RADIO PROPAGATION THEORY Flashcards
(37 cards)
What is Attenuation ? What are the two components of attenuation ?
Attenuation. Attenuation is the term given to the loss of signal strength in a radio wave as it
travels outward from the transmitter. There are two aspects to attenuation:
–Absorption. As the radio wave travels outwards from a transmitter the energy is absorbed and
scattered by the molecules of air and water vapour, dust particles, water droplets, vegetation,
the surface of the earth and the ionosphere. The effect of this absorption, (except ionospheric)
increases as frequency increases and is a very significant factor above about 1000 MHz.
–Inverse Square Law. The EM radiation from an aerial spreads out as the surface of a sphere so
the power available decreases with increasing distance from the transmitter.
There are five propagation paths of which four need to be considered for aviation purposes
NON-IONOSPHERIC
–Surface Wave 20 kHz-50 MHz (Used 20 kHz-2MHz)
–Space Wave >50 MHz
IONOSPHERIC
–Skywave 20 kHz-50 MHz (Used 2-30 MHz)
–Satellite (UHF, SHF)
–Ducting <20 kHz
Principle of propogation of Surface wave.
Surface wave propagation exists at frequencies from about 20 kHz to about 50 MHz (from the upper end of VLF to the lower end of VHF). The portion of the wave in contact with the surface of the earth is retarded causing the wave to bend round the surface of the earth; a process known as diffraction.
–The range of surface wave propogations depends on _________ and _________.
– As the frequency increases the surface wave range ____________
– A _________ polarised signal has greater range in surface wave transmission.
The range achievable is dependent on several factors: the frequency, the surface over which
the wave is travelling and the polarisation of the wave. As the frequency increases, surface
attenuation increases and the surface wave range decreases; it is effectively non-existent above
HF. The losses to attenuation by the surface of the earth are greater over land than over sea,
because the sea has good electrical conductivity. Hence greater ranges are attainable over the
sea. A horizontally polarised wave will be attenuated very quickly and give very short ranges;
therefore, vertical polarisation is generally used at these lower frequencies.
–Surface wave propogation is the primary propagation path used in the ………. ………..
–An approximation to the useable range achievable over sea and land for a MF transmission at a
frequency of 300 kHz is given by:
Sea: range ≈
Land: range ≈
–This is the primary propagation path used in the LF frequency band and the lower part of the
MF frequency band (ie frequencies of 30 kHz to 2 MHz).
–An approximation to the useable range achievable over sea and land for a MF transmission at a
frequency of 300 kHz is given by:
Sea: range ≈ 3 x √Power
Land: range ≈ 2 x √Power
For example, a 300 kHz transmitter with a power output of 10 kW would give a surface wave
range of about ……… nm over the sea and ……… nm over the land.
for example, a 300 kHz transmitter with a power output of 10 kW would give a surface wave
range of about 300 nm over the sea and 200 nm over the land.
–Space wave. The space wave is made up of two paths, a ………. and a ………… wave.
– At frequencies of VHF and above radio waves start to behave more like …….. and as we
have a ……….. horizon with light we have a ……… horizon with the radio frequencies.
–So the only atmospheric propagation at these frequencies is …………..
–Space wave. The space wave is made up of two paths, a direct wave and a reflected wave.
– At frequencies of VHF and above radio waves start to behave more like visible light and as we
have a visual horizon with light we have a radio horizon with the radio frequencies.
–So the only atmospheric propagation at these frequencies is line of sight.
–What is atmospheric refraction ?
–The amount of refraction _________ as frequency increases.
–Range (nm) = __________
HTX : Transmitter height in feet
HRX : Receiver height in feet
–At VHF and above it does not matter ______ the transmitter is, if the receiver is below the
line of sight range, it will receive nothing.
–Regardless of the possible propagation paths, if a receiver is in line of sight with a transmitter, then the …………………….
