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Chelmsford Amateur Radio Society Foundation Course (6) Propagation

Chelmsford Amateur Radio Society Foundation Course (6) Propagation. 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

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Chelmsford Amateur Radio Society Foundation Course (6) Propagation

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  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. Earth HF and the Ionosphere • 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 400km 70km

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