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8a. Propagation

Definitions

  • Propagation is happens to the radio signal between it leaving the transmitter aerial and it arriving at the receiver aerial
  • Refraction is the deflection of a ray of light etc when it passes obliquely from one medium (water, air, glass etc) to another
  • Sun Spot appears as a dark patch on the sun and causes increased ionisation around our world
  • Skip distance is the distance between the transmitter and the nearest point on earth where the signal is receivable after having been refracted back from the ionosphere
  • Fading causes large changes in the strength of a signal at the receiver aerial due to the signal arriving via 2 or more paths
  • Automatic gain control (AGC) is a closed-loop feedback regulating circuit, the purpose of which is to provide a controlled signal amplitude at its output, despite variation of the amplitude in the input signal
  • Selective fading will occur when the refractive effect of the ionosphere varies with frequency
  • Critical Frequency is the highest frequency signal that, when transmitted straight up, will be reflected (total refraction) back to earth at the same point
  • Maximum Usable Frequency (MUF) is maximum frequency that can be used to communicate between two stated places. MUF includes path and time of the day.
  • Temperature inversion happens in the atmosphere when warm air rises (due to smoke) and cold air is above ground

Diagrams

Notes

Ground wave

  • simple case of the signal traveling across the ground (or sea) to the receiving aerial
  • actual distance possible will depend on the conductivity of the terrain
  • example:
    • best: over salty water
    • worse: over sandy desert
  • distance:
    • 2MHz travels 100s km
    • 21MHz travels 10km - 1km

Low frequencies are so good for world wide communication. Why are they not used more often?

  • The frequency is too low to carry the bandwidth of speech modulation
  • Being low, there are not many frequencies available

Ionosphere

Radio waves are refracted by the ionosphere

  • The well known long distance (DX) Amateur Bands 14MHz, 16MHz, 21MHz, 24MHz, 29MHz
  • Variables:
    • strength of the suns rays
      • day / summer --> higher ionisation
      • night / winter --> lower ionisation
    • height, thickness and density of the ionosphere
    • position and density of the ionosphere dictate which frequencies will be refracted and by how much
    • Sun Spot activity varies over the years - peaking and dipping (troughing) every 11 years
ionosphere layer Position Description
D 60km - 85km - Absorbs rather than refracts radio waves
- Absorption is inversely proportional to the frequency
E 100km - 120 km - Lowest ionised layer that can usefully be used for ionospheric propagation
- Refract (bend) radio waves during daylight hours, virtually disappears at night
- Does not normally result in vast distances of communication (100s of miles, not 1000s of miles)
F 140km - 400km

150-200 kms (F1) disappears at night		 - Height varies from vary rare to non-existent
- Ionises very quickly at sunrise to reach a maximum just after midday
- decay is so slow that the ionised layer remains present all through the darkness hours
- F2 layer is most important

Sky waves

  • sky waves can be single or multi-hop
  • In multi-hop propagation, the radio wave is refracted back to earth where It is reflected up again
  • This is the way to communicate over the longest distances; to the opposite side of the world
Skip distances
  • actual skip distance depends on:
    • the time of day
    • season
    • frequency
Frequency Noon skip (km) Midnight skip (km)
1.9 MHz 0 0
3.6 MHz 0 0
7.1 MHz 0 500
10.1 MHz 350 1000
14.1 MHz 600 1600
16.1 MHz 1200 Not possible
21.1 MHz 1300 Not possible
24.9 MHz 1600 Not possible
26.5 MHz 2000 Not possible

Fading

  • Both single and double hop sky waves are received but the path lengths are very different
  • two (or more) signals are most likely to arrive out of phase
  • At one moment the signals will add and a little later they will tend to cancel each other out
  • A receiver with good "automatic gain control" (AGC) can greatly overcome the problems of fading
  • Selective Fading
    • severe modulation distortion will take place
    • speech may become unreadable

Fade out

  • A major flare eruption on the sun will rapidly increase the ionisation of the E layer
  • Its absorption will dramatically increase for a period of up to two hours
  • Many radio signals will suddenly disappear during this time

Other types of Propagation

1. VHF Propagation (above 30MHz)

  • Don't get refracted by the ionosphere those going up, go on and on to get lost in space
  • Signals tend to travel more or less in straight lines
  • VHF aerials are usually designed to transmit the VHF signal in a narrow beam, parallel with the earth’s surface
  • terrain becomes very important
  • Tip: If you intend to operate on the VHF/UHF Amateur Bands you should try to live on top of a hill
  • Obstacles can be an advantage as a reflection to the possible destination

2. Tropospheric ducting

  • Sometimes there might be a layer of warm air in the atmosphere above the cool air on ground
    • temperature inversion
    • smoke rises
  • Temperature inversion will conduct VHF signals over long distance

3. Aurora / Northern lights

  • ionised waving curtain also reflects VHF radio signals
  • The Northern Lights result from large solar flares on the Sun
  • Disadvantage causes large Doppler frequency shift
  • Tip makes Morse necessary rather than ghostly speech

4. Meteor tail

  • When meteors pass through the earths atmosphere they create short term ionised tails behind them
  • It may last only for a few seconds but contacts over 2000KMs are possible
  • Disadvantage Lasting only a few seconds
  • Tip Speeded up Morse is usually sent and a tape recorder used at the receiving end to slow it down again

5. Moon bounce

  • requires high gain aerials
  • high power sensitive receivers
  • Disadvantage Moon is not an easy target to hit with a radio wave as it keeps moving

6. Amateur satellites

  • amateurs using satellites must follow them across the skies with their aerials
  • Disadvantage They are constantly on the move relative to the earth in non geostationary orbits
  • Unlike the "passive" moon an Amateur Satellite re-transmits the received signal on another Amateur Band