–There is some atmospheric refraction which causes the radio waves to bend towards the surface
of the earth increasing the range slightly beyond the geometric horizon.
–The amount of refraction decreases as frequency increases.
–Range (nm) = 1.23 x (√HTX + √HRX)
HTX : Transmitter height in feet
HRX : Receiver height in feet
–At VHF and above it does not matter how powerful the transmitter is, if the receiver is below the
line of sight range, it will receive nothing.
—-Regardless of the possible propagation paths, if a receiver is in line of sight with a transmitter, then the a space wave will be received.
What is the maximum range a receiver at 1600 ft can receive VHF transmissions from a transmitter
at 1024 ft?
1.23 x (40 + 32) = 88.6 nm
–The electrons are continually attempting to reunite with the ions, so the highest levels of
ionisation will be ……………
–In ………… season the ionisation levels will be higher than in ………..
– Ionisation levels will …………. as latitude decreases, again because of the ……………..
–The levels of ionisation ……… with increase in altitude because………
–The electrons are continually attempting to reunite with the ions, so the highest levels of
ionisation will be found shortly after midday (about 1400) local time, when there is a balance
between the ionisation and the decay of the ionisation with the electrons rejoining the ions and
the lowest just before sunrise (at the surface).
–In summer the ionisation levels will be higher than in winter,
– Ionisation levels will increase as latitude decreases, again because of the increased intensity of the solar radiation.
–As the incoming solar energy is absorbed by the gaseous atoms the amount of energy available
to ionise the atoms at lower levels reduces and hence the levels of ionisation increase with increase in altitude.
Expalin the three layers of the Atmosphere (pg 24)
The lowest of these layers occurs at an average altitude of 75 km and is known as the D-region or Dlayer.
This is a fairly diffuse area which, for practical purposes, forms at sunrise and disappears
at sunset. The next layer, at an average altitude of 125 km, is present throughout the 24 hours
and is known as the E-layer. The E-layer reduces in altitude at sunrise and increases in altitude
after sunset. The final layer of significance is the F-layer at an average altitude of 225 km. The
F-layer splits into two at sunrise and rejoins at sunset, the F1-layer reducing in altitude at sunrise
and increasing in altitude after sunset. The behaviour of the F2-layer is dependent on time of
year, in summer it increases in altitude and may reach altitudes in excess of 400 km and in
winter it reduces in altitude.
Why dawn and dusk are critical times for Radio Nav aids ?
The structure of the ionosphere gives stable conditions by day and by night. Around dawn
and dusk, however, the ionosphere is in a transitional state, which leads to what can best be
described as electrical turbulence. The result is that around dawn and dusk, radio navigation
and communication systems using the ionosphere are subject to excessive interference and
disruption.
What is total internal refraction of a radio wave ? (pg 25)
–The ionisation levels in the layers increase towards the centre of the layer. This means that as a
radio wave transits a layer it encounters an increasing density of ions as it moves to the centre
of the layer and decreasing density as it moves out of the layer. If the radio waves travel across
the layer at right angles they will be retarded, but will maintain a straight path. However, if the
waves penetrate the layer at an angle they will be refracted away from the normal as they enter,
then back towards the normal as they exit the layer.
–The amount of refraction experienced by the radio waves is dependent on both the frequency
and the levels of ionisation. If the radio wave refracts to the (earth) horizontal before it reaches
the centre of the layer then it will continue to refract and will return to the surface of the earth
as skywave; this is total internal refraction at the layer.
–What is critical angle ?
–What is the skip distance ?
– As skywaves occur in the ________________ bands there will also be some surface wave present.
– What is dead space ?
–The minimum angle between the vertical and the radio wave at which total internal refraction occurs and the wave returns to the surface. This is known as the first returning skywave and the angle (measured from the vertical) at which this occurs is known as the critical angle.
–The distance from the transmitter to the point where the first returning
skywave appears at the surface is known as the skip distance.
– As skywaves occur in the LF, MF and HF frequency bands there will also be some surface wave present.
