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pointing on IS 904 problem

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Posts: 10
Dec 13th, 2008 at 2:40pm  
I am in west Africa and i have to antenna pointing on IS 904 , so my azimut is 92° and the elevation is  7.35° , I have a 1.8 m antenna   with 22.3°  offset .

When i substract  7.35-22.3  i get  a negative  value .

How to point  it on IS 904

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USN - Retired
YaBB Moderator

Posts: 837
Kentucky (USA)
Reply #1 - Dec 13th, 2008 at 3:55pm  
See , and remember to consider magnetic deviation when arriving at your azimuth number.

But out of curiosity, I looked at the frequency plan versus footprint maps of IS904 on . Perhaps I missed something, but I don't see any active transponders associated with West Africa coverage.

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Eric Johnston
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Posts: 2109
Reply #2 - Dec 13th, 2008 at 4:51pm  
To set 7.35 deg beam elevation angle (1.8m dia, 22.3 deg offset dish).

Get a suitable weight on the end of a 3m long string and dangle it down the front of your dish from the top front edge. Tie the back end so it stays put.

First set your dish so the front face is vertical.  The string will just touch at the bottom.   The beam elevation angle is now 22.3 deg.  This is too high. You need to go down by an amount of 14.95 deg.

Get a steel measuring rule tape to measure the gap at the bottom edge between the dish and the string.  There will be no gap right now.

Tip the top of the dish forwards and the gap at the bottom will widen.  Adjust till the gap is 48 cm.  Your beam elevation angle is now 7.35 deg.  Note sin(14.95 deg) = 0.258 = opposite(measuring rule)/hypotenuse(186cm).  I assume the vertical diameter of your dish is 186 cm (my guess).  

Use Start, All Programs, Accessories, Calculator, Scientific Mode for other angles.

The above might work.  It is possible however that the dish is not designed to go as low as 7.35 deg beam elevation angle and the bottom of the dish may be hitting the pole.   In this case you might have to undo the dish from its steel work and turn the dish upside down so that the feed arm is at the top.  As above, adjust till the front of the dish is vertical.  The beam direction is now -22.3 deg elevation, i.e. down into the ground.   You need to increase the angle upwards by 22.3 + 7.35 deg so you need to tip the top of the dish backwards by an amount of 29.65 deg.

Best regards, Eric.
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