FC90P/SG2100 Observations

[wejones]

Just received an FC90P dish and SG2100 motor yesterday, and rather than rush to put the thing up, I decided to take some random measurements and do some experiments. I've started a web page, which I'll be adding to in the future, showing pictures of what I'm doing, ie:
http://eskerridge.com/bj/FC90-sg2100.html

However I thought I'd post a couple of the pictures here. First, I made some measurements to try to evaluate how accurate the latitude and elevation scales on the SG2100 motor are, and whether the motor shaft offset angle is actually 30 degrees as the specs say.
Well what I found is that first of all, the offset angle is indeed 30 degrees. Secondly, I found a flat surface on the motor, ie the surface just above the coax connectors, which is perpindicular to the rotation axis of the motor, where you can rest an inclinometer to verify settings. See below:


Note line "B" shows the rotation axis, line "A" is a crack in case perpindicular to this.

The above show a covenient surface for measuring angle perpindicular to the rotation axis.

Using this flat surface, I set up the motor elevation to correspond to a latitude setting of 44 deg and elevation of 46 degrees, then checked to see what the scales indicated. The elevation reading is shown below:

I've added a little line to the image to show where the proper angle seems to be indicated. The latitude scale seems to read at the same point, ie just forward of the little indicator point. But overall, the SG2100 motor angle scales seem to read fairly accurately, as others have suggested.

The forum software says I have too many images, so I'm going to continue this post in a second message dealing with the FC90P dish.

[wejones]

As a followup to the last post, this post deals with the FC90P dish.
I've been concerned about the dish angle scales on both the "U" version and "P" versions of these dishes, because Sadoun seems to include the same bracket with all dishes despite the fact that the different dishes have different offset angles. In fact, my FC90P came with a parts box apparently intended for an FC80 dish (the instruction sheet actuall said FC80, and no parts other than the dish and lnbf arm looked like what I had), which has a different offset angle than the FC90. So I did an experiment to measure the dish elevation reading when the dish was vertical (ie apparent aim of zero, which should give an elevation reading equal to the dishes offset angle). Well the results seemed to indicate that when vertical, the elevation scale would read approximately 23.5, which is closer to the 22.75 spec for the FC80 than the 24.62 spec for the FC90. Either that or they have a generic scale in between the two. But this would be responsible for some error in setting the dish elevation. I suspect that there is even more error when using the "U" version mount, but I don't have that one to test.
I then decided to try to estimate the actual offset angle of the dish, and also try to verify the focal point. To do this, I needed to measure both the actual aim of the dish and the apparent aim of the dish. To do this, I decided to aim the dish at the sun. I was able to measure the actual elevation of the sun by watching the shadow of a yard stick aimed at the sun, and I had an inclinometer resting on the yard stick.
Determination of when the dish was aimed at the sun was more difficult, however I decided to use 5 little stick on mirrors, which would reflect the sun to the lnbf. I decided to use a fake lnbf so as not to hurt my real lnbf. See one of the stick on mirrors, ie the little diamond shape thing in the picture:

Also see that I've positioned them up,down,righ,left,center, and installed a fake wood lnbf:

When aimed at the sun, I got the following results:

As you notice, there is a slight problem, in that all the spots don't focus at the same point. I managed to find a better focal point, where 4 of the 5 spots focused at one point, but that was about an inch above the actual LNBF position, and would require bending the lnbf arm, which I didn't want to do at this point.
However, using this focal point, I measured the apparent aim of the dish with an inclinometer at the center of the dish, and obtained an approximate offset angle of 25 degrees, which is pretty close to the published spec of 24.62.
Anyway, I'm not sure what good all this will be, but I think it will help give some confidence to initial angles that the dish and motor are set to. However it does introduce some doubt with respect to just how good the surface of these dishes really are.
Some of the discrepancy may possibly be due to the fact that my little mirrors are about 1/8" thick, and so the reflected surface is 1/8" above the actual surface of the dish. I may try to see if I can repeat the test with something thinner, like aluminum foil or something. I also had not yet tightened down all the bolts on the dish mount, so it's possible that the dish might flex a bit after tightening the bolts.
But anyway, I thought the experiments were interesting.

[pmb1010]

Interesting analysis.

Cute dog.

[wejones]

Originally Posted by pmb1010

Interesting analysis.

Cute dog.

Dog is one of our 3 collies. Other two are regular Lassie types, but this one is what's called a "white collie". I had never seen one before. She's still a puppy.

[satnutwillb]

I haven't had any experiences with offset ku installations,but to me it looks harder to setup than a prime focus big dish.

[rainman]

Originally Posted by satnutwillb

I haven't had any experiences with offset ku installations,but to me it looks harder to setup than a prime focus big dish.

not really harder just different.just remember to use the listed angles as a starting point and not the last word and you will be ok.

