Thread: Focal Length distances

a member sent me this link which is kinda cool. Shows focal length distance for a bunch of C-Band dishes




Focal Length Distance

Originally Posted by Iceberg

a member sent me this link which is kinda cool. Shows focal length distance for a bunch of C-Band dishes


Focal Length Distance

Thanks! The Orbitron PDF is particularly helpful. I've never been positive exactly what model I had (or that there were that many models even). I've always calculated my focal lengths from the equations, but it wasn't real exact because of the center plate that keeps you from measuring exactly to the center of the dish, plus the rim around the edge makes it hard to measure the dish depth or even the diameter exactly. Well that PDF not only tells the actual focal length, but tells how much (1/4") the center plate stands above the center of the paraboloid. Turns out that my estimates were pretty close, but it's nice to get confirmation.

And that reminds me... last time I measured my focal length, I attached a metal yard-stick between the feedhorn and the center of the dish. Then, I forgot to remove it. That yardstick has been partially blocking the entrance to my feed for probably 3 months now. I kept meaning to go back and work on it, and remove it, but the bugs were too bad to venture up on the ladder or front end loader. I guess I should do that now, as the bugs are finally starting to die back a bit.... particularly after my blue-sticky-head deer fly catcher has caught well over 25 dozen deer flies, just along my driveway and road frontage. If anyone ever has deer fly problems, paint a plastic flower pot blue, attach it to a stick and smear some sticky gunk called "tanglefoot" on it. It is really amazing how the flies go for this "blue head" instead of YOUR head, and they end up stuck to it. This time of year, I don't leave home without it.

I have a Unimesh dish measuring 92" in 2 directions at 90d from each other,almost 8',more than 7 1/2' ??It's 14" from the string to the center plate and is about 35 3/8" from the closest surface of the center scalar ring to the center plate,if this helps.It doesn't show on that PDF chart.Any ideas what size it is?The strings touch 'gently'.
B.J. I made a crude metal container out of part of an old gas tank,welded 3 legs onto it and make a small fire in it from wood scraps to keep the flies away.I seems to work for anything but Circle flies.They find me anyway.Are you in Labrador?

I've never really understood why the bother of calculating focal depth at all when all you have to do is adjust it until you get the best SQ. If the best SQ happens to be achieved at a level other than the calculated value I doubt we'd opt to use the calculated level instead.
Is it something some people do just for a point of reference?

C band is new to me but I agree with you.I assume there might be a point to it?I'm doing a lot of reading .I have the above dish and procured a 10' one last week.I just need to find the time to set them up.

trommy, well I would say I'm fairly new but I've spend ALOT of time and jumped in pretty fast.
The same thing goes for declination and elevation and fine tuning. There is a elevation and declination value for your area, you set your angles to this and get onto the arc but once there you can't just rely on those "said" angles to be the "be all and end all"
Frankly, I don't care too much what those numbers are because I'll peak to the best SQ regardless anyway. But, you should be pretty close to those value to maintain proper SQ at both ends of the arc (or all the way across the arc anyway).
You can peak your dec and elev on your true south bird, then get all the way over to a far east or west bird and find you have to keep mickey mousing with things to get good SQ. Thats when you can usually be sure that you have to revisit those angles. So having the proper spec is a good starting point.
I still go back to them the odd time and start over again when I feel I've made too many adjustments.

Originally Posted by jaray

I've never really understood why the bother of calculating focal depth at all when all you have to do is adjust it until you get the best SQ. If the best SQ happens to be achieved at a level other than the calculated value I doubt we'd opt to use the calculated level instead.
Is it something some people do just for a point of reference?

Well, it may depend quite a bit on the specific dish/feed you have, but in my case, it is extremely difficult to adjust the focal length. With my dish, it's virtually impossible to move the feed in/out while watching S/Q. I have tried dozens of times over the past 14 years since I got this darn dish, and every time I have given up, and gone back to setting the FL according to the calculations. Basically, with the quad feedhorn support arms, to change the FL, you have to get to where each arm connects to the dish, and change the length of that arm, keeping each arm the same. This is hard enough that I never do it let alone the idea that it can't be adjusted real time, but rather set it, then check the S/Q. An alternative way of setting FL with my dish is to move the feed throat in and out via the scalar ring, however if you do this, you're changing the F/D setting for the feedhorn, plus, when you loosen the allen screw to do this adjustment, the throat cocks one way or the other plus you have to be careful not to change the polarity setting, plus it can't be done without standing directly in the way of the signal, so if I move at all, it affects the signal. So the signal is bouncing all around when I try this.
There is another way that I've tried adjusting the FL, and that is by twisting the whole feedhorn/scalar, so that the quad arms aren't aiming at the center, but are more tangential. This pulls the whole feed in closer to the dish changing the focal distance without affecting the F/D setting of the feed. The only problem with this is that doing this changes the polarity, so that I can't really rely on real time adjustments while moving the feed in and out, but instead have to make a change, then go back inside, adjust the polarity via the receiver, check to see if it improved things, go back outside, and try again, and again.

But perhaps the biggest issue with respect to adjusting the FL by peaking S/Q is that if you do this on C-band, the changes aren't significant enough to observe the S/Q changing more than the normal variations, so it's more accurate to peak on Ku. The problem with peaking FL on Ku is that if your dish is even very slightly off aim, you will get maximum S/Q at the wrong FL, because the signal pattern becomes more of a donut shape when you're at the wrong FL, and if you peak the signal when off FL you'll peak on the side lobe, and both the FL and position will be peaked at the wrong place. Ie basically, you need to be changing both the aim of the dish and the FL at the same time, which with a dish controlled by a receiver inside the house makes any real time adjustment extremely difficult. I've basically given up the idea of peaking S/Q, and just set the FL to the calculated values. However usually about once a year, I get all enthused and try again..... but each time I give up and set it to specs.
NOW, if I had the button-hook feed I had on my first dish, I think I could peak the FL. But not with this quad feed arm arrangement. Anyway, it's not something I "do just for a point of reference". I just can't figure out a way to do it that's better than using the calculated values.

That's one heck of a explanation. But it makes good sense.
Its a good thing then, that FL doesn't make huge differences to the point that they always need to be heavily adjusted during peaking.

This comes in handy when I do get a real BUD.