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VSAT technology and installation >> Dish pointing and alignment >> polar mount main axis angle
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Message started by voyager1 on Apr 21st, 2008 at 12:40am

Title: polar mount main axis angle
Post by voyager1 on Apr 21st, 2008 at 12:40am
Dear people of the satsig forum.
My name is Sebastian from the Netherlands and this is my polar mount.

(original image file http://www.jol-oudieplas.info/polarmount.jpg )


Normally I would ask the dutch sat4all board for help, but it seems all the  knowledge is gone. Most people use diseqc motors over here. The big plate on the back is a convertor plate I made. The polar mount is used  for a 1.2 echostar dish, but it has a  strange problem I think a have the wrong feed arms (to short) so a made this to fit my channelmaster. I believe if done it right, center of the dish is correct.

I’am trying to setup a 1.2 channel master offset dish, this is my first polar mount 36v motor setup. I can setup a diseqc motor in a few minutes but this thing I can’t figure out.

I have read your “Setting up a polar mount satellite dish - offset feed dish” But Iám afraid it’s over my head and I hope you can help.

I need to know my polar mount main axis angle. My declination angle I always figure out by moving the dish up and down, watching the signal and BER levels on a sat receiver. But if you can give me the right angle I will be most pleased ;-)

This is my data I have so far :

Latitude = 52.6297, Longitude = 4.8560
Lat    = 52 degrees,   37.8 minutes   North
Long = 4 degrees,   51.4 minutes   East
If you want to point a satellite TV dish at Satellite= 5.1 East orbit longitude
Dish elevation= 29.9, Azimuth= 180.9 (magnetic compass), Polarisation= -0.2

We use Sirius at 5 east to find the south its really handy.

Your program gives me the following data :
Satellite orbit 5.1
Site latitude 526297
Site longitude 4.8560
Dish Azimuth ( true north)  179.69
Dish Azimuth (magnetic north) 180.86
Disk elevation 29.86
Slant range 38626.92
Polarization till -0.19
Polar main axis angel 53.3
Polar mount dish offset till 6.86
Hope you can help me with my angle numbers.

Kind Regards Sebastian

Title: Re: polar mount main axis angle
Post by Eric Johnston on Apr 21st, 2008 at 9:35am

Main polar axis angle
Your main axis angle is about 60 deg.  This is too large. Reduce to 53.3 deg.

Small downward tilt (declination) angle
Achieved by making C larger than A.

Measure lengths A and B

Assuming the size of the L shaped metal brackets cancels the dish offset angle...
I see that A and C are presently the same, so your downward dish tilt is now zero deg.   You need to lengthen C or shorten A, so that C is longer than A

Note tan(6.86) = 0.12 = opposite/ adjacent

C = B* tan (tilt) + A
C = B * 0.12 + A

Work this out and then adjust distance C.  I think you need to do this in two places. Make the same adjustment on each side.

Any problems, come back here.

Best regards, Eric.

Title: Re: polar mount main axis angle
Post by voyager1 on Apr 22nd, 2008 at 12:35am
The first thing I did was to put a small metal plate on my main axis.

The only thing I could get my hands on was a geo triangle, the metal plate on the back of the geo showed my disk angle. It was something like 57 dec. I changed it back to 53.

Then I put the declination angel down till I got the best signal on my digisat pro. I know I have to measured  it but I wanted to try it out this way first.
The signal on Sirius was about 96% on my signal meter before, but now I only got 70% max.

I was confused so I took my topfield 5000 with me on the roof and looked at the Bit Error Rate. And it was good. 100%. I turned the disk manually using a commodore 64 joystick en turned it to  Turksat 1c @45 dec E. made a very small adjustment to the declination angle.  Turned it to hispasat @ 30 dec w and the quality of the signal was still 100%

Still I have the feeling I did something wrong. Did I put the 0 on the geo triangle at the correct location?

Why is the Digisat pro signal meter telling me the signal strength is down by 20%

All with All, it was a useful day, it a lot better now.

Thanks for helping me.

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