Offset LNBs: Theory & Practice

[MJFlash]

Hi, Folks!

I'm in the process of setting up a second LNB on a 90cm dish, and thought that it would be worthwhile creating a thread that seeks to understand both the theory and practice behind adding additional LNBs to a dish. For folks that aren't familiar with this, the goal of doing so is to receive additional satellites with a single fixed dish. First of all, I'd like to express my thanks to "Anole" and "Smith, P" for getting me started on the theoretical aspects in this thread! Second, please understand that I'm no expert - I'm just posting as I try to learn all of this!

Taking a step beyond the approximations in the other thread, here's one which I believe might be even better. Let's use an example where I'm interested in receiving both 110 and 119 - I'll use these only because they're easy to aim at, since they have strong transponder signals. To begin with, I'll use a satellite calculator to figure out where I need to aim to hit either bird. Based on my location, I get:

119: Azimuth 174.9 Elevation 46.5
110: Azimuth 160.5 Elevation 44.8

We then need to figure out where to point the dish. Let's say that we decide to point the dish to 119, so that the dish has azimuth 174.9 and elevation 46.5. Now, where should the second LNB be placed to attempt to receive 110 from the same dish? My (revised) approximation to the proper position is:

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

My satellite dish is a Fortec Star 90cm, which has a focal length of 510mm. The offsets for me are therefore:

Horizontal: 510mm * sin(174.9 - 160.5 [degrees]) = 127mm
Vertical: 510mm * sin(46.5 - 44.8 [degrees]) = 15mm

[EDIT: We interrupt this post... to let you know that I originally had the sign bit flipped on the east/west correction! That's the bad news. The good news is that it was found by actually measuring optimal LNB placement, as documented later on in this thread. Numerically, the formula really works! The following paragraph has been corrected to fix the initial error.]

To translate these numbers, a positive horizontal offset means to move the new LNB to the west (i.e. left as you face the dish), while a negative horizontal offset means to move it to the east (right as you face the dish). For positive vertical offsets, move the new LNB upward, while a negative result means to move the LNB downward. So, in this case, I would need to place the 110 LNB 127mm (or 5 inches) to the left of the old LNB, and mount it 15mm (or ~0.6") higher.

Regardless of these guesses, I'd greatly appreciate your input, for I'm sure that better approximations will be possible. For example, these approximations all assume that the dish is spherical, even though it's actually parabolic. With a spherical dish, it may well be that moving the LNB one degree will change the target by one degree. With a parabolic dish, though, is this really the case? My gut tells me that the F/D ratio (the degree of curvature of the dish) might have something to do with a proper estimate. Any ideas as to how this might affect the results?

[EDIT2: It appears that the formula works fine, with no adjustment necessary for the F/D ratio, etc. It just works!]

What will make this most interesting will be to compare this theory to actual practice. Once I get around to actually installing my dish, I'll let you know how well this works out!

Cheers!

P.S. While some folks think that theory's a waste of time, it's worth comparing this prediction to the approximation listed in the other thread. In that case, the estimated location for the new LNB is found as:

Horizontal offset: focal_length * sin(new_orbital_position - old_orbital_position)
Vertical offset: horizontal_offset * tan(my_skew_angle)

Using the same example setup, the numbers would work out to:

Horizontal: 510mm * sin(9) = 80mm (~3.1 inches)
Vertical: 80mm * tan(6.7) = 9mm (~1/3 inch)

In the other thread, there's then some special casing to figure out which directions to move the LNBs. Once I install my dish, it'll be fascinating to see which method is closest!
​

 

[MJFlash]

Hi, Folks!

To be clear, please note that I keep saying "approximation"! No matter what you calculate, it's important to be able to tweak things for maximum signal, no matter what the numbers say. My goal with this thread is just to be able to get as close as possible to the correct position beforehand!

Have Fun!

 

[Anole]

In the other thread, we assumed one LNB was aiming at a weaker bird, and centered at the focus.
We then calculated the absolute distance the #2 LNB was to be located from the prime LNB.
This would be the length of a bracket used to attach the #2 LNB to the first.
And we disregarded the direction.

Then, we discussed which side of the primary LNB (east or west) to put the #2 LNB.

And finally, we discussed the angle of the bracket which held the #2 LNB to (or relative to) the #1 LNB.
This stands in for the skew.

Hopefully, all the calculations were done correctly and based on sound science.
I thought that presentation was easy to understand, measure, and fabricate.


There is no actual skew in the individual LNB when dealing with circular polarization, but it becomes a factor if you apply this idea to linear LNBs.

 

[MJFlash]

Hi, Anole!

It was very easy to understand and extremely helpful, and I thank you again for it! The assumptions that I'm making here are identical, except that I believe that the original calculations ignored the actual delta azimuth and elevation, which I use here. I may well be mistaken, but I'm guessing that using the user's actual deltas will yield a more accurate estimate. Time will tell! If only it'd stop raining here, I'd go on to the experimentation stage.

