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Offset Dish Elevation Angles

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Mar 10th, 2006 at 3:27pm  
Could anyone advise me please what the normal angle is between the dish and the LNB, i belive Eric mentioned its approx 22 degrees, is this the case for every type of offset dish (i'm in Europe).

Thanks

Martyn.
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Eric Johnston
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Reply #1 - Mar 10th, 2006 at 5:07pm  
The offset angle is the elevation angle of the beam if the front top and bottom edges of the face dish are vertical.

Dish designs vary and so do the offset angles.  All it means is that designer has put the focal point to the side of the beam - there are many options. The idea is to stop beam blockage and scatter from the feed and feed support struts.  The problem is that cross-polarisation quality goes down, particularly with shorter dish/diameter ratios (<= 0.6).  With longer feed/diameter ratios (0.8 ETSI) the cross pol is restored but you need a larger diameter feed horn window and the long arm is mechanically problematic and unsightly.

Here are some ideas:

Lay a thin, stiff strip such as a ruler, on top of the feed horn and under the lower edge of the dish, the slope of the strip will approximately represent the elevation angle towards the satellite.  Similarly if you position your eye under the rear edge of the dish and sight over the top of the feed horn that also will be the approx direction of the satellite.  All this assumes that the designer positioned the feed just outside the beam.   If the designer put the feed a bit closer in to the beam the above method is wrong by a few degrees.

Put a small mirror (preferably triangular) at the centre of the dish and a small bulb at the centre of the feed window.    View the bulb via the mirror and measure the reflection angle.  Offset = half the reflection angle.

Ask the manufacturer - and at the same time ask for reference measurements from the centre of the feed window to the top and bottom edges of the dish.  Measurements may alternatively be available for the top and bottom edges of the feed to the top and bottom edges of the dish.

Find an identifiable  satellite and align accurately.   Measure the angle of the front face of the dish using a plank of wood and an inclinometer or weight and string.  Calculate the elevation angle for your location.  https://www.satsig.net/sf.htm

If you have an antenna where the feed arm has come loose or the focal length is adjustable and unknown you are in big trouble.  Good luck.

Best regards, Eric.
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Reply #2 - Mar 11th, 2006 at 9:13am  
Eric

Thanks for the reply, very interesting, food for thought !!!!

I am living in Spain on the east coast 40.214 Lat 0.232 Long, we mainly use a 1.35m offset dish with a 0.3 LNB and the best STB we have in Europe for Sky TV which is a PACE 430DN.

The signal from Astra 2D is very weak in Spain but during the day we do get resonable signal strength and quality but in the early evening we tend to loose a few key channels on both H&V polarisation (the ones with the soaps on). Could this problem be due to "beam blockage" and would it be any advantage to install prime focus dishes.

Many thanks.

Martyn.
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Eric Johnston
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Reply #3 - Mar 11th, 2006 at 12:50pm  
The Astra 2D coverage is not intended for reception in Spain.  See https://www.ses-astra.com/corpSite/siteSections/ASTRAFleet/astra2d/

The satellite beam pointing towards the earth moves very slightly (perhaps +/- 0.05 deg) over the day. (ref https://www.st.northropgrumman.com/capabilities/SiteFiles/technicallibrary/aiaa_... ) For receive sites within the intended coverage area this slight movement has negligible effect as the beam contour levels vary little over +/- 0.05 deg.  However in the region outside the main beam the contour levels vary rapidly with angle. There are deep nulls.  See this example satellite beam cross section pattern:
https://envisat.esa.int/dataproducts/asar/aux-files/ephimg-1945529.gif ;  I've chosed it because it shows the deep nulls clearly.   Astra 2D will have detailed downlink contour patterns also.

If you are in the region outside the main beam you need a larger dish (perhaps 10 times or more the normal diameter) and need to be prepared for large variations in level with negligible movement of the satellite beam pointing.   Other sites perhaps 50km from you might have quite different downlink levels.

Note:
1.12 x normal diamter = +1 dB
1.4 x normal diameter = +3 dB (twice the area)
3.16 x normal diameter = +10 dB (ten times the area)
   10 x normal diameter = +20 dB. (hundred times the area)

Make sure your pointing is perfect.

Best regards, Eric.
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Reply #4 - Mar 11th, 2006 at 4:39pm  
Eric

Yes i am aware we are outside the main beam area, considering we have such a weak signal allignment is very key to the success of the installation.

One final point would prime focus dishes give any advantage in our scenario.

Many thanks for your replies.

Martyn.
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Eric Johnston
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Reply #5 - Mar 11th, 2006 at 4:58pm  
A prime focus, circular, axi-symmetric dish with the feed in the middle and beam blockage from the feed/LNB and support struts will give you no improvement, even if it is slightly larger than your existing dish.

A dish that is much larger (either axi- or offset) will give a worthwhile improvement.

You need some big improvement that comes from a much larger dish, not some trivial improvement from using an offset rather than axi- dish, both of the same diameter.

Best regards, Eric.
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