[Sadoun]

Bill, very nice analysis.

The focal point should be optimized at the Dipole pins inside the LNBF. If you take the plastic cap of a real LNBF, you will see the dipole towards the back of the LNBF.

Let us know the results once you tighten the bolts and use flat mirrors.

[wejones]

Just as an update on my experiments, I did manage to find some thinner mirrors (described in web page listed on first message of this thread). I put these thinner mirrors on my dish, and repeated the experiment. I still got 3 of the 5 mirrors focusing at different places. :-( see below:



I am bit confused that some of the squares gave bright reflections and some were dim, however it does seem that 2 mirrors reflected in the same place and 3 were off center.
I then moved the aime of the dish around, which moves all 5 spots around in different directions (really interesting, and anyone who thinks that you can put a 2nd lnbf on one of these dishes and get anywhere near a good focal point should try this), and to make a long story short, I found one spot where I got all 5 mirrors to focus on the same point! See below:


In the picture this appears to be a wider area, but the picture was a closeup, so it is really pretty good convergence.
I was hoping to do some experiments comparing these two focal points on an actual sat signal, but I ran into some other problems that I'm put in another message. At this point, I made an adapter for the lnbf holder, which holds the lnbf in the area where the off center focal point was found. I installed this, and connected the lnbf to my old lifetime receiver, and started searching for a suitable sat signal to test reception on, but I got messed up by an unrelated problem, and didn't finish the experiment, although I do seem to be getting relatively good reception using BOTH focal points.
Anyway, I'm going to continue with these experiments and try to compare the two focal points when I get time, but I thought it was interesting that it might be possible to improve the performance of these dishes by finding a different focal point for the lnbf.

Edit: Was having problems displaying images this morning. Sorry if you loaded this message while I was editing it.

[Sadoun]

Bill

It seems from your experiment and pictures that the focal point for the "sun light" is higher than the LNBF holder location. Could that be because you are using the light from the sun which is higher than the satellites and not over the equator? Thus the angles will all be different.

Remember, the dishes are designed for satellite signal with fixed satellite location (vs variable for the sun).

[wejones]

Originally Posted by Sadoun

Bill

It seems from your experiment and pictures that the focal point for the "sun light" is higher than the LNBF holder location. Could that be because you are using the light from the sun which is higher than the satellites and not over the equator? Thus the angles will all be different.

Remember, the dishes are designed for satellite signal with fixed satellite location (vs variable for the sun).

I don't think I explained well enough what I was doing. I was hand-aiming the dish at the sun, trying to get it to focus on the lnbf holder. No matter where I aimed it, I could not get all 5 mirrors to hit the same spot. My moving the dish, I could get any one of them hitting the spot, but not all 5.
However, by moving the dish, I WAS able to get all 5 to converge on that spot 2.5 inches away from the lnbf holder. If I tried moving the dish, to move that converged spot down and right, it would immediately break up into 5 separate spots.
So it's not that I had the dish mounted following satellite arcs, I was hand holding the dish, aiming at the sun. That point above and left of the lnbf holder appears to be the true focal point of the dish, it's just that the lnbf holder doesn't put the lnbf there.
It's possible that the lnbf arm is slightly bent, although I was careful with it. It does look like it isn't quite perpindicular to the dish surface. I'll have to measure it's position relative to each side of the dish to be sure.

[freeview]

Based on the comments I read from this thread and some of the experiments wejones has done, I became skeptical of the angle readout scale on the offset dish. For reference, I am installing a FortecStar FC90U.

I mounted a pole through the dish U-bolt mount and measured the angle using an adjustable set-square (a handy tool to measure angles like the protractor). If I make the pole parallel to the LNB arm, where it attaches to the back of the dish, I read about 20 degrees on the *dish's* readout scale. I then angled the pole to have a readout of Zero on the *dish's* scale. At this point my adjustable *set-square* reads 17 Degrees. Neither of these measurements seem to correlate with the 24.62 Degrees specified by the manufacturer. Does the LNB arm and holder angle make the offset angle verification not possible by simple means? Ideally I would like to use my adjustable set square, as it has 0.5 degree resolution -- but, for that to work, it is necessary to understand the offset angle concept and how the dish manufacturer has established the 0 degree mark on the readout scale

[wejones]

Originally Posted by freeview

Based on the comments I read from this thread and some of the experiments wejones has done, I became skeptical of the angle readout scale on the offset dish. For reference, I am installing a FortecStar FC90U.

