INFLUENCE OF CLUTTER

In the main, the antenna locations for fixed links will be chosen to be well clear of clutter such as trees and buildings. If this is not possible, considerable clutter loss may be produced. In the case of buildings, which are good absorbers, this is usually treated by uplifting the terrain heights at appropriate places by representative values for the building heights, effectively assuming the buildings are opaque. Chapter 8 contains more detailed methods for accounting for buildings. In the case of trees, however, there will be significant penetration which will depend strongly on the frequency and on the types of trees. The simplest way to account for this is to estimate the length of the path which passes through the trees, and to multiply this length by an appropriate value for the specific attenuation [decibels per metre]. The specific attenuation depends strongly on frequency, and to a lesser extent on the polarisation, with vertical polarisation being more heavily attenuated due to the presence of tree trunks. To an even lesser extent, the attenuation is increased by the presence of leaves on the trees. Typical values for the specific attenuation are given in Figure 6.28.

10¹ 10² 10³ 10⁴

10^-3 10^-2 10^-1 10⁰

Frequency [MHz]

Specific attenuation [dB m-1]

Horizontal polarisation Vertical polarisation

Figure 6.28: Specific attenuation due to trees, from [ITU, 833]

134 Antennas and Propagation for Wireless Communication Systems

It is often found in practice, however, that the specific attenuation is a function of the path length, so more accurate models account for the path length explicitly. One empirical approach is the modified exponential decay model [ITU, 236],

L¼ fc d^g ð6:70Þ

with fcin [MHz] and d in [km], where ¼ 0:187; ¼ 0:284 and g ¼ 0:588 are parameters of the model, valid for frequencies between 200 MHz and 95 GHz. Other approaches, based directly on the relevant propagation mechanisms, can also be applied, e.g. [Tamir, 77].

6.10 CONCLUSION

The prediction of propagation over fixed links is a mature art and reasonably accurate predictions may be made in most cases, although the accuracy will depend on the data available and the time available for computation, since the more exact methods can be very computationally intensive. Here are the main steps in predicting the propagation loss:

1. Locate the positions and heights of the antennas.

2. Construct the great circle path between the antennas.

3. Derive the terrain path profile; this step can be done by reading contour heights from conventional maps, although it is increasingly common to use digital terrain maps.

4. Uplift the terrain profile by representative heights for any known buildings along the path.

5. Select a value for the effective Earth radius factor appropriate to the percentage of time being designed for; modify the path profile by this value.

6. Calculate the free space loss for the path.

7. If any obstructions exist within 0.6 times the first Fresnel zone, calculate diffraction over these obstructions, using canonical or full-wave methods depending on the application, and add the resulting excess loss to the link budget.

8. Compute the path length which passes through trees and add the corresponding extra loss.

For systems which require very high availability, the time variability of the signal due to multipath propagation from ducting and reflections must also be accounted for.

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[Bullington, 77] K. Bullington, Radio propagation for vehicular communications, IEEE Transactions On Vehicular Technology, 26 (4), 295–308, 1977.

[Causebrook, 71] J. H. Causebrook and B. Davies, Tropospheric radio wave propagation over irregular terrain: the computation of field strength for UHF broadcasting, BBC Research Report 43, 1971.

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[Craig, 91] K. H. Craig and M. F. Levy, Parabolic equation modelling of the effects of multipath and ducting on radar systems, IEEE Proceedings, Part F, 138 (2), 153–162, 1991.

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[Deygout, 66] J. Deygout, Multiple knife-edge diffraction of microwaves, IEEE Transactions on Antennas and Propagation, 14 (4), 480–489, 1966.

[Dinapoli, 79] F. R. Dinapoli and R. L. Davenport, Numerical models of underwater acoustics propagation, Ocean Accoustics, Topics in Current Physics, Vol. 8, Springer Verlag, New York, pp. 79–157, 1979.

[Dockery, 91] G. D. Dockery and J. R. Kuttler, Theoretical description of the parabolic approximation/Fourier split-step method of representing electromagnetic pro-pagation in the troposphere, Radio Science, (2), 381–393, 1991.

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[Hviid, 52] J. T. Hviid, J. Bach Andersen, J. Toftgard and J. Boger, Terrain based propagation model for rural area – An integral equation approach, IEEE Transactions on Antennas and Propagation, (43), 41–46, 1995.

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Propagation in non-ionised media, CCIR XVth Plenary Assembly, Dubrovnik, 1986.

[ITU, 833] International Telecommunication Union, ITU-R Recommendation PN.833-1, Attenuation in vegetation, Geneva, 1994.

[ITU, 638] International Telecommunication Union, Recommendation P.638-7, Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz, Geneva, 1997e.

[ITU, 453] International Telecommunication Union, ITU-R Recommendation P.453-6, The radio refractive index: its formula and refractivity data, Geneva, 1997a.

[ITU, 526] International Telecommunication Union, ITU-R Recommendation P.526-5, Propagation by diffraction, Geneva, 1997d.

[ITU, 530] International Telecommunication Union, ITU-R Recommendation P.530-7, Propagation data and prediction methods required for the design of terrestrial line-of-sight systems, Geneva, 1997c.

[ITU, 834] International Telecommunication Union, ITU-R Recommendation P.834-2, Effects of tropospheric refraction on radiowave propagation, Geneva, 1997b.

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PROBLEMS

6.1 An omnidirectional antenna is placed 10 m above the Earth’s surface.

(a) What is the distance to the horizon, assuming a smooth Earth and median refractiv-ity?

(b) How would the antenna height have to be increased in order to double the visible coverage area?

(c) What would be the horizon distance with a 10 m antenna height under super-refractive conditions with ke¼ 1:5?

6.2 A ray is launched at an angle of 1 to the Earth’s surface. The refractivity is constant at 300 N units between the ground and a height of 50 m, when it abruptly decreases to 250 N units. What is the new elevation angle of the ray?

6.3 The refractivity in a certain region decreases by 80 N units per kilometre of height. What is the effective Earth radius?

6.4 Calculate the total attenuation for a 4 km path, with equal height antennas, and three knife-edges with excess heights of 50 m, 100 m and 50 m (in that order) equally spaced

Terrestrial Fixed Links 137

along the path at 100 MHz, using (a) the Deygout method and (b) the Causebrook method.

6.5 In Problem 6.4, how would the total loss change if the central knife-edge were replaced by a cylinder with the same effective height but a radius of 5 m?

6.6 When a diffracting obstacle is below the direct path between a transmitter and a receiver, the excess diffraction loss is lower at higher frequencies. Why? Does this imply that the total loss is lower?

138 Antennas and Propagation for Wireless Communication Systems