Propagation Flashcards
What is propagation
Propagation is the behaviour of the radio wave as it travels from one point on earth to another.
Propagation Tree
Types of propagation- sky wave and ground wave- surface wave and space wave- direct wave and ground reflected wave
What is Surface wave propagation
Surface wave propagation flows along the earths surface supported by currents that have been induced into the ground by the transmitting attena.
What is Attenuation
Is when the radio wave starts to lose energy when travelling from transmitter to receiver. This can be caused due to lack of power or obstacles/ features on the ground soaking up the radio wave.
Factors affecting Surface Wave
Height- antenna must be as close to the ground as possible
Polarity- antenna must be vertically polarised
Ground-wet ground is best
Frequency- lower frequencies travel further
Terrain- flatter terrain is best
Power- increasing the power increases the range
Noise- man made noise will cause interference
Range- dependent on frequency and ground
AE Efficiency- this is maximised by using correct Ae lengths, using counter poise and earthing the antenna
What is Space wave propagation
When the radio wave travels from transmitter to receiver in a line of sight (LOS)
Direct (Radio line of sight)
Ground reflected waves
What is the range for space wave and what frequency do they use?
Ranges up to 100km and It is predominantly at VHF (very high frequency) UHF (ultra high frequency)
What does VHF depend on?
Ae height- the higher the better
Sitting and Terrain- ensure Ae has clear take-off in the wanted direction
Ae efficiency- ensure Ae is efficient. Try vertical cut dipole as alternative if problems occur
Power- increasing the power increases the range. The bowman EVHF can operate at 50W power output
Minimum Effective Height at 30 MHz
10 metres
Minimum effective height at 50 MHz
6 metres
Minimum effective height at 75 MHz
4 metres
What is sky wave propagation
It’s when the radio wave travels from the transmitter up into the ionosphere and refracts back down to the receiver. Sky wave enables long range communications up to 3000km and is used with HF (High Frequency)
What is ionosphere
Ionosphere plays a big part for us to communicate using sky wave propagation. Ultra violet radiation from the sun positively charges ions situated within the ionospheric layers which then allows a radio wave to be refracted back to earth.
Factors affecting sky wave
Polarity- horizontal polarised
Height- angle off take off
Frequency- affect range and must be lower at night
Sitting- requires open ground
Power- increases range
Time of day- noisier at night
Seasonal- longer/ shorter days
Geographical- closer to the equator (higher temperatures)
Sunspot cycle- ionisation intensity
Sporadic E- loss of signal path
Sudden ionospheric disturbance (SID)- loss off signal path- (solar flares, higher UV rays)
Ae efficiency- as always maximise efficiency by using the relevant equation to calculate Ae length
Ae sitting- ensure that the separation between masts is sufficient to accommodate the lowest freq on a 24 hr link.
Lost Comms- ensure that an effective and workable procedure is included in the FCS. It’s important that out stations know when to implement the lost comms procedure.
What are the free channel modes off VHF ?
Fixed frequency (FF)
Frequency hopping (FH)
Free channel search (FCS)
Clear Hail
