Quote:
Originally Posted by reho33
I recently acquired a commercial 6 foot fiberglass offset dish that was orginally used by a radio station to receive a Ku feed from ABC News. It measures 74 inches high by 71 inches wide. I bought a voltage polarity controlled C-Band LNBF (13/18 V H/V) and put it in place of the KU LNBF that was there. I tried to recieve C- Band but to no avail. So I guess my questions are these? |
? Should have worked, provided that you put the C-band lnbf where the Ku-lnbf was located, although you might need to play with the focal length a bit, because I think the "actual" focal point of a C-band feed is closer to the front of the feed (ie about 1/4" inside the front surface), compared to an offset feed, for which it looks like the real focal point is deeper inside the front surface.
Quote:
Originally Posted by reho33
1. Could I use the dish as a prime focus dish on C-Band and what would the approximate placement and focal depth of the LNB be ?
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There is only one focal point of any dish. You CANNOT put the feed in the middle of an offset dish and expect it to work. The focal point is where the offset feed is located. Actually, this whole term prime focus vs offset is misleading, because both prime focus and offset dishes have the feed at the prime focus, again, because there IS only one focal point. The best way to think of an offset dish, is to think of it as say a 4 section prime focus dish which is missing 3 of it's sections. Ie it's only the upper 25% of the dish. But the actual shape of an offset dish is the same as a prime focus dish that would have the other 3 sections.
Quote:
Originally Posted by reho33
2. Could it be used as an offset dish with the C-Band LNBF in some configuration other than what I have ( the old config that doesn't work )
The attraction was that it was free and that it was used in the commericial service which may indicate better quality. I am not looking to receive 4DTV or analog , just DVB. ....
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It SHOULD work the way you did it. If it was used for Ku, then it should be an excellent dish. Does it have a 3 leg support for the lnbf? Ie if a single leg, maybe the weight of the lnbf is bending it away from the focal point? But if it's a commercial dish, then I'm sure it must be a stable mount, however like I said above, it may not be convenient for getting the front surface of a C-band feed at the right focal point. So I'd recommend just playing with the focal length. My guess is that you have the front of the feed too close to the dish.
Another issue is that the C-band feed may not be matched for a high F/D dish, however in this case it would result in a little more noise that the feed sees from around the dish, and wouldn't be a situation of losing signal. Once you find where the focal point is, it might be possible to get a conical scalar ring to improve signal, but you really should be able to get this to work just by getting the lnbf a little further out... I think.... and by "out" I think it should be along a line between the inside of the old lnbf and the bottom edge of the dish... which might not be easy.
Anyway, sounds like a fun project. That should be a nice dish. What sats/transponders have you tried to hit???
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6 Foot offset Dish for C Band ?
Linear Mode
