Optical limits
7.4 SATELLITE EARTH STATION ANTENNAS
The standard antenna type used in satellite earth stations is the parabolic reflector antenna as described in Section 4.5.8. These allow almost arbitrarily large gains to be produced by simply increasing the diameter of the dish, and gains well in excess of 30 dBi are often required in order to provide sufficient fade margin for high-reliability earth satellite links, which often carry signals for broadcast to many users on the Earth. Most high-gain antennas, even for geostationary satellite systems, have to be automatically steered to track irregula-rities in the satellite orbit and to overcome the effects of wind loading.
There are two basic feed types employed in the design of parabolic reflector antennas, namely, prime focus and Cassegrain (Figure 7.24). In the prime focus case, the feed antenna, usually a horn antenna, is located directly at the focus of the parabola and illuminates the parabola with circular wavefronts which are converted into plane waves by the reflector curvature. The Cassegrain case, by contrast, has the prime feed located at the apex of the main reflector, and it illuminates a secondary subreflector placed close to the main focus. If the subreflector curvature is chosen to be hyperbolic, then essentially the same far-field radiation pattern is produced as in the prime focus case. The Cassegrain configuration has less feeder loss due to its reduced length, and permits easy access to the feed horn. When used in receive mode, any spillover associated with the feed horn receives noise from the relatively low-noise Table 7.4: Comparison of ionospheric effects
Frequency
Effect 1 GHz 3 GHz 10 GHz 30 GHz dependence
Faraday rotation [ ] 106 12 1.1 0.1 f2
Propagation delay½ s 0.25 0.028 0.003 0.0003 f2
Dispersion [ps MHz1] 400 15 0.4 0.015 f3
160 Antennas and Propagation for Wireless Communication Systems
sky, whereas the prime focus horn receives spillover noise from the noisy ground. However, the blockage of the aperture by the feed arrangement (and consequent efficiency reduction) is much less in the case of the prime-focus antenna, so the Cassegrain is usually only used for systems requiring beamwidths less than around 1 . See [Stutzman, 81] for more details.
In recent times, small-aperture earth terminals are increasingly implemented using arrays of printed dipoles, which are relatively cheap to manufacture and smaller in volume and wind loading than parabolic reflectors, although the radiation efficiency is usually lower.
7.5 CONCLUSION
Satellite fixed links provide high reliability communications by relying on line-of-sight paths and high-gain antennas, which avoid the obstruction losses encountered in terrestrial links and permit communication over very much greater distances than would be possible when the curvature of the Earth is the limiting factor. Nevertheless, propagation mechanisms due to atmospheric particles are highly significant in creating attenuation and time-variant fading of the signal, which must be characterised in a statistical manner in order to permit low outage probabilities over time periods measured in years.
REFERENCES
[Allnutt, 89] J. A. Allnutt, Satellite-to-ground radiowave propagation, Institution of Elec-trical Engineers, ISBN 0-86341-157-6, 1989.
[Belloul, 98] B. Belloul, S. R. Saunders and B. G. Evans, Prediction of scintillation intensity from sky-noise temperature in earth-satellite links, Electronics Letters, 34 (10), 1023–1024, 1998.
[Budden, 61] K. G. Budden, Radiowaves in the ionosphere, Cambridge University Press, Cambridge, 1961.
[Clarke, 45] A. C. Clarke, Extra-terrestrial relays – Can rocket stations give world-wide radio coverage?, Wireless World, pp. 305–308, 1945.
[Howell, 92] R. G. Howell, R. L. Stuckey and J. W. Harris, The BT laboratories slant-path measurement complex, BT Technology Journal, 10 (4), 9–21, 1992.
[ITU, 372] International Telecommunication Union, ITU-R Recommendation PI.372-6, RadioNnoise, Geneva, 1994.
[ITU, 531] International Telecommunication Union, ITU-R Recommendation P.531-4, Ionospheric propagation data and prediction methods required for the design of satellite services and systems, Geneva, 1997a.
