"Dish 500" out of a True-Focus 1.8M?

[MJFlash]

Hi, Anole!

This is a really good start towards understanding how off-axis works with our dishes. Most folks have had success with elliptical dishes, but as I try to apply these results to normal - presumably parabolic - FTA dishes, the results are fascinating!

Doing a Google search for "off-axis parabolic focus" yielded a fascinating resource by Mark Christensen: Ray Tracing of a Parabolic Mirror. In this excellent work, Mark offers a spreadsheet that computes how the satellite beam is defocused as a result of being off-axis. Now, this is isn't quite what we're looking for, since we want to know how the off-axis position is focusing on an off-axis distant object, but it's interesting nonetheless!

When I enter typical LNB offsets into the spreadsheet, it shows directly how the primary (aiming point) object is defocused at the new LNB position. Hopefully, I can continue diving into this enough to be able understand how the new LNB position focuses on off-axis targets. By the way, if anybody gets this first, please let us know! My math's rusty enough that it'll probably take me a long time (if ever) to figure out the next step!

Have Fun!

 

[Anole]

I pointed MJFlash to this thread for solving the 9º spacing for a Fortec Star 90cm dish.
Not what you need in Alaska nor Mexico with the 4..6 foot dishes...
But pretty good for Canadians with really bad weather, or places like Florida with lots of rain-fade!

As for the defocusing, you either put both LNBs equal distance from the center and take a little hit on both, or as in the Mexico problem, put the weak bird on the centerline, and the 2nd LNB off to the side.
(I'll edit in a link for the Mexico thread later, if I remember)

 

[MJFlash]

Hi, Anole!

After poring over that spreadsheet, I'm clearly out of my league, mathematically speaking. Consequently, I did some Googling to see if I could find the Mark Christensen who wrote that article and spreadsheet. I may well have the wrong person, but I just sent him this note:

First of all, please forgive me for contacting you out of the blue, especially since you may not be the right person! In scouting the web for solutions to what happens to focus with off-axis detectors on parabolic reflectors, I ran into a terrific spreadsheet on the FVAstro astronomy club site. Might you be that same Mark Christensen who created this? If so, I would be very grateful if you might possibly consider recommending me to someone - perhaps a grad student (?) who might be willing to help me answer some questions for a small stipend (this is "just" a hobby project ;-) ). If my Google search has led me to the wrong person, once again, please accept my most sincere apologies!

I'm trying to apply this off-axis spreadsheet to a problem where a parabolic reflector itself is vector normal to a first target, but where additional detectors and their distant targets are off-axis to that vector norm. The specific application is for home Ku band satellite dishes, where hobbyists like me attach multiple "detectors" (LNBs) to a single parabolic dish, in order to receive multiple satellites simultaneously. It's my goal to attempt to quantify in some manner the relative efficiency of such an approach. Pragmatically, many folks are doing this now, and it certainly works, but I'd love to understand the theoretical impact on relative efficiency while doing so. Might you possibly have any recommendations as to whom I could work with to help solve this problem?

Regardless, I sincerely appreciate your even having read this far! Further, if you are indeed the "Mark Christensen, Ph.D" from FVAstro, please accept my congratulations and thanks for such superbly lucid writeups on optics!

It'll be fascinating to see if I hear back from him!

Have Fun!

 

[MotleyFool]

Wow, ray tracing, haven't done that since optics class oh so many decades ago. Oh those were the days, the 1980's, star wars programs, 1M diameter high energy UV excimer lasers, sigh.

 

[Smith P.]

The setup LNBFs for 1m or 2m dish doesn't required high precision calculation, better left some margin for manual adjustment for real tune.

 

[Anole]

Yea, no kidding.
This ain't brain surgery.
As I said above, just tune for maximum smoke.
Agonizing over the littlest detail is a waste of time.
Get on with it!

Then report your findings, and we can all say: "aaaaahhhhh"

 

[MJFlash]

Hi, Anole!

Well, I'm the kind of guy that seeks to understand both the theory and the practice. I'll be experimenting with this today, at the same time that I'll seek to better understand the math. I've made some progress, and have come up with a better position approximation than that shown above. On the other hand, I don't want to derail this thread, so I'll start a new one.

Thanks again for getting me started on this!

 

[MJFlash]

Hi, SForgue!

I've made some progress on coming up with a more accurate formula for estimating offset LNB positions. Based on your zip code, the sat calculator I'm using says that the satellites are at:

119: Azimuth 149.17 - Elevation 13.58
110: Azimuth 139.83 - Elevation 11.28

The formula is actually quite simple:

Horizontal offset = dish_focal_length * sin(dish_azimuth - new_azimuth)
Vertical offset = dish_focal_length * sin(dish_elevation - new_elevation)

So, with your dish having a focal length of 43.92", and being aimed at 119, the offset for the second LNB for 110 would be:

Horizontal offset: 43.92" * sin(149.17 - 139.83) = 7.128"
Vertical offset: 43.92" * sin(13.58 - 11.28) = 1.76"

For horizontal offsets, positive means to move towards the left as you face the dish (i.e. west), and negative means to move to the right. For vertical offsets, positive means to move upwards, and negative means to move downwards. In your case, the centerpoint of the second LNB's window should be 7-1/8" to the left/west, and 1-3/4" upward from that of the first LNB. Since you are pointing at circularly polarized signals, skew is completely irrelevant, so just ignore it.

For more details, see Offset Focus: Theory and Practice.

Please let us know how things work out!

