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Originally Posted by wahroonga farm
Hi,
My question relates to the f/D specification of a Ku offset lnb.
f/D is a relationship of a dish focal point and diameter. It is used to specify the 'look' angle of the lnb which matches that dish. It is most typically used to specify prime focus dish installations. C-band prime focus f/D's are typically in the range of 0.3 - 0.4. The look angle of the matching C-band lnb is approcimately 90 degrees ie a perfectly circular 90 degree conical view.
f/D is also relevant to offset designs where the figure is in the 0.6 - 0.7 range. This means that the offset lnb now has a 'sharper' approximately 60 degree view.
Now for my question.
Does the offset Ku lnb also share a 'perfectely circular 60 degree conical view'?
I guess I could easily determine this by cutting the windows of a variety of offset ku lnb's to study the scalar rings, but I'm sure this has been done before. |
I don't know the answer to your question, but what you're discussing is really a function of the feed not the lnb, ie the "F" of the LNBF. But I think any feed needs to be matched to the F/D of the dish. Ie it needs to illuminate as much of the dish as possible, but no more. On my big dish, the feedhorn is adjustable, ie you can move it in or out to match dishes of different F/D ratios. I'm a bit surprised that the offset dishes are actually 0.6-0.7 as you say, and I wonder if that figure is based on the physical dimensions of the dish, or on the dimensions that the dish would have if it were all there and a prime focus. I suspect the former.
But yes, the angle from a typical offset lnbf would seem to illuminate at a narrower angle than your typical prime focus dish. I suspect that this is why when I put a DBS lnbf on my big dish, that I don't get any more signal than when it's on an 18" dish. Ie the DBS lnbf only sees a fraction of the big dish, and that fraction isn't shaped as perfectly as a small dish.
Also, when you see eliptical shaped offset dishes, the lnbf's opening is often eliptical too, matched to the view of the dish that it sees. One interesting aspect of all this, is that with a prime focus dish, the feedhorn aims at the center of the parabola, which is also the center of the dish, however with an offset dish, the center of the parabola is down off the bottom edge of the dish. If the lnbf aimed at the center of the parabola, it would be illuminating a lot of space where the dish wasn't located, so they tilt the lnbf to aim at the center of the dish, instead of the center of the parabola. One interesting aspect of this is that apparently with big dishes, the feedhorns receive more efficiently from the center of the dish than from the edges. A year or two, I was trying to optimize the positioning of my feedhorn while aimed at my southern most satellite, which I didn't realize at the time had been partially blocked by a couple trees that had grown up since I installed the dish. What had happened was that the bottom half of my dish wasn't receiving any signal at all because it was blocked by the trees, so when I tried optimizing the feedhorn, I found that I got the best signal when the feedhorn was tilted toward the upper half of the dish. This really confused me, until I turned around to see where the dish was
aiming and noticed that the big tree was blocking my view.
It always amazes me how signal even gets into a feedhorn from the edges of a low F/D C-band dish. My previous dish had a fairly high F/D, and thus the feed had a lower illumination angle, and more efficient view of the dish. However my current dish has a very low F/D, which I don't like at all. The signal reflected from the edges of the dish come almost from a 90deg angle as you say above, and it's really hard to imagine how the signal even makes it into the feedhorn. It's almost like the circular rings around the center cylinder act as an eyepiece to further bend the signal down into the feed.
Anyway, interesting topic. I wish I understood it better.
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