Quote:
Originally Posted by q12345
This message is for wejones.
I found a link of yours in another post. You used small mirrors taped to your FC90 to find the focal point. I have a 90cm dish fitted with a multi-lnbf bracket. I tried your experiment and found that the point at which the reflections from the mirrors converged was an inch closer to the dish than the lnbf holder was set to.
So I adjusted the bracket to put the face of the feedhorn on the focal point to see if I could get a better signal (i.e. better performance from my dish). It turns out that instead of an inch, half an inch gave the best result. I used G25 for my test.
I wonder if the focal point has to be at the face of the lnbf or at the "cross-hairs" near the back of it. I can post some shots if there is interest. |
Nice to see that there is someone else crazy enough to put mirrors on their dish.
It was really educational for me.
Re where on the lnbf the focus should be... I'm not sure about these little lnbfs, but on the big C/Ku feedhorns, I think that they say the focal point should be something like 1/4" inside the front of the tubular throat, not at the back where the probes are.
When I did my mirror experiments, I had something like 5 different mirrors, and except for the one main focal points, when you got off center, the reflections from the different mirrors were zipping around in different directions as you moved the dish around, and moved the feed (in my case I used a fake feed) closer and further from the dish. It seemed like there were multiple places where a couple of the mirrors would hit the same place, but no other place where they all hit. I got the impression that if I had 10 mirrors, I'd see even more variability. Ie, if you only had 2 or 3 mirrirs, you well may have found a spot 1" in that looked good for a couple of the mirrors, but was worse for other points on the dish. Basically there is only ONE focal point, so when you go off center you're just trying to find a spot that works OK, but not perfectly. In my case with 5 mirrors, I found that my LNBF was off position by about 2 inches, but the actual reception at the place where the lnbf was, vs the real focal point, wasn't enough to see a difference in the quality levels on the Fortec receiver I had connected at the time. Ie even though the 2" off focus seemed a lot worse than the real focal point with 5 mirrors, apparently the reflections from all the places that I didn't have mirrors made the reception not all that worse when measured with a receiver. Anyway, I didn't even bother bending my feed to the proper focal point.