Cheers!
Mark

 

[sforgue]

Guys;
I just finished re-reading the whole post, as I'm getting close to putting everything together. I'm digging a hole to mount the support pole, and it's slow going. We had a really cold winter up here in North Pole, so the ground is still frozen. I figure I'll go down about four feet, and have the dish about 10 feet up (gotta clear the peak of the house and some trees). I bought an Accu-track 22 and some other supplies from Sadoun, and they should be at the house soon. I'm in the military, and we're currently deployed to Guam for a while, so hopefully the ground will be thawed when I get back. I'll let everyone know how things are going when I get back to Alaska and can get everything hooked up.
One more thing: everybody down there in the Lower 48 should count their blessings that they can use one little 20" dish and get everything that requires a 6', 75 lb reflector! -Steve

 

[Anole]

Steve -

We're happy to hear from you again.
Was wondering how things worked out for ya.
Keep us informed, and do post pictures.
Keep adding to this thread - it's a wonderful resource for the next guy with a similar problem.

And you know... if it gets too chilly up there for ya, we've got plenty of room for you down here in the lower 48. :up

....where dishes are smaller, and aiming is easier...

 

[sforgue]

A quick update on my progress... I've been working on the dish install off-and-on for the last three days, and I'm making some real progress. The pole is in (four feet below ground, 10 feet above), and I got the reflector mounted yesterday (thanks for the help Keith). Next step is to make an RG6 test lead (I'm using quad-shield with compression fittings), and see if I've got the reflector pointed at the right satellite. I did a quick peak last night after getting the reflector mounted, but I'm not sure which satellite I'm looking at. A quick peek at lyngsat shows about five satellites near 110W, and my meter doesn't differentiate between satellites.
Does anybody have any tips on installing compressions fittings on RG6? I'm using EX6XL compression fittings with an ultraseal ripley crimper.

 

[sforgue]

Big Setback

Seems I'm having quite a bit of trouble peaking the dish. I've got the dish pointed to roughly where I computed the satellite should be, and I'm getting a strong signal, but I can't get the receiver (311) to lock on and show any signal strength. I've got the tilt really close (I bought the dish from a guy who was using it to get 110W), and I've tried slewing it all around the sky, but no luck. The Accutrack22 is showing a signal of anywhere from 33 to 60, which I then have going through an in-line amplifier (13 dB). By the time it comes out of the RG6, the signal is around 80, which is about the same relative strength as I'm getting on our 119W dish.
I've run a check switch multiple times, and the box says that there is a DP Dual connected, but that it isn't locked to a satellite and can't get any signal (red bar w/0 signal). Does anybody have any hints/tricks I might try?

 

[Smith P.]

You got wrong sat - try to move left-right up-down for get initial lock and attributing right sat; 119W could be first, then search fro 110W.

 

[sforgue]

Little Help

I'm still trying to get this thing peaked, and I got some outside help yesterday. In the early afternoon, I got a service-call visit from a local dish installer, and he gave me some helpful pointers. It looks like I already have the reflector really close to where the 110W sats are, and it might be something as simple as too many tall, leafy trees in the way. I kinda verified this today while I was standing in front of the dish playing with the LNB: I could hear all of the trees (100+ feet away) rustling in the wind (the sound was all being collected by the reflector)! So the next step is to start selectively cutting down the trees that are the most likely culprits. The service guy also said that, for the next few days, the 110W satellites would be directly beneath the sun at 1140 a.m. It was too cloudy to check today, but I'm hoping for clear weather tomorrow, and I can double check my sight-line to the satellites...

 

[Smith P.]

Oh, don't waist your time with sun eclipse - use school protractor - make it by yourself in 15 min.

 

[sforgue]

Death to Trees!

Spent most of Saturday cutting down the trees that were in the way of the satellites. After dropping three big 'uns, the meter reading on the Accutrack went up by two points, but I still couldn't get a lock. BTW, is there a minimum reading on the receiver strength meter before I'll get the thing locked on? The strength I'm getting from (what I think is) 110W is pretty close to the strength from the 119W slot. Do I need to unplug the 119W, run a check switch, plug in the 110W, and do a check switch?

Oh, and the sun was right where I figured (hoped) it would be at 1140 a.m.

 

[Smith P.]

Avoid use a switch - just straight cable ( use barrel if it need ). It will lock after SS 38. Also, use one ( main) LNBF and find both sats; mark angles, then using middle values and right skew try to finalize this setup.

 

[Bountykiller]

I too have done this before. In general if you can't skew the dish (I don't advise skewing a heavy dish like a 1.8 or 2.4 even if you could, you rotate the LNB's accordingly to match the polar arc. However, it should noted that if you are in an area that NEEDS a 1.8 meter dish to receive either 119 or 110, you will not be able to share/split it like this. The reason being that the resolution of the dish is dropped big time when you do this. In the instance of a 6 footer, it becomes the resolution of a 4 footer or just under.

 

[sforgue]

Smith, P., & Bountykiller;
Thanks for the insight. Right now I've just got one LNB mounted in the middle, and 100 feet of RG6 running straight to the receiver. I initially had the DP34 in the loop, but I'm trying to simplify the installation and remove any potential bottlenecks.
I think that part of the problem right now is that the dish was installed with the hope that I would be able to get both 110 & 119 from it. The line of sight to the 110W slot is kinda-maybe obscured by a tree. The dish is optimised for 113 (the pointing angle for a D500 up here). I'm thinking that I might have to dig up the pole, move the whole thing over to 110, and just have two dishes getting the two slots. I'd rather have just one dish, but if I can't get signal...

 

[Smith P.]

Well, since you use true-focus ( prime focus) dish, drill a hole in a center and look into it removing LNBF - you'll see if three is the obstacle.