Quote:
Originally Posted by glen4cindy
80cm is not near big enough to receive any C-Band, ***maybe*** with a 120 but that would be absolute minimum and even with that, you would only get the strongest signals. Sorry!
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Just to add to the above, not only is 80cm too small with respect to signal strength, but the resolution of an 80cm dish on C-band is approximately 7 degrees. Ie, you'd be seeing about 5 satellites at the same time, so that low signal strength would be overwhelmed by interferrence from adjacent sats.
I was about to suggest that perhaps he was reading those overly optimistic tables found on the Lyngsat web page. Ie I would look at the C-band footprints at Lyngsat, and then use their chart to estimate how big of a dish you need, it would indicate that I could receive many C-band sats with an 80 CM, which is obviously wrong. However, I just went to Lyngsat to try to find an example, and I see that Lyngsat doesn't seem to have active "BEAM" links on most of the C-band sats any more. I checked about 10 C-band sats before I found one that still had an active link, ie AMC-11..
AMC 11 at 131.0°W - LyngSat Maps
You notice for this sat, most of the US is in their EIRP=41 zone, and the chart at the right suggests that a 70-90CM dish should be adequate. I wonder how many people have used those charts, and made purchases based on that? Perhaps people have complained, and that's why Lyngsat removed most of the beam links?
Also interesting that the Lyngsat links used to point to the sat owner's web pages, but now, where they have links, they are links to their own page. Perhaps they got into Copywrit problems with the EIRP maps?
EDIT: After posting the above, I noticed that although the Lyngsat beam links were no longer active at the main satellite page,
that they can still be accessed via the http://www.lyngsat-maps.com/america.html page. So my curiosity over the
motives for the links going away was likely wrong.
With respect to the prime focus lnbf vs offset lnbf, the main problem is with respect to illumination. Most lnbfs are made to function with a certain range of F/D values for the dishes they are on, ie basically the angle that they "see". On a dish with a low F/D ratio, the lnbf needs to see a wider angle, because it is closer to the dish, whereas on a high F/D dish the lnbf can look at a smaller angle. Most small offset dishes have a high F/D, so the lnbf sees a very small angle view. Most big C-band dishes have relatively low F/D ratios. If you put an lnbf from a small offset dish on a big C-band dish, the lnbf only sees the middle part of the dish, not the edges, so the size of the dish is wasted. If you put a big dish lnbf on a small dish, it sees more than just the dish, and picks up noise from around the outside of the dish.
Thank you!!!
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