Technical info behind ku motor setup

[DruzeTito]

I finally took some time to look into the "technical" stuff of why we have to setup motors in a specific way. The setup is so simple and dumbed down that we never find out why things are done the way they are. Knowing the technical information behind it can save you hours of frustration of motor setup and arc tracking. http://www.geo-orbit.org is an awesome resource to use although most of the stuff highly leans toward c-band. I still have some unanswered questions and i will list them later.

Facts:

- An HH-motor (Horizon to Horizon) tracks satellites in the sky in an imaginary belt called the "Clarke Belt."

-Most television satellites on earth are 22,000 miles above the equator with varying distance between one and another. Since they are in an orderly fashion, the motor is designed to track them exactly based on what elevation you have set it to (also latitude setting) and if you have pointed it to your true south heading.

-When the motor moves, it moves azimuth, elevation, and skew all at once. This is why your lnb does not need adjusting but should be set to it's neutral position.

- When setting up the motor, there are three initial adjustments you must make: First is pointing your motor to your true south, then setting the elevation scale on the motor to your latitude (this is the elevation setting) and finally adjusting the declination angle on your dish.

- When fine tuning your dish for signal and quality, only adjust where the motor is pointing to (left.right) and the elevation setting on your dish (declination angle)

-The reason behind pointing your motor to true south is because this is used like a reference point for the motor. It is the position where your dish will aim at it's highest point on the belt.. this is called the "zenith".

- When adjusting the elevation on your dish in a motor setup, you are not really adjusting elevation (that has already been set on the motor) but you are adjusting the declination angle. To find out just what that means, read this very informative information here: http://www.geo-orbit.org/sizepgs/decchartp.html.

Looking at this image helps greatly to understand the meaning of these adjustments and the clarke belt.



As you can see, when we adjust the elevation of the dish (really the declination angle) we are kind of adjusting the "width" of how the motor will track the belt.

Please note that according to this image, we do not have to worry about elevation at all because it has been set on the motor scale to your latitude. What we have to worry about when peaking the signal is declination and north/south axis.

The importance of declination angle comes into play when the motor moves further east or west to lower elevations and higher skew angles. For example, when you have your motor pointing to the satellite closest to your true south, making declination adjustments to the dish have barely any effect to the signal/quality (given that you keep it in the reasonable range of your calculated value) because the basics have already been adjusted: elevation (on your motor) and azimuth (the pointing of your motor to true south).

Conversely, when the motor aims further east or west, if your declination angle setting is not right, the dish will aim into deep space somewhere and totally miss the satellite.

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Now I do have some interesting questions for people if they can answer. I find that, for my motor, SG2100, to "calculate" the declination setting of the dish, I must do (30 - declination angle of my latitude (found on the chart) ) .. my question is WHY 30?? Is this the average offset angle of a dish it is taking into account?

I ask about offset angle because, when I calculate it, not matter where you are, it goes from about 20 degrees to 29 degrees. Mine turns out about 25 degrees. I find that when my dish is at 25 degrees to it's scale it is a perfectly upright position at 0 degrees. The fortec 90 cm dish has about 24.62 degrees offset and the 80 cm is 22 degrees offset. This means that when either of them are pointed to this degree setting they are really aiming 0 degrees when measure with an angle finder.

So does declination angle set the dish to 0 degrees? Or does it calculate it to aim down about -4 degrees to not aim off into deep space? Because all declination angles are negative values on earth.

Or does this then all depend on how the motor is designed?

To calculate my declination angle, should I do (my dish's offset angle - my locations declination angle) to get a perfect setting?

This is what i am theorizing and thinking about. I haven't found any written info on this.

BTW... I hope new users find this info helpful.. if anyone finds any factual errors or bad wording in sentences please do tell!

[jackp]

Hello and thank you for asking the question.Good comment/you explaied correctly.Keep up the good work/so us newby can learn.I have been to that Geo site many time.Made 14 page copy.Now I have 2 drive motor(sg2100 and pm900)I am having a problem with my hotdish75 bracket.I remove the long bolt at the top/so the pm900 shaft can go in.Inside of the shaft,it don't line up.I have to used a smaller size bolt to go thru the shaft and bracket.Doing this made the elevation adjustment smaller/the shaft is hitting the boom arm top and bottom. Help

[DruzeTito]

it's quite difficult to visualize what your problem is because I have never seen or used a hot dish before. Mabybe sending pictures can help if you can.

[bobkat]

Damn Skippy !!!! Nice post. I'm making this baby a Sticky !!

kat

[DruzeTito]

thanks kat you are the greatest :-)

[DruzeTito]

I finally found the answers to my own questions after thinking about it for hours. =D>

The reason we have various motors having let's say 30-declination angle or X-declination angle is because 30 is the angle of the pole on the motor!!!

For the SG2100A's case, the pole on the motor is bent down precisely 30 degrees. Why is it bent? Well, if the pole was perfectly vertical then you wouldn't be able to set that slight negative declination angle because you cannot lower the dish less the around 5 degrees when it is on let's say a perfectly vertical pole!. So in a way, once the dish is on the motor, the dish's scale is off by -30 degrees!

