Chapter

2

Radio Propagation Theory

Introduction							19
Factors Affecting Propagation . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	. 19
Propagation Paths . . . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	. 21
Non-ionospheric Propagation . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	. 21
Ionospheric Propagation . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	. 24
Sky Wave . . . . . . . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	27
HF Communications							32
Propagation Summary . . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	34
Super-refraction							35
Sub-refraction							35
Questions . . . . . . . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	37
Answers . . . . . . . . . . . . . .	. .	. .	. .	. . . . . . . .	. .	.	. 40

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Theory Propagation Radio 2

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Radio PropagationTheory		2

Introduction

In the context of radio waves the term propagation simply means how the radio waves travel through the atmosphere. Different frequency bands use different propagation paths through the atmosphere; the propagation path often determines the uses to which a particular frequency band can be put in either communication or navigation systems. The different propagation paths associated with particular frequencies can also impose limitations on the use of those frequencies.

Factors Affecting Propagation

There are several factors which affect the propagation of radio waves and need to be considered when discussing the propagation paths:

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. For example, if, at a certain distance from a transmitter, the field intensity is 4 Wm-2 at double the distance that energy will be spread over an area of 4 m2 and the field intensity will be 1 Wm-2. That is, power available is proportional to the inverse of the square of the range.

Radio Propagation Theory 2

	1 m	1W	1W
	4W	1 m	2 m
			1W
		1W
		2 m

R

2 × R

Figure 2.1 Inverse Square Law

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Theory Propagation Radio 2

P 1

R2

The practical effect of this is that if it is required to double the effective range of a transmitter then the power would have to be increased by a factor of 4.

Static Interference

There is a large amount of static electricity generated in the atmosphere by weather, human activity and geological activity. The effect of static interference is greater at lower frequencies whereas at VHF and above the effect of interference is generally negligible. However, radio waves travelling through the ionosphere will collect interference at all frequencies. Additionally the circuitry in the receivers and transmitters also produces static interference. The static, from whatever source, reduces the clarity of communications and the accuracy of navigation systems. The strength of the required signal compared to the amount of interference is expressed as a signal to noise ratio (S/N) and for the best clarity or accuracy the unwanted noise needs to be reduced to the lowest possible levels.

Fading

Transmissions following different paths can occur for a number of reasons, e.g. reflections, and can arrive at a receiver simultaneously; however, the two signals will not necessarily be in phase. In extreme cases the two signals will be in anti-phase and will cancel each other out. Signals going in and out of phase are indicated by alternate fading and strengthening of the received signal.

Power

An increase in the power output of a transmitter will increase the range, within the limits of the inverse square law. As noted above, to double the range of a radio transmitter would require the power to be increased by a factor of 4.

Receiver Sensitivity

If internal noise in a receiver can be reduced then the receiver will be able to process weaker signals hence increasing the effective range at which a useable signal can be received. However, this is an expensive process.

Directivity

If the power output is concentrated into a narrow beam then there will be an increase in range, or a reduction in power required for a given range. However the signal will only be usable in the direction of the beam.

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