I mounted a pole through the dish U-bolt mount and measured the angle using an adjustable set-square (a handy tool to measure angles like the protractor). If I make the pole parallel to the LNB arm, where it attaches to the back of the dish, I read about 20 degrees on the *dish's* readout scale. I then angled the pole to have a readout of Zero on the *dish's* scale. At this point my adjustable *set-square* reads 17 Degrees. Neither of these measurements seem to correlate with the 24.62 Degrees specified by the manufacturer. Does the LNB arm and holder angle make the offset angle verification not possible by simple means? Ideally I would like to use my adjustable set square, as it has 0.5 degree resolution -- but, for that to work, it is necessary to understand the offset angle concept and how the dish manufacturer has established the 0 degree mark on the readout scale

I did something similar to what you described above. There are a couple additional pictures in the URL I mentioned above, ie:
http://eskerridge.com/bj/FC90-sg2100.html . My dish came with a different mount, ie I got the pole mount version. My mount, however apparently came from an FC80, and was off a bit but was pretty close to being at the proper angle. My guess is that the U-bolt version is a generic mount, and is even further off. However I am also not convinced that any part of the dish mount is really appropriate for making these measurements. On my dish, the lnb arm is pretty close to being perpindicular to the "apparent" aim of the dish, so I was hoping that it would be a good place to make measurements to confirm the scale on the mount, but I've seen some dishes where the mount surfaces on the back of the dish are off center, so I'm not sure that the manufacturers really made any attempt to make these surfaces perpindicular to the apparent aim. WHen I measured the offset angle using the lnbf arm, I think I got 23.5 degrees, but when I estimated it using the mirrors, the angle was closer to the 24.6 specification for the dish. At least with the FC90P, it is close to being accurate, ie within a degree.
However, the setting of this dish elevation scale only needs to be approximate, because in all liklihood you'll end up changing it when you peak on a satellite anyway. When I set up my system, I set the angle on my motor mount, before I ever set it up, and have never touched it again. I also set up the dish elevation to match what I measured in my experiments here, however I quickly started changing this setting during alignment. However once I was aligned, I looked at where it had ended up, and it was within a degree of where it was supposed to be. So the FC90P seems to come with a mount that is relatively accurate, while apparently that isn't true for the FC90U version.

[freeview]

Originally Posted by wejones

I did something similar to what you described above. There are a couple additional pictures in the URL I mentioned above, ie: http://eskerridge.com/bj/FC90-sg2100.html .

Very interesting article. I am doing something similar to reduce the installation procedure to a methodolgy and pre-deterministic values as oppose to trial and error. I will be mounting the dish about 22 Feet high on the side of the house to get a clear shot of the southern sky -- trial and error should be avoided at all cost that high up on a ladder!

Originally Posted by wejones

My dish came with a different mount, ie I got the pole mount version. My mount, however apparently came from an FC80, and was off a bit but was pretty close to being at the proper angle. My guess is that the U-bolt version is a generic mount, and is even further off. .................. WHen I measured the offset angle using the lnbf arm, I think I got 23.5 degrees, but when I estimated it using the mirrors, the angle was closer to the 24.6 specification for the dish. At least with the FC90P, it is close to being accurate, ie within a degree.

From your methodolgy and findings, it seems possible to recalibrate the FC90U readout scale or any dish for that matter. If we can establish parallelism between the pole and the LNBF arm as 0 degrees, the readout scale should should give you the offset angle at this condition.

Originally Posted by wejones

However, the setting of this dish elevation scale only needs to be approximate, because in all liklihood you'll end up changing it when you peak on a satellite anyway. When I set up my system, I set the angle on my motor mount, before I ever set it up, and have never touched it again.

It is a good and easy practice to set the motor angle. I have used the set-square to set this angle, and the feeling of confidence is great!

Originally Posted by wejones

I also set up the dish elevation to match what I measured in my experiments here, however I quickly started changing this setting during alignment. However once I was aligned, I looked at where it had ended up, and it was within a degree of where it was supposed to be. So the FC90P seems to come with a mount that is relatively accurate, while apparently that isn't true for the FC90U version.

This statement is what makes me think it is possible to calibrate out the erroneous scale on the dish mount. If parallelism between the pole and the LNBF yields the offset angle specified by the manufacturer, you can then take the difference between the readout on the scale and that of the offset angle specification as the new readout calibration factor.

[Salukian]

This is the stuff that keeps me coming back to this forum. Interesting analysis and procedure WE. Here is a pic of my first motorized install of an FC-80cm dish. The elevation scale on the dish was off and gave me fits until I broke out the old digisat II plus sat finder. The correct setting for my location should be 40.6. As you can see from my sloppy ham-handed adjustments in freezing temperatures, it took a few tries to find the correct elevation. Since then, I have done a few more for family/friends and the scale was near perfect or only a short-hair off.