Case study 1
To achieve acceptable reception quality a certain satellite signal requires an earth station G/T of 24 dBK. It is known that the LNC to be used has a noise temperature of 200K and the (clear sky) antenna temperature can be assumed to be 50K. The satellite signal operates at 12.5 GHz. For realistic range of dish efficiency values determine the range of dish sizes that will achieve the G/T requirement and comment on which is likely to be preferred.
Solution 1
For the purpose of this exercise we can assume that Tsys is approximately assumed to be equal to TLNC + Tant
Realistic values of lie in the range from 30% to perhaps 80%. The higher value though applies only to small offset fed dishes.
so we find
efficiency (%) Dish diameter (m) 30 3.5 40 3.0 45 2.9 50 2.7 55 2.6 60 2.5 65 2.4 70 2.3 75 2.2
Dishes tend to come in standardised diameters. 2.0, 2.4, 3.0, 3.7m are of most interest for this example. Desirably the dish should be as small as possible because of cost, mounting and transportation issues. Therefore 2.4m should be chosen even though the efficiency is at the upper end of what is commonly available.
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