Hi All,
some comment about size of the dishes and Space Diversity:
The limiting factors for choosing size of dishes could be:
Size of the supporting structure and its current loading level
The need to design link to a specific reliability parameters (eg 99.999%)
Certain regulations of Licensing Authorities in a particular country (for instance antenna minimum size and minimum path for certain frequency bands)
Chosen frequency band – generally on higher bands smaller antennas could be used
Capacity of the transmission traffic (generally higher capacity links require higher reliability therefore bigger antennas)
Other factors
A space diversity technique is used to overcome multi-path propagation issues – eg during abnormal propagation radio rays can be reflected and also bended away from direct path therefore signal loss is present due to this. In simple terms, without going much into theory by having 2 antennas in space diversity configuration, separated say by 15metres, you would have better signal in one antenna, while the other is fading. So the output (resultant of the 2 signals) would be always better than in the situation with the single antenna. Also important that say for a particular case without space diversity you would need say 3m dish to achieve required availability, but if you use 2 x 1.8m space diversity antennas not only you would get better link availability, but also you could spread the loading on the structure more evenly…
Frequency diversity is using similar principle – when link fades on one particular frequency – you get another frequency as a back up.