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Post subject: Antenna dish for satellite
Posted: Tue Jul 15, 2008 12:47 am
Joined: Tue Jul 15, 2008 12:42 am
I am quite new for the satellite staffs. Could anyone give me some explanation about the size of antenna dish? Why the dish for Ku band is smaller than C band?
Posted: Tue Jul 15, 2008 3:45 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
The gain formula is approximately Gain = 17.5 + 20 * LOG( F * D)
where F is the frequency in GHz and D is the diameter in metres.
Therefore as F increases for a given dish diameter, the gain increases. You might think then that if you choose a 4 feet (1.2m) dish size, you just get more gain at Ku gain, and that's good right?
Well, no, because the consequence of greater gain is narrower beamwidth. At some point the dish pointing accuracy requirements are too severe, in other words the antenna beamwidth is too narrow. Slight pointing errors will result in flutter of the signal, for example due to wind vibration.
Each satellite service has a given almost constant gain requirement, because the link parameters are stable with time, varying only by a few dB due to rain or due to sun noise. You therefore need just enough gain to satisfy the link loss budget, and having more gain achieves nothing terms of system performance. It just costs more bucks, and gives more alignment headaches.