Cheers!

 

[MJFlash]

Hi, Anole!

To restate the difference in methods in a different way, the original approach used a fixed value of 9 degrees, which was the difference in orbital positions. However, for most folks and most pairs of satellites, the angular difference between the two satellites won't match the orbital spacing. For example, if I were much further south, the angular azimuth difference that the dish sees might be just 1 degree, not 9 degrees. Similarly, at that position, the delta elevation would be much higher. That's why I use the actual delta azimuths and elevations for this computation.

Have Fun!

 

[MJFlash]

Hi, Anole!

I just thought of a good thought experiment that'll prove that this is a more accurate method. Imagine that you are on the equator itself. The original technique would still recommend a horizontal offset of focal_length*sin(9), but the right answer is 0, which this approach will yield.

Best Regards!

 

[Anole]

Your argument escapes me.
I will re-read your comments in other threads in case the clue is there.

I'll agree that the satellite spacing (in this case) is 9º , if viewed from the core of the earth.
So, from the surface, it may not be quite the same.
However, I suspect the difference is insufficient to argue over.

Somehow, I imagine your point is something else, and will look forward to better understanding.

 

[RVD420]

I have just done the point the dish at one bird, then with a sat meter hold the LNB with my hand and move it around until I hit the sweet spot. Then zip ties and scrap angle iron comes in handy.

 

[MJFlash]

Hi, Anole!

I don't know if this will help or not, but the formulas from the other thread will give perfect results if you are at the North pole (since the azimuth angle will exactly match the orbital spacing), but will yield increasing error as you head south, with a worst-case error at the equator, and the difference can be very significant. The approach that I've outlined here has no such error.

I'll try hard to think of another way of communicating this more effectively. My apologies!

 

[Wescopc]

I agree that the angle will be different from the satellite spacing. Here in Oregon AMC 4 (101°W) and IA 5 (now G-25 at 97°W) site in at 132.9°M and 128.0°M - a difference of almost 5 degrees vs 4 degrees difference in orbital position.
Bob

 

[MJFlash]

Hi, Bob!

Exactly! In fact, in the example I gave for 110 and 119 from my home, the azimuth angle is a whopping (174.9 - 160.5) = 14.4 degrees, and from Acapulco, it'd be 18.5 degrees, even though the satellites are "only 9 degrees apart". The error keeps getting bigger as you go further south - particularly when the satellites are near your longitude.

Have Fun!

 

[Anole]

However, for most folks and most pairs of satellites, the angular difference between the two satellites won't match the orbital spacing. For example, if I were much further south, the angular azimuth difference that the dish sees might be just 1 degree, not 9 degrees. Similarly, at that position, the delta elevation would be much higher.

I believe the azimuth would be higher, now lower, and the delta elevation would be less, not higher.

I'll agree that the satellite spacing (in this case) is 9º , if viewed from the core of the earth.
So, from the surface, it may not be quite the same.

...in other words, the center of the Earth.
Of course, the north pole would work, too.

the formulas from the other thread will give perfect results if you are at the North pole (since the azimuth angle will exactly match the orbital spacing), but will yield increasing error as you head south, with a worst-case error at the equator,

I guess for reasonable values in the USA, I was assuming trivial error.
Of course, the original formula was based on an Alaskan location (closer to the North pole).

Regardless of any error with either approach, the idea is to decide roughly where a second LNB needs to be located relative to the first.
Once the general answer is known, the user can "tune for maximum smoke" and find the sweet spot.

Further, for better performance on any given dish, it would actually be wiser to offset both LNBs half the distance (in opposite directions), and at the skew angle, to achieve best results.
A good example is the GeoSatPro LNB holder, which moves each LNB +2º or -2º to achieve 4º satellite spacing.
This assumes a dish you cannot skew, of course, as many larger ones are.

 

[MJFlash]

Hi, Anole!

Let's use your example where I aim the dish halfway between the satellites. Only this time, we'll go after 119 and 129. As a result, we'll aim at a hypothetical satellite at the "124 orbital location", splitting the 10 degree difference. For my location (zip code 94306, in what is actually central California, though we call it Northern California out here), we'd aim the dish at:

124: Azimuth 183.1, Elevation: 46.6

The sat calculator tells us that the following are the proper aiming points for the 119 and 129 birds here:

119: Azimuth 174.9, Elevation: 46.5
129: Azimuth 191.2, Elevation 46.0

Now, let's make a bracket for our new LNB locations. Since we're splitting the difference, the bracket will have a hole in the middle, and then holes for the +/- 5 degree orbital locations. Using the original method at my location, it'd predict that the two LNB holes should be located at horizontal offsets of:

510mm * +/-sin(5) = +44.4mm, -44.4mm

So, we make up a new bracket, where the LNB holes are 88.8mm apart.

With the new method, the holes would be located at:

119: 510mm * sin(183.1 - 174.9) = +72.7mm
129: 510mm * sin(183.1 - 191.2) = -71.9mm

In this case, the LNB holes would be 144.6mm apart.

