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Polar Mount in Burgundy, France

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Feb 24th, 2008 at 8:23pm  
Dear All

I'm a newbie to forums and to satellite TV, so apologies in advance if i make a few gaffes (as they say locally) with technical terms and lack of attempts at witty vernacular.

My name is Toby, I live in Burgundy, France, and i bought a satellite kit to receive digital and analog signals from a manufacturer called Metronic, using a motorised mount.

Despite the clarity and absence of jargon, i am too much of a f@€k-wit to get the kit working.  Hence this request for help.

My longtitude is 4.5165, lat is 48.4623, which gives me the following: "Polar mounts only: Main angle= 47.2, Downward tilt= 6.3, Motor drive sideways angle= 9.5" (using the fancy algorithms and Eric's bizarrely translated page at http://www.satsig.net/maps/pointage-plat-television-satellites-france.htm)
My set up looks very similar to the picture in reply 6 on the discussion entitled "Pointing dish with motor".

Now, despite my incompetence, my mounting pole is vertical, it is solid and i have been fiddling with mount angles and the receiver's settings for many hours now.

I am not sure what part of my installation is faulty, the wiring seems good - the motor is turned by the receiver, there used to be a signal that i received before a worker twisted the mount two months ago, and i have managed to pick up a Lebanese music channel from one satellite (i know not which) in the last day or so, whilst fiddling.

I have also had problems in reconciling the mounting angles quoted above with the data given in the manufacturer's guide booklet (http://www.metronic.com/pdf/notices/450907_UK.pdf) ; My main angle seems to be a good five degrees different between the fancy algorithms on this site and the guide book.  Also, downward tilt, seems to produce an upward tilt when looking at the profile of the mounted dish.

I can produce any number of pictures on request, but given the long-winded nature of this request, it seems better if i end now, with gratitude in anticipation of any replies.

Regards
Toby.
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Reply #1 - Feb 25th, 2008 at 12:07pm  
This image below shows what is the polar mount main axis angle M
...
The polar axis is the green sloping line through the motor bearing, at right angles to the joint ring.  The polar axis line points approximately towards the north pole star.

Using my bizarrely translated page at Parabolique antenne pointage a la France please check your lat and long again.  Select Hotbird at 13 east, for the sideways motor angle.

The cranked arm has a minus 30 deg bend in it.   If the dish bracket has a scale it may read 20, 30, 40.  If this scale exists then it refers to the beam elevation angle when the dish as being used without a motor and directly attached to a vertical pole.  If there is a dish scale set it to 23.7 deg. (i.e. 30 - 6.3 = 23.7).  If no dish scale do as per the instructions and change the angle in 2 deg steps and sweep across each time.  I like the bit where is says it is normal to have to do up to 20 sweeps across the sky to find anything!

Remember the following:  
Pre-setting the main polar axis angle and the small downward dish angle is critical.
With the motor central, you turn the entire assembly on the pole to point exactly south.  You will then be pointed exactly at an imaginary satellite due south of you, at the top of the orbital arc across the sky.
The Metronic guide is clever to point out that you can pre-set the motor sideways by a calculated amount and then turn the entire assembly (on the pole) to line up on that particular satellite (e.g. Hotbird at 13E)  Make sure your receiver is tuned to some known Hotbird carrier.

If you have an inclinometer you may be able to apply it to set up the polar axis angle.  You could remove the dish and apply the inclinometer along the cranked tube.  Allowing for the 30 deg bend.  You may or may not find the result agrees with the "Elevation" and "Latitude" scales on the motor bracket.  Don't worry, my figures will work OK.

If you are any good at French any suggested improvements to my France page would be appreciated.

You are welcome to send a high resolution image similar to the image above, so I can read your scales. Send to eric@satsig.net

Best regards, Eric.
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« Last Edit: Feb 25th, 2008 at 3:12pm by Admin1 »  
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Reply #2 - Feb 25th, 2008 at 1:17pm  
I don't know this specific system either. But when mechanically tracking several satellites across a known orbital arc, shouldn't the initial Az/El/Pol angles be set for the center of the arc? The arc itself is a constant, and the tracking geometry of the dish is a constant. The only variable is where the dish is located.

So it seems to me you'd want to establish the center of both arcs, based upon where you live. You said longtitude is 4.5165, lat is 48.4623. Excellent. So due south of you over the equator is  4.5165 lon and 0.0 lat. I'd want to calculate  look angles based upon an imaginary satellite orbiting due south of you at 4.5165 lon (true). Fortunately, magnetic deviation for your area is only 1.25 degrees, so these coarse settings should get us in the ballpark.

Then determine which satellite closest to this point from which you actually want a signal. Let the hardware turn the dish to that point in the sky. Fine tune Az/El/Pol for signal strength and polarity, you should be done. From that point the motor will change the Az for you, and the mechanical geometry of the mount should change the El and Pol accordingly.

Eric's already addressed the hard part, which is to interpret polar offset relative to indicated pointing angles. Once that's clear in your mind, the rest should fall in place.

//greg//
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Reply #3 - Feb 25th, 2008 at 6:48pm  
Gentlemen, you are both.  And scholars.  It's been a while since i studied trig, and it was tough getting through the various technical parts of the explanations i found around your site.

Eric seems to have just uncovered my most serious error - which was with the angle for the cranked arm - somewhere i misinterpreted the guides and assumed it was cranked at 40 and not at 30.  there is a scale on the dish bracket - and once i have made another adjustment, i will take some highres pictures and email them.

My french is good-ish, but only up to the point at criticising the efforts of a computerised translator, not actually doing the translation myself.  I will ask French wifey to have a go...

Am off on a short trip at the moment, but by mid-week, i'll be back home with the bit between my teeth and ready to get my gazillion tv channels.

Thanks again.  Very clear and erudite responses.
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Reply #4 - Feb 28th, 2008 at 3:04pm  
...
Image above shows your complete system now.  I have measured the angles from the image.
It shows polar axis main angle =  53 deg.  This is wrong, decrease to 47.2 deg.  Take the reading on the 'ELEVATION" scale and move by the difference 5.8 deg.  Is now reading 38 deg so change to read 43.8 deg.  I've no idea why they call that scale ELEVATION since that would only make sense with a straight rather than bent pipe and an axisymmetric rather than offset fed dish.  Anyway it appears that ELEVATION might = 90 minus the polar axis main angle.  Try moving the scale to the extreme ends 0 and 90 and see it that makes sense.  If so, use the scale set to 42.8   In this case make sure the bracket it attached to your vertical pole accurately.  Getting the bracket accurate to better than 0.5 deg is not easy.  A spirit level might help.

The dish scale shows a reading = 25 deg.  This is wrong.  Set it to 23.7 deg.  Note the scale has tick marks at 4 deg intervals.

...
Image above shows your dish scale, which would set the beam elevation angle if the pole were vertical.  Your pole is bent 30 deg so to get -6.3 downward tilt you need to set to 23.7 deg.

I may seem an overkill to labour the point of getting the above angles so accurate, but it is only if they are accurately set will you ever get to see all the possible satellites.

If you can get the main axis angle and the small downward tilt set right, the next stage is to align the motor due south.  You can point to an imaginary satellite due south or pre-set the motor sideways and line up on a satellite.  It is not easy as the scales are so crude.

It is quite normal for first time installers to take several weeks getting it right.

Best regards, Eric.
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« Last Edit: Feb 28th, 2008 at 7:21pm by Eric Johnston »  
 
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