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However, here is what they call out for the BSC-621's:
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- finding
focal point for Ku and C-band
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This web page doesn't make much sense to me. As mentioned by rv1pop below, a dish has 1 focal point, and in theory, it should be the same point for either C or Ku, but the diagram they have at the above link shows them being different, and makes it seem like somehow the Ku from the edge of the dish can go through the sides of the feed to get to a probe deep inside. I don't have a clue about what they are trying to suggest here.
From a broadcaster engineer's statement...."The Ku antenna and the C antenna have to have the same focal point to work. It would be very difficult* to get both focal points at the same distance in a straight cylinder." In short he would never use one. The dish has one focal point. I do not understand much about this issue, but I will go with separate dishes.
* His description of difficult was too graphic for a PG 15 forum.
Yes, the dish has one focal point, in theory, however I have seen a couple things that make it appear that C and Ku are focusing differently.
One thing is that C-band dishes can work well on C-band, but be very imperfect on Ku. When I put little mirrors on my 10' dish, and aimed at the sun, the darn spots were all over the place, most not coming anywhere near the throat of the feedhorn. This tells me that with a consumer C-band dish, you're just lucking out at finding the spot which gets the most reflections from the dish, and this may or may not be at the actual theoretical focal point of the dish.
The 2nd thing, is that even with a perfectly shaped dish that DOES focus the same on C and Ku, one can appear to get C and Ku to focus at different places, particularly if the feed isn't at the correct focal length. From what I've observed, this seems to be because when you are not at the correct focal length, the pattern of a Ku signal at the feed becomes shaped somewhat like a donut, while the pattern of a C band signal just gets a bit blurred. I think that if you were further away from the focal length (proportional to the different wavelengths), that the C band signal would also be like a donut, but the Ku signal distorts sooner due to it's shorter wavelength.
This donut thing is sort of analogous to side lobes, except that the center lobe goes away. You can see the same sort of thing with a magnifying glass focusing light from the sun. Ie you can see rings instead of a spot when not at the right distance.
Anyway, what I've observed, is that if the focal length is wrong, the maxima for C-band will still be centered at the proper point across the arc, but Ku will tend to have maxima that are off by several tenths of a degree or more, and will have a minimum where the sat should be located. This is why, as mentioned by someone earlier in this thread, that you should aim at C-band first. By aiming at C-band first, you insure that you're aiming at the sat. Then if you vary the focal length distance of your feed to maximize the Ku band signal, which will be changing the donut shaped pattern into a spot.
Anyway, I may be all wet on this explanation, but it seems to fit what I've observed, and by going through the above process, on several occasions, I gone from a dish that seemed to have Ku focusing nearly a degree different from C-band, into a situation where they both focus at the same point.
The adjustment is often difficult to make, however, so often I tend to give up and just put the feed so that the measured theoretical focal point is about a quarter inch inside the throat of the feed, and that usually works OK, at least as a starting point.
But the bottom line is that it can be sometimes difficult to get C and Ku on a C-band dish, and it can be either because the shape of the dish isn't perfect, or because the feed isn't at the proper focal length, however I don't believe the suggestion that C and Ku really focus at a different point. In theory the focal points should be the same, but things can make them appear to focus at different points.
