Propagation of radio waves Ways of travelling Propagation

Propagation of radio waves

Ways of travelling • • Propagation in ionosphere Propagation in troposphere Special ways of reflecting Propagation depends on – the properties of the medium – frequency – weather, time of day, sun activity

Propagation in ionosphere • Solar radiation (UV, X-ray) partially ionize the atmosphere • Ionization depends on the density of the gases • Waves reflect (refract) from the ionized layers • Directly dependent – on day of time, most neutralized just before sunrise – actitivity of sun • sun spot number • solar flux

• Ionization level and reflection properties divide the ionosphere in different layers: – D: 55 -90 km – E: 90 -150 km – F: 150 -400 km

D layer • lowest layer: exist only during daylight • unsuitable for contacts, absorbs radio energy • 7 & 10 MHz can travel through at high angles

E layer • A bit more stable layer, disappears at night • Skip from E layer is limited by D – hard to distinguish from F layer propagation – longest one skip range is ~2000 km • home for other interesting ways of propagation – Es – Aurora, meteor scatter

F layer • Divided in two during daytime F 1, F 2 – F 2 more important, never absent – one skip almost 4000 km, multihops even longer • reflects from ground, E-layer once or several times • Controlled by sun activity

Propagation in troposphere • VHF, UHF & microwaves • weather effects • tropospheric scattering primary form – rain, fog, dust, snow, clouds • ducting • Radiation inversions: – air layers with different temperatures and dew points – can travel long (1500 km) – calm summer evenings

Other forms of propagation • • • Aurora EME Meteors Sporadic E Satellites

Aurora • High-energy particles flow into ionosphere during geomagnetic storm ionize E layer • 28 – 432 MHz • K index: magnetic activity 0 -9 • Signals sound distorted, sometimes only CW at VHF-UHF (A=Aurora)

EME • Earth-Moon-Earth • 50 MHz to 10 GHz • Free space loss, ½ degree target, Doppler shift, Faraday rotation. . . not that easy • e. FFe-mobiili

Meteor scatter • Ionize a column of air at E layer – stays ionized for a few seconds to a minute just enough time for brief contacs • 50 MHz, 144 MHz • High speed Morse code, pieced together

Sporadic E • Ionization level gets abnormally high • 28, 50, 144 MHz • Usually at summer time at morning and again at early evening • Might dissipate quickly or move

Satellites • Works as repeater station