few feet because of multipath propagation - the signal arrives at your antenna via multiple paths (direct path plus reflections from buildings, terrain, etc.), and these paths can constructively or destructively interfere with each other.
When multiple signal paths combine, they can add together (reinforce, making the signal stronger) or cancel each other out (making the signal weaker or disappear). Moving the antenna even a few feet can change the relative phase of these paths, dramatically changing the received signal strength. This is why VHF signals can be strong in one location and weak just a few feet away - you're experiencing different combinations of the multipath signals.
Exam Tip
VHF signal variation = Multipath. Remember: When VHF signals vary greatly over short distances, it's due to multipath propagation - signals arriving via multiple paths that interfere with each other.
Memory Aid
"**V**HF **V**ariation = **V**arious **P**aths (think 'VV = VP' = Various Paths, multipath)"
Real-World Application
You're operating mobile on 2 meters, and your signal strength to a repeater varies dramatically as you drive. In one spot, you have full quieting. Move 10 feet, and the signal drops to barely readable. Move another 10 feet, and it's strong again. This is multipath - your signal is bouncing off buildings and terrain, and these reflected signals are combining with the direct signal, sometimes reinforcing and sometimes canceling.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Water vapor concentrations don't change significantly over a few feet. This wouldn't cause the rapid signal strength variations described.
Option B: Incorrect. VHF ionospheric propagation is not the primary mechanism for local VHF communications. Multipath from local objects is more relevant.
Option D: Incorrect. Not all options are correct. Multipath propagation is the primary cause of rapid signal strength variations over short distances.
题目解析
The correct answer is C: Multipath propagation cancels or reinforces signals. VHF signal strengths can vary greatly when an antenna is moved only a few feet because of multipath propagation - the signal arrives at your antenna via multiple paths (direct path plus reflections from buildings, terrain, etc.), and these paths can constructively or destructively interfere with each other. When multiple signal paths combine, they can add together (reinforce, making the signal stronger) or cancel each other out (making the signal weaker or disappear). Moving the antenna even a few feet can change the relative phase of these paths, dramatically changing the received signal strength. This is why VHF signals can be strong in one location and weak just a few feet away - you're experiencing different combinations of the multipath signals.
考试技巧
VHF signal variation = Multipath. Remember: When VHF signals vary greatly over short distances, it's due to multipath propagation - signals arriving via multiple paths that interfere with each other.
记忆口诀
**V**HF **V**ariation = **V**arious **P**aths (think 'VV = VP' = Various Paths, multipath)
实际应用示例
You're operating mobile on 2 meters, and your signal strength to a repeater varies dramatically as you drive. In one spot, you have full quieting. Move 10 feet, and the signal drops to barely readable. Move another 10 feet, and it's strong again. This is multipath - your signal is bouncing off buildings and terrain, and these reflected signals are combining with the direct signal, sometimes reinforcing and sometimes canceling.
错误选项分析
Option A: Incorrect. Water vapor concentrations don't change significantly over a few feet. This wouldn't cause the rapid signal strength variations described. Option B: Incorrect. VHF ionospheric propagation is not the primary mechanism for local VHF communications. Multipath from local objects is more relevant. Option D: Incorrect. Not all options are correct. Multipath propagation is the primary cause of rapid signal strength variations over short distances.
知识点
Multipath propagation, VHF signals, Signal fading, Constructive and destructive interference
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.