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Propagation Paths

There are four propagation paths of which four need to be considered for aviation purposes:

PROPAGATION

NON-IONOSPHERIC IONOSPHERIC

Surface Wave			Sky wave
20 kHz-50 MHz			20 kHz-50 MHz
(Used 20 kHz-2 MHz)			(Used 2-30 MHz)
Space Wave			SatComm
> 50 MHz			Direct Wave
			(UHF, SHF)

Figure 2.2

Ionospheric propagation is propagation affected by the properties of the ionosphere. At this stage it is only necessary to discuss sky wave, satellite propagation will be considered in conjunction with global navigation satellite systems (GNSS) in Chapter 18. Knowledge of propagation below 30 kHz is not required.

Non-ionospheric propagation covers the other propagation paths.

Non-ionospheric Propagation

SurfaceWave

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.

Radio Propagation Theory 2

Figure 2.3 Surface Wave

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The range achievable is dependent on several factors: the frequency, the surface over which the wave is travelling and the polarization 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 polarized wave will be attenuated very quickly and give very short ranges; therefore, vertical polarization is generally used at these lower frequencies.

10 k

LAND SEA

100 k

f (Hz)

1 M

10 M

100 M

1

10

100

1000

10 000

NM

Figure 2.4

This is the primary propagation path used in the LF frequency band and the lower part of the MF frequency band (i.e. frequencies of 30 kHz to 2 MHz).

An approximation to the useable range achievable over sea and land for an MF transmission at a frequency of 300 kHz is given by:

Sea: range ≈ 3 × √Power

Land: range ≈ 2 × √Power

So, 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.

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Radio PropagationTheory		2

SpaceWave

The space wave is made up of two paths, a direct wave and a reflected wave.

Radio Propagation Theory 2

Figure 2.5 Space 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.

RX

TX

Figure 2.6 Maximum theoretical range

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. Since the diameter of the earth is known and the atmospheric refraction can be calculated it is possible to determine the maximum theoretical range at which a transmission can be received. The amount of refraction decreases as frequency increases but for practical purposes for the EASA syllabus the line of sight range can be calculated using the formula:

Range (NM) = 1.23 × (√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.

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For example:

What is the maximum range a receiver at 1600 ft can receive VHF transmissions from a transmitter at 1024 ft?

Range = 1.23 × (√1600 + √1024) = 1.23 × (40 + 32) = 88.6 NM

Note 1: Regardless of the possible propagation paths, if a receiver is in line of sight with a transmitter, then the space wave will be received.

Ionospheric Propagation

Before studying ionospheric propagation it is necessary to know about the processes which produce the ionization in the upper atmosphere and the properties of the ionosphere that produce sky wave.

The Ionosphere

The ionosphere extends upwards from an altitude of about 60 km to limits of the atmosphere (notionally 1500 km). In this part of the atmosphere the pressures are very low (at 60 km the atmospheric pressure is 0.22 hPa) and hence the gaseous atoms are widely dispersed. Within this region incoming solar radiation at ultra-violet and shorter wavelengths interacts with the atoms raising their energy levels and causing electrons to be ejected from the shells of the atoms. Since an atom is electrically neutral, the result is negatively charged electrons and positively charged particles known as ions.

The electrons are continually attempting to reunite with the ions, so the highest levels of ionization will be found shortly after midday (about 1400) local time, when there is a balance between the ionization and the decay of the ionization with the electrons rejoining the ions and the lowest just before sunrise (at the surface). In summer the ionization levels will be higher than in winter, and ionization levels will increase as latitude decreases, again because of the increased intensity of the solar radiation.

Increased radiation from solar flares is unpredictable but can give rise to exceptionally high levels of ionization, which in turn can cause severe disruption of communication and navigation systems, particularly those which are space based. It is not unusual for communication (and other) satellites to be shut down during periods of intense solar flare activity to avoid damage.

As the incoming solar energy is absorbed by the gaseous atoms the amount of energy available to ionize the atoms at lower levels reduces and hence the levels of ionization increase with increase in altitude. However, because the normal atmospheric mixing processes associated with the lower levels of the atmosphere are absent in the higher levels, gravitation and terrestrial magnetism affect the distribution of gases. This means that the increase in ionization is not linear but the ionized particles form into discrete layers.

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F LAYER

E LAYER

Km

D LAYER

e

Figure 2.7 Effect of ionisation with height

The ionization is most intense at the centre of the layers decreasing towards the lower and upper edges of the layers. The characteristics of these layers vary with the levels of ionization. The lowest of these layers occurs at an average altitude of 75 km and is known as the D-region or D-layer. 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.

Radio Propagation Theory 2

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Figure 2.8 Layers of the ionosphere

Although, overall the levels of ionization increase from sunrise to midday local time and then decrease until sunrise the following morning, the levels are continually fluctuating as the intensity of high energy radiation from the sun fluctuates. So it would be possible for the ionization levels to decrease temporarily during the morning, or increase temporarily during the afternoon.

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.

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