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 F1, F2– F2 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• eFFe-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