Chelmsford Amateur Radio Society Foundation Course (6) Propagation - PowerPoint PPT Presentation

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Chelmsford Amateur Radio Society Foundation Course (6) Propagation

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Waves Nominally Travel in Straight Lines ... include Meteor Scatter, Aurora, multipath, fading and Rain/Aircraft Scatter for microwaves. ... – PowerPoint PPT presentation

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Title: Chelmsford Amateur Radio Society Foundation Course (6) Propagation


1
Chelmsford Amateur Radio Society Foundation
Course(6) Propagation
2
Mechanisms Effects
  • Waves Nominally Travel in Straight Lines
  • Diffraction - Waves can spread out/around hills
    and obstacles. Such as after passing through
    narrow gaps and around corners
  • Reflection - Waves (esp at UHF) can bounce off
    buildings
  • Refraction - VHF can be bent by high/low
    pressure - often termed lifts or ducting. HF is
    bent by the Ionosphere
  • Other mechanisms include Meteor Scatter, Aurora,
    multipath, fading and Rain/Aircraft Scatter for
    microwaves.

3
VHF/UHF
  • VHF/UHF has almost line of sight propagation
  • A clear path is much more effective to get a good
    signal than a 10 or 100 times increase in power.
  • For example Satellites can be accessed with low
    power at great distances if there are no
    obstructions
  • Refraction/Diffraction over the Horizon does
    occur but is limited. Buildings/Hills will cause
    shadows and path loss
  • In towns UHF reflects/scatters off buildings
    better.
  • Higher antennas are better than high power, and
    outdoor antennas perform much better than indoor
    ones

4
HF and the Ionosphere
400km
70km
  • Ionosphere is layers of Ionised air 70-400km
    above earth
  • Layers vary day to night - and with sunspot
    cycles etc
  • HF is bent by ionosphere (refraction) - VHF
    passes through
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