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Polar mount: Elevation calculation problem

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Ex Member
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Sep 22nd, 2008 at 12:40pm  
Hi

I've recently purchased a secondhand Channel Master 1.2m dish with polar mount and I need help in establishing the correct elevation. I've read your information pages and numerous threads, though I'm still having problems locating the satellites. The polar mount isn't a CM, but some other make/brand and there's very little I can change on the polar mount apart from azimuth and elevation (see below).

I've calculate the Main Polar axis angle as approx. 59/60 deg and according to the calculations, this should be 60deg. However, I'm unsure on how to interrupt the Polar mount dish offset tilt what does is actually mean?

The dish has a reflector offset of 22.6 deg.

The scale on the polar mount ranges from 50 to 70 degrees and to be quite honest the pointer is useless.

The main satellite I'm interested in is Astra though I've tried setting up the dish towards Thor 2/3 1W that has an elevation requirement of 20 deg, but again no luck!

I have a photo of the polar mount, though I'm unsure how to attach them to the post/thread.

Your help will be greatly appreciated,
Anthony

Based on my location (Stockholm) the set up should be as following:
Latitude: 59.4 deg
Longitude: 17.9 deg

Satellite orbit: 28.2E Astra 2A, 2B, 2C, 2D
Elevation: 22.0 deg
Azimuth (true): 168.0
Azimuth (magn.): 164.1
Polarisation tilt (deg): -6.04 (though for Astra satellites + 7 ) = 1.04

Polar mount main axis angle (deg): 60
Polar mount dish offset tilt (deg); 7.42
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« Last Edit: Jan 18th, 2019 at 6:55pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Sep 22nd, 2008 at 1:50pm  
Please send several images to eric@satsig.net by email and I will add them in here.

Amongst the images a full side view, while the mechanism is centralised would be helpful.

Amended:

These are the key adjustment angles for your polar mount:
Polar mount main axis angle (deg): 60
Polar mount dish offset tilt (deg); 7.42
... ...
The main axis angle is straightforward. It is the red angle I have marked in the image. At present it is set to 71 deg, so it needs some adjustment.

The small downward tilt of the dish is less clear as I can't quite see how to do it. There are two possibiities:

1. The small downward angle is to be achieved by setting the top bolt C slightly longer than bolt A. Does it look like this is the intended method ? Is there sufficient thread on the bolts to allow the top distance C to be increased a bit and bottom distance A to be decreased? If so, measure distance B in mm and multiply by tan(7.42). i.e. 0.13 x B mm. The result is the required difference in lengths of bolts A and C. Bolt C must be the longer. You need to get the gap to 7.42 deg. At present, the gap is more like 0 deg.
There is a similar explanation here: http://www.satsig.net/pol_ex1.htm

2. If the two bolts are not adjustable, consider part D. Is there some way this part may be lengthened ? For example is there a a slot or slots where it attached near the top of the axis. There may even be scale, range 0 to 10 deg on the slot. If there is a scale use it. If not, adjust till you find the due south satellite. Can you adjust so that the purple angle = 22.6 +7.4 = 30 deg ?

Note that the generally triangular shaped bracket arrangement behind the dish is probably intended to cancel out the beam offset (22.6 deg) so that the line along B is at right angles (90 deg) to the beam direction. Is the purple angle 22.6 deg ?  Do the elevation angle calculation for an imaginary satellite at 17.9E and you will be spot on.

Concentrate all your efforts on finding and peaking up on the satellite nearest to due south (I suggest Astra at 19.2E), with your motor mechanism almost centralised, with the motor pre-set just 1.2 deg to the left (while facing south). Use this page http://www.satsig.net/maps/satellite-tv-dish-pointing-uk-ireland.htm and drag the map across till you get to Stockholm.
...
wxw
Best regards, Eric.
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« Last Edit: Jan 18th, 2019 at 6:56pm by Admin1 »  
 
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Reply #2 - Sep 22nd, 2008 at 7:48pm  
Thanks Eric,

With respects to option/possibility:

1. I don't know whether there is enough thread on the bolt to achieve the 7.42 degrees and I'm not sure how easy it will be to replace that particular bolt, as that is the swivel point.

2. Unfortunately there's no way of adjusting the length of D, however, what about if I was to use longer bolts where the polar mount is bolted on to the back of the dish?
The length between the upper and lower bolts is 210mm, thus the gap required on the upper would need to be in the order of 27.3mm.  One idea is..if there is enough room... is to use 4 nuts on the long bolts (in between the polar mount plate and the dish back plate) to enable me to obtain the required angle and then allow me to tighten them up.

Do you think? How stable or risk will this be during windy conditions?

Regards,
Anthony


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Eric Johnston
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Reply #3 - Sep 22nd, 2008 at 10:16pm  
Both nuts must be fully threaded onto the bolts for security. The rose (spherical) joint bearings on the ends are intended to allow the axis to be altered slightly. There should be no need for new hardware. There must be some intended way of setting it.

The small downward adjustment angle varies over a range of about 0 to 9 deg, according to latitude.

Maybe the purple angle already incorporates some portion of the 9 deg. e.g. 22.6 + 4 deg. That would allow the length of the bolts to be adjusted +/- 15mm say.
What is the purple angle ? If the back of the dish is parallel with the front of the dish then 22.6 deg is a probablility as this would cancel the offset angle. In other words with the rear line B vertical the beam is horizontal.

Did any extra spacers or washers come with the antenna ?  There are 8 bolts involved attaching the dish to the plate then to the bracket. The upper pairs of bolts could have washers added on the underside (your idea above).

With the polar axis set correct the question is: can you now adjust the dish far enough forwards and downwards to see the Astra satellite?.

Do you have any instructions with the antenna ?

Best regards, Eric.
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« Last Edit: Jan 18th, 2019 at 6:57pm by Admin1 »  
 
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Reply #4 - Sep 23rd, 2008 at 11:37am  
Hi Eric,

No additional instructions or extra spacers/washers came with the antenna. As I mentioned, I brought it second hand.

I calculated the purple angle to be approx 30 degrees.

And with regards the rose (spherical) joint bearings, I could only lengthen the bolts by 15mm, giving me an angle of approx 6.3-6.5 degrees - slightly short of the 7.42.

I'm sending a new photo, with dimensions...
...

Anthony
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Reply #5 - Sep 23rd, 2008 at 4:34pm  
Assumes you have pre-set the polar axis first and operated the motor so it has stopped with the dish about 1.2 deg to the left of central. You need the actuator motored to half way and then use the clamp sleeve on the actuator body to set the dish 1.2 deg left of central. Swing the whole unit sideways on the pole in azimuth till you are peaked on Astra 19.2E. The head unit on the pole will now be accurately north-south.

With the purple angle already about 30 deg (i.e. 22.4 dish offset + 7.6 deg downward tilt) you will only need to make very small adjustment of the nuts to peak up the beam (in elevation) on Astra 19.2E.

Turn the underneath top nut slowly while reading the signal strength and peak up. The weight of the antenna will make it sag down and eliminate backlash. Then lock the other nut.

Best regards, Eric.
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« Last Edit: Jan 18th, 2019 at 6:57pm by Admin1 »  
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