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

• Skip distance, skip zone

  

  Image credit: Electrician training

• Ionosphere height

  

  Image credit: image

• Ionosphere day and night

  

  Image credit: Wikipedia

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