–From the point where the surface wave is totally attenuated to the point where the first returning skywave appears there will be no detectable signal, this area is known as dead space.
– The height at which full internal refraction occurs is dependent on ________
–Frequencies up to 2 MHz will be refracted at the __________ and from 2 – 50 MHz at
the ________.
–Skywave is only likely to occur above 50 MHz when there ____________,
– VHF frequencies used for Navigation systems ________ produce ___________.
– The height at which full internal refraction occurs is dependent on frequency,
–Frequencies up to 2 MHz will be refracted at the E-layer and from 2 – 50 MHz at
the F-layers.
–Skywave is only likely to occur above 50 MHz when there are abnormal ionospheric
conditions associated with intense sunspot or solar flare activity, therefore,
– VHF frequencies used for Navigation systems do not produce skywaves.
What is the Effect of change in ionisation intensity on sky wave propogation ?
At a given frequency, as ionisation increases the refractive index and hence the amount of refraction affecting the radio waves will also increase. This means that refraction will take place at a smaller critical angle and the skip distance and dead space will decrease. Conversely, a decrease in ionisation will result in an increase in critical angle, skip distance and dead space.
What is the Effect of change of frequency in sky wave propagation ?
For a given ionisation intensity, the amount of refraction of radio waves decreases as frequency increases, because as frequency increases the energy contained in the radio wave increases and therefore refraction decreases. So, as frequency increases, the critical angle will increase and the skip distance and dead space will also increase. As frequency increases, the surface wave range will decrease, so there is an increase in dead space caused by both the increase in skip distance and decrease in surface wave range. Conversely, a decrease in frequency will give a decrease in critical angle, skip distance and dead space.
What is the effect of Height of the Layers in sky wave propagation ?
-The skip distance will also be affected by the altitude of the refracting layers. As the altitude of the layer increases then the skip distance will also increase and greater ranges will be experienced by refraction at the F-layer than the E-layer.
What is the impact on LF and MF skywave propagation at night ?
During the day the D-region absorbs radio energy at frequencies below about 2 MHz (LF and MF bands). At night the D-region is effectively nonexistent so, at these frequencies, sky waves, refracted at the E-layer are present. This means the skywaves at LF and MF are not reliable for continuous long-range use and the presence of skywaves at night at the relatively short ranges associated with these lower frequencies will cause interference with short range navigation (and broadcasting) systems relying on surface
wave reception. This affects ADF
–The maximum range for sky wave will be achieved when ________________
–The average maximum range for refraction from the E-layer at 125 km is ______ nm, and the average maximum range from the F-layer at 225 km is _______ nm.
–The maximum range for sky wave will be achieved when the path of the radio wave is tangential at the surface of the earth at both the transmitter and receiver.
–The average maximum range for refraction from the E-layer at 125 km is 1350 nm, and the average maximum range from the F-layer at 225 km is 2200 nm.
What is multi-hop propagation ?
Multi-hop skywave occurs when the wave is refracted at the ionosphere then the sky wave
is reflected back from the surface of the earth to the ionosphere etc. Multi-hop skywave can
achieve ranges of half the diameter of the earth.
What is MUF and OUF ?
The maximum usable frequency (MUF) for a given range will be that of the first returning
skywave and this is the ideal frequency for that range because it will have had the shortest path
through the ionosphere, and therefore, will have experienced less attenuation and contain less
static interference. However, since the ionisation intensity fluctuates, a decrease in ionisation
would result in an increase in skip distance and hence loss of signal. So a compromise frequency
is used, known as the optimum working frequency (OWF), which by decades of experimentation
and experience has been determined to be 0.85 times the MUF.
What is the impact of night time on the usable frequencies and why ?
Since ionisation levels are lower by night than by day it follows that the frequency required for
use at a particular range by night will of necessity be less than the frequency required for use by
day. A good rule of thumb is that the frequency required at night is roughly half that required
by day.
