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VSAT technology and installation >> Dish pointing and alignment >> changing longitude

Message started by eyeonyou on Sep 15th, 2011 at 1:48pm

Title: changing longitude
Post by eyeonyou on Sep 15th, 2011 at 1:48pm
I have a Technomate 6800 satellite receiver.
The problem I have is my longitude is -0.1636, but  I have to set my longitude to at least 1.2 east.
I cannot receive satellites with a good signal at my correct longitude.
I alsoo cannot receive any satellites west of Hotbird at 13 east.
If I try Hotbird at my correct longitude the signal is weak,at 1.2 east it comes in strong.
I used to be able to get satellites upto 5 degrees west.
There are no obstacles blocking the path of the signal.
Could my dish be out of alignment?
I would be grateful for any suggestions to try and rectify things.

Title: Re: changing longitude
Post by Eric Johnston on Sep 15th, 2011 at 4:38pm
Yes, it is possible that your dish is mispointed.

Do you have a steerable motor powered dish on polar mount ?

If yes, is it possibe that you could send me pictures, to ?  

What is the latitude and longitude of your site location ?

What is the name (and orbit position) of the satellite that you can receive successfully ?

Best regards, Eric.

Title: Re: changing longitude
Post by USN - Retired on Sep 15th, 2011 at 4:43pm
It's difficult to say without knowing your latitude, but it could be as simple matter of magnetic deviation.


Title: Re: changing longitude
Post by eyeonyou on Sep 15th, 2011 at 6:03pm
I have a one metre motorised polar mount dish.
My longitude is -0.1636 degrees west and my latitude is 52.9517 degrees north.
I can receive  turksat at 42 degrees east,Hellas sat at 39 degrees east ,Eutelsat w4 at 36 degrees east,Eurobird 3 at 33 degrees east,Astra 2A at 28 east, Badr/Arabsat at 26 east,Astra 1D at 23.5 east, Eutelsat W6 at 21.6 east Astra 1E-H at 19.2 east, Eutelsat W2 at 16 east and Hotbird at 13 east.
After Hotbird I can't receive a ny satellites, though in the past I could pick up satellites upto Atlantic Bird 3 at 5 degrees west.
I will try and send photos of my dish.
Thanks for your advice.

Title: Re: changing longitude
Post by Eric Johnston on Sep 15th, 2011 at 7:28pm
Just one thought.  Can your dish see low down in the sky towards the south west or is there an obstruction ?

It is likely that your dish is misaligned slightly.

This diagram shows a view of the sky, facing south. The red line and red dots represent satellites. The vertical red line is exactly due south of your longitude and this should be the directlion your dish is pointing when the motor is centralised.

The green line shows the probable path of your dish.  Two things are wrong. Your centre position is slightly to the right (west), maybe 1 or 2 deg off. The overall width of your curve is also too wide, this means the polar axis is slightly wrong.

Before contemplating and  making any adjustments note that setting up a polar mount is highly skilled job.  You only need to make a small random change and you may find yourself spending several days getting back to the starting position, if at all.  Be warned !!.

Your dish pointing angles are here:

Ideally you need to align your dish, initially using accurate scale measurements, and then by pointing to the satellite nearest to your own longitude to peak up your pointing with the motor almost centralised. Getting the whole mechanism accurately aligned to due south is difficult.

The nearest satellite you mention is Hotbird at 13 east (which is rather far away for this purpose).  Line up on 13E exactly using the motor.  Can you now read or measure the sideways angle movement of the motor ?  It should read 14.5 deg.  If it does not read 14.5 deg you need to drive the motor until it does read 14.5 deg exactly. You will no longer be pointed at Hotbird 13 east.  Now loosen the mount on the pole (or otherwise rotate the pole) and turn the entire mount and antenna manually till Hotbird at 13 east is again pointed accurately.  This procedure should set your antenna pointed exactly south when the motor is driven back to the central position.

The above is only one step in the alignment process.  If you have any doubts about making adjustments do call a local satellite TV installer with polar mount expertise, to help you.  It is remarkably easy to mess it up and never get it right.

There are two further settings, the main motor axis angle (53.6) and the small downwards tilt (-6.9) of the dish. These two angles combined make up the  beam elevation angle for an imaginary satellite which due south of you.
Best regards, Eric.

Title: Re: changing longitude
Post by eyeonyou on Sep 16th, 2011 at 9:53am
Dear Eric,

Thank you for your very detailed, informative and expert advice.
I think I should, as you suggested, contact someone who is more skilled than I am in these matters to do the work for me.
I am very pleased for your help and expertise.

Best Regards,


Title: Re: changing longitude
Post by USN - Retired on Sep 16th, 2011 at 7:28pm
Just for the record, magnetic deviation in your location - the error induced in a compass by local magnetic field - is currently estimated at roughly "1.67 west"  (see ). So even though a calculation may produce an azimuth of ~163.7 degrees to HB13 for example, a compass would actually read ~165.4 degrees when pointing at the same spot. And magnetic deviation for any given location changes over time.  So - in addition to Eric's advice -  it wouldn't hurt to determine if the dish controller needs to be provided with  your current magnetic deviation to optimize pointing accuracy.


Title: Re: changing longitude
Post by Forum Admin on Sep 17th, 2011 at 9:48pm
This is a picture of the polar mounted dish:

It is likely that the view of the orbit is obstructed in a westerly direction by the house.

For a polar mount to see the whole range of the geo orbit above the horizon it is necessary in this case to have clear view of the sky to the south at an elevation of 30 deg, and almost down to the horizon in approx south east and south west directions (actually true bearings of 117 and 247, both with elevations of 9 deg for satellites from 60 deg east to 60 deg west orbit longitude positions).

The direction of the beam may be assessed by putting your eye just behind the lower edge of the dish beside the rear end of the feed support strut and sighting across the top of the feed horn window (red line). That is the approximate direction of the lower edge of the cylindrical beam from the antenna.

Best regards, Eric.

Title: Re: changing longitude
Post by eyeonyou on Sep 20th, 2011 at 3:17pm
Thank you for your advice and the photo showing the correct position of my satellite dish.
It is most appreciated and I can now receive satellites upto 5 degrees west.

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