We both agree that we should always test before making anything. However, especially if your goal is to espouse "sound science", why would you intentionally push for an estimate that yields, in this case, 63% error, when you know that there is a simple way of getting a much more accurate answer?

 

[Anole]

Sounds good.
Build it, and they will come.
Would be nice to get some experimental results to confirm.
Hope the weather clears up there , soon.

edit: after cleaning up the numbers above, you have made a much more persuasive case.
any sort of theory deserves a good beating to see how it stands up.

post edit: and for birds closer to your own longitude, the angular distance between them is more than if they are off at your horizon.
meaning, if you design for two birds directly south of you, and re-aim at two with the same degree separation but at the horizon, you won't be optimized.
for me, 110/119 are 16º apart, but 82/91 are only 10º apart, requiring different adjustments of the LNB locations.

 

[MJFlash]

Hi, Anole!

Thanks for the very nice post! Unfortunately, it's raining again, and we're scheduled for three more days of rain before we get a break. I'm tired enough of being held back, though, that I may just risk dragging out the FTA receiver and the TV today if we get a short break. I may not get the chance to actually mount one of my two new dishes, but I'd at least like to try some experiments with offset LNBs on my existing 80 cm steerable dish.

I really did go a little nuts on my dish setup. I started in FTA just about 2 weeks ago, and I'm already upgrading my motor and my steerable dish from an SG2100 on an 80 cm dish to an HH120 on a 100cm dish, plus increasing the size of 3 out of the 4 dishes I have installed. I think I'm hooked!

I'll post the results as soon as I get a chance to test this out.

Have Fun!
Mark

 

[dem0nlord]

This has been one of my peeves for a while, the advertisement of things like LNB holders with a specified degree spacing, or making statements like the T90 dish can see X degrees of the arc. Anyone that actually sits down and thinks about this has to realize that these are nothing but generalizations with large possibilities of error, depending on geographic location. Good to see someone else thinking this through MJFlash. :up

 

[MJFlash]

Eureka! It Really Works!!!

Hi, Folks!

Yippee! It really works! There's just one small caveat: I had the sign bit wrong on the left/right correction. How I tested this was to rest the LNB on an old speaker with a varying pile of books on it, and placed this next to my motorized 80cm Fortec Star's LNB. I then moved things around until I got a good signal. At that point, I'd go inside and see if my Viewsat Ultra could identify the satellite. Finally, I measured the approximate position of the LNB. The measurements aren't super-accurate, since I had to eyeball them with a tape measure, but they're close.

With the motorized dish pointed at 82, I moved around the LNB until I found the 91 bird at ~3-1/4" horizontal offset, and ~2" vertical offset. Note that the correction for this was down and to the right (not down and to the left, as I originally wrote in the first post). With the Fortec Star 80, which has a focal length of 492mm, the calculated position for the LNB is a horizontal offset of 80mm, and a vertical offset of 51mm. Having measured the equivalent of 84mm x 51mm, this is definitely spot-on, within the limits of my measuring capability!

I then found another satellite that was 3" to the left, and up about 2-1/2", but my Viewsat couldn't identify it, even though I had a very strong signal. I had to come back inside to figure out what it must have been. I'm reasonably sure that it must have been DirecTV-5 at 71.5 degrees. The theory tells us that this should be at an offset of 75mm horizontally and 60mm vertically. Having measured the equivalent of 76mm x 64mm, this formula is really, really close, if not dead-accurate.

So, with the exception of blowing the left/right correction sign bit, this looks like a very accurate estimator for optimal LNB placement!

I'll go ahead and modify the first post to correct the left/right sign (making it very clear that I had it backwards initially!), and I'm declaring this ready for use!

Hallelujah! Thanks again for your feedback and advice, folks, with special thanks to Anole and "Smith, P" for getting me started on this!

Cheers!
Mark

 

[MJFlash]

HI, Dem0nLord!

Thank you very much! I appreciate the kudos!

Best Wishes!
Mark

 

[MJFlash]

Hi, Gang!

As an interesting observation, when I say left/right or up/down motion, that's exactly what I mean. In the tests above, my motorized dish was practically laying on its side due to the very low elevation of the 82 degree satellite here. No matter which way your dish is oriented, the LNB offsets are plain left/right and up/down. As Anole pointed out earlier, you'll need to remember to rotate the LNB to match the proper skew if you use this technique for linear LNBs, but the offset distances will still be identical.

Have Fun!
Mark

 

[Anole]

wrap up

just two things:

I like to use east/west instead of left/right, as noted in the Alaska thread, because you never know if the guy you are talking to is in front of or behind the dish.
But, he always knows which LNB is east or west of the other.
The up/down description is usually easy to convey.

Now that you've got a better estimation technique, you might want to return to the Alaska thread, mention this thread, and recalculate the LNB distance for the guy.
(and maybe for the Mexico thread, too)