So if you set the dish elevation on the scale to 30 when it's on the motor, then the real angle of the dish is 0, the perfect upright position. That is why in order to get that slight declination angle, you must do (0 - declination angle for your location) and it will turn out a negative number. Add 30 to that number and viola!! set your dish to that! Of course you know 30 - dec angle is much easier but I explain the opposite way so show why everything works out. It has nothing to do with the offset of the dish at all.

So in an example install with latitude of 28.0 N and 82.5 W , first let's take a look at some parameters for this location.

A real good site to calculate this stuff is www.satlex.de

Just input the satellite's longitude as your own longitude and you will get your area's declination angle and your highest elevation value!

- The declination angle for this location is -4.65
- The highest point, or the "zenith" of this location is 62 degrees, this does not include the declination angle. If we include the declination angle, it will be (62 - 4.65 = 57.35 ) This says that if there was a satellite on 82.5 W longitude that I needed to point to as a fixed dish, I would set the elevation on the dish's scale to 57.35. My elevation on a fixed dish would NEVER go higher than that because it is the zenith of my location or highest point.

So now, if I wanted to include a motor here is what the goal is:

The goal is to point the motor to my true south, adjust the motor's elevation bracket to my latitude, and finally adjust the declination angle on the dish to get the resultant pointing of the dish to be 57.35!

Here's how we get there: First we point to true south; Then I adjust the motor's bracket to my latitude to 28.0 N, on the other side of the scale I get 62 degrees! How does latitude translate into degrees? Here's how: 180-90-28= 62!! Dont ask me how i got this! It has something to do with the coordinate system.

Ok, so we have 62 deg, but we need a final value of 57.35 deg elevation.

So now we put the dish on. Since the motor's arm is bent down 30 degrees, we need to make the dish point to -4.65 for our declination. Add 30 degrees to -4.65 and you get 25.33. So set the dish to this value and we now finally have our 57.35 degrees elevation! Of course, if you take offset into account, the dish is pointing 24.62 degrees lower than the incoming signal at 57.35 degrees.. Assuming the dish offset is 24.62 degrees: (57.35 - 24.62 = 32.73) But the scale is adjusted to match the real incoming elevation.

I hope someone can understand this! :mozilla_smile:

[redman2]

Tito,

I read your "how to" post and I have some questions. I tried to install a sat. system: Winegard 76cm, Invacom LNB and GS120(SG2100) motor. My location: zip 92009 (USA/Calif - Lat 33.09 N/ Long 117.26 W) I used the declination table and got 5.38. Therefore , the dish angle should be 30-5.38 = 24.62 approx 25. The angle for the motor should be 33 (matches the lat.) Total = 25+33= 58. Here comes my problem: I cannot set the angle of the dish to 24. The lowest I can go is 30 as the motor mast hits one of the LNB installation screws. Maybe I didn't install the dish support right? And, even at those 30 deg. the mast of the motor is paralel with the LNB support bracket(at the back of the dish). Can't I make up the 58 deg using the motor's elevation bracket? Therefore, considering that the motor's pole is bent at 30 deg. I should set the motor elevation at 58-30=28 deg. Am I correct? Will this work?
Thanks in advance for your help!

[DruzeTito]

well I know the winegards are weird. I cannot imagine why anyone wouldnt make a offset ku dish go less than 30 degrees as there are plenty of satellites that need less than 30 degrees.

no i dont think you can set the motor elevation to compensate because the elevation must be set to your latitude or 57 degrees.

What the dish is doing is setting the declination to actually "hit" the satellites because they are relatively close to the earth, if no declination was set and the dish was set level with the motor's elevation it will miss the satellites and aim into deep space.

as i said, the SG2100's motor is bent down 30 degrees, because if it wasn't bent at all, it would be impossible to put the dish at a negative value to set the declination!

I'm not sure what other chances you got, but all i can think of now is maybe you should get a SMD312 motor, because it's arm is bent down even more: about 42.5 degrees i think. That just may cut it for your dish.

[redman2]

Thanks for such a quick answer! To be honest, I was hoping for a quick fix! Or, at least a motor kit available for this dish.Oh well... What I don't understand is how come this combination (Winegard 76 & SG2100/GS120 ) is such a popular package: there are many companies that sell it as a package (including the receiver also). By the way, what other sites do you you know that will show you how to install the dish? I am not sure I got the TRUE SOUTH correctly. This is how I did it: I aligned the North index line with the North magnetic needle and the 150 indication on my compass points to TRUE SOUTH. Is that correct? By the way, are the other calculations of mine (see previous message) correct? Thanks again!

[DruzeTito]

AH!! You live by San Diego! One of my favorite cities i've seen and someday i want to move there. You must be rich.. lol.

Well yes, most of your calculations are correct. Only don't add 25+33 to get 58. The declination setting of the dish is independent and in your case it conicidentally equals to the elevation setting of the motor.

It three simple steps: set the motor to your latitude, set your declination angle on dish to 25 and point the whole motor/dish by hand to 180-13= 167. Point it to 167 on your compass. However method you use, make sure it turns out like that.

Now as for your winegard not going under 30, there must be something wrong.. hmm. What is the lowest number that the dish's bracket says?

And you say "the motor mast hits one of the lnb installation screws" I am confused by this statement. The LNB is all the way in the front of the dish, how can it interfere?