Input
Feed horn
Input Hyperbolic
subreflector Prime
focus
Cassegrain
Figure 7.24: Feed configurations for parabolic reflector antennas
Satellite Fixed Links 161
[ITU, 618] International Telecommunication Union, ITU-R Recommendation P.618-5, Propagation data and prediction methods required for the design of earth-space telecommunication systems, Geneva, 1997b.
[ITU, 676] International Telecommunication Union, ITU-R Recommendation P.676-3, Attenuation by atmospheric gases, Geneva, 1997c.
[ITU, 838] International Telecommunication Union, ITU-R Recommendation 838, Specific attenuation model for rain for use in prediction methods, Geneva, 1992.
[ITU, 840] International Telecommunication Union, ITU-R Recommendation P.840-2, Attenuation due to clouds and fog, Geneva, 1997d.
[Maral, 93] G. Maral and M. Bousquet, Satellite Communications Systems – Systems, techniques and technology, 2nd edn, John Wiley & Sons, Ltd, Chichester, ISBN 0-471-93032-6, 1993.
[Marshall, 48] J. S. Marshall and W. M. K. Palmer, The distribution of raindrops with size, Journal of Metrology, 5, 1965–1966, 1948.
[NASA, 83] W. L. Flock, Propagation effects on satellite systems at frequencies below 10 GHz, NASA Reference Publication 1108, 1983.
[Pruppacher, 71] H. R. Pruppacher and R. L. Pitter, A semi-empirical determination of the shape of cloud and rain drops, Journal of The Atmospheric Sciences, 28, 86–
94, 1971.
[Stutzmen, 81] W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, John Wiley &
Sons, Inc., New York, ISBN 0-471-04458-X, 1981.
[Tatarski 61] V. I. Tatarski, Wave propagation in a turbulent medium, McGraw-Hill, New York, 1961.
PROBLEMS
7.1 Calculate the free space loss for a satellite operating in geostationary orbit, in commu-nication at 10 GHz with an earth station which observes the satellite at zenith.
7.2 Would you expect tropospheric scintillation to be greatest at high or low elevation angles? Why?
7.3 Compute the rain attenuation not exceeded for 0.001% of the time in non-Mediterranean European regions at 30 GHz with an elevation angle of 30 .
7.4 Compute the total atmospheric gas attenuation in Problem 7.3.
7.5 An earth station receive antenna, located at a latitude of 55 , with 3 dB beamwidth of 0.1 at 10 GHz is pointed directly at the Moon with an elevation angle of 45 . Assuming a rainfall rate of 28 mm h1 and feeder loss of 3 dB, calculate the system noise temperature.
7.6 Assuming that the variation of electron density N with height r is given by NðrÞ ¼ N0ekð1rÞ, where N0¼ 1 1012and k¼ 1 105m1, calculate the following parameters for a zenith path: total electron content, Faraday rotation, propagation delay and dispersion for a 30 GHz wave.
7.7 For a 28 GHz ordinary wave, and given the electron density equation in 7.6, determine the critical frequency and critical angle.
162 Antennas and Propagation for Wireless Communication Systems
8 Macrocells
8.1 INTRODUCTION
This chapter introduces methods for predicting the path loss encountered in macrocells. Such cells are commonly encountered in cellular telephony, where they are the main means of providing initial network coverage over a wide area. However, the propagation models are also applicable to broadcasting, private mobile radio and fixed wireless access applications including WiMax. For such systems, in principle, the methods introduced in Chapter 6 could be used to predict the loss over every path profile between the base station and every possible user location. However, the data describing the terrain and clutter would be very large and the computational effort involved would often be excessive. Even if such resources were avail-able, the important parameter for the macrocell designer is the overall area covered, rather than the specific field strength at particular locations, so models of a statistical nature are often more appropriate.
The models presented in this chapter treat the path loss associated with a given macrocell as dependent on distance, provided that the environment surrounding the base station is fairly uniform. In consequence, the coverage area predicted by these models for an isolated base station in an area of consistent environment type will be approximated as circular. Although this is clearly inaccurate, it is useful for system dimensioning purposes. Methods will be indicated at the end of this chapter and in Chapter 9 for improving the reality of this picture.