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T3A01

RADIO WAVE PROPAGATION · Radio wave characteristics: how a radio signal travels, fading, multipath, polarization, wavelength vs absorption; Antenna orientation

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

  1. AThe signal path encounters different concentrations of water vapor
  2. BVHF ionospheric propagation is very sensitive to path length
  3. CMultipath propagation cancels or reinforces signals
  4. DThe Doppler effect causes slight frequency shifts which result in changes in signal strength
Why At VHF a signal usually arrives at your antenna by several paths at once: the direct path plus reflections off buildings, hills, towers and vehicles. Those copies arrive with different phase, so they add up in some spots and cancel in others, creating a standing pattern of strong and weak spots. Since a wavelength at 146 MHz is only about 2 meters, moving the antenna a few feet can shift you from a null to a peak. That is why mobile stations hear rapid flutter, or picket fencing, while driving.
Watch out Ionospheric propagation is rarely a factor at VHF for local contacts, and a few feet of path length would be utterly insignificant to skip anyway; Doppler shift changes frequency, not strength, and requires real motion between stations.
Short waves, short nulls: at 2 meters a peak and a null can be a few feet apart. Move the antenna, change the multipath mix.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of T3A

  1. T3A01 Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
  2. T3A02 How does vegetation affect UHF and microwave signals?
  3. T3A03 What antenna polarization is normally used for long-distance CW and SSB contacts on the VHF and UHF bands?
  4. T3A04 What is the effect of antenna cross-polarization over a line-of-sight VHF or UHF path?
  5. T3A05 When using a directional antenna, how might your station be able to communicate with a distant repeater if bu…
  6. T3A06 What is the meaning of the term "picket fencing"?
  7. T3A07 What weather condition might decrease range at microwave frequencies?
  8. T3A08 What is a likely cause of irregular fading of signals propagated by the ionosphere?
  9. T3A09 Which of the following results from the fact that signals propagated by the ionosphere are elliptically polar…
  10. T3A10 What effect does multi-path propagation have on data transmissions?
  11. T3A11 Which region of the atmosphere can reflect HF radio waves?
  12. T3A12 What effect does fog or rain have on 10-meter and 6-meter band signals?
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