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Microwave propagation

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Problems in microwave propagation

The microwave propagation is subject to extremely complex physical laws. Even to deal with these in an approximate way would be exceed the scope of this website, especially as all phenomena and conditions are not only closely dependent on the frequency and transmission distance used but also on the modulation scheme and transmission capacity. Furthermore all of the relevant factors have mutual effects upon each other. Therefore an attempt has been made in the following to touch upon only the most important problems.

Unimpeded lines-of-sight are absolutely necessary between two microwave nodes. In addition the first Fresnel zone must be free from all obstructions. Panes of glass or plastic blinds cause additional attenuation and affect the beam directionality of the antenna. Special laws apply in the antenna near field. Furthermore, for longer distances, the curvature of the earth and the refraction of the "microwave beam" in the atmosphere along with the latter's deviations throughout the day and the year must be taken into consideration. In practice two-dimensional path profiles are created that enable an experienced microwave link planner to evaluate the transmission path.

For expanding please click on the images

Path profile prepared by software-tool of Aircom International

A decisive influence on the microwave propagation is exerted by meteorological phenomena in close interaction with the topographical conditions in the transmission field. In long-haul microwave technology, refraction, reflection and various types of fading have a marked temporary effect on the quality of the microwave link. In contrast to this, in short-haul microwave links, the propagation is impaired above all by heavy rain. The transition between these two causes of impaired quality is however fluid and extremely complex. For this reason the achievable quality of a microwave link can only be comprehensively analysed by an experienced specialist on the spot and be calculated in advance by him.

In practice, values of between 0.1 and 0.001 %, (as determined statistically over the period of one year) for non-availability arising in connection with the microwave propagation of a single link are achievable. However, achieving very high values of availability requires a disproportionately high cost, above all for antennas. For long-haul microwave links it can even be necessary for each location to install two antennas at a certain distance from each other (space diversity).

Calculating the achievable availability of a microwave link is carried out on the basis of statistical methods and takes into account observations and measurements carried out over tens of years. Currently there are now available software solutions from a number of companies (e.g. Teoco Corporation, ATDI, Comsearch, LS telcom, Pathloss/CTE), which enable the microwave link planner to calculate and configure rapidly and effectively the microwave link. All of this software is based on uniform guidelines of the International Telecommunication Union (ITU).

At this point it must be emphasised once again that the best software tools — no matter how comprehensive the topographical data on which they draw upon — cannot replace the visual inspection and evaluation by an experienced microwave link planner! This is particularly true for long-haul links, or over built-up areas, or for unusual topographical conditions (e.g. over large expanses of water or flat terrain).

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Copyright 2006-2017 Reinhard Wagner