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two focal points KU band 5.6 m

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Posts: 18
Aug 13th, 2013 at 9:37pm  
I have a 5,6 m Ku band antenna and have the following problem.

I do have two focal points EL and Az on this antenna.
When I set my sub reflector perfect on the AZ pattern then my EL pattern is very bad  no nulls at all. If I set the EL perfect deep nulls then the AZ is very bad.  When I set the sub in the middle of both the I got nulls  2-3 db below the side lobs.
What is the problem here.
It is a build together antenna.  We even put a theodolite in and get it as good as possible.

Hope that soem one can help me out.

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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Aug 14th, 2013 at 4:23am  
Could be "saddle" distortion:  When the dish is tilted to point at a satellite the top and bottom distort forwards and the sides distort backwards. With the dish at its operational elevation, check using crossed tight strings, sideways and up/down across the front. Measure the gap.  The strings should just touch in the middle.

If that is the problem please say what rib / panel adjustment facilities you have.

Best regards, Eric
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USN - Retired
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Posts: 837
Kentucky (USA)
Reply #2 - Aug 14th, 2013 at 5:48pm  
You didn't specify if this was a tracking dish or a fixed dish. Might be that the sub-reflector has adjustments to be set during dish assembly, adjustments that are to subsequently remain a constant. That way you only have to peak using main reflector Az/El adjustments. Might not hurt to refer to the installation manual regarding (semi-)permanent sub-reflector operating position(s)

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Reply #3 - Aug 14th, 2013 at 6:47pm  
Here is how to adjust the sub-reflector

But, this does assume that you can first get the dish in focus (feed to sub-reflector distance) with good nulls for both az and el simultaneously.

I've seen this problem (no nulls on one axis or the other) on a very "floppy" 4.5m dish. It is a real pain if you have a dish with no adjustments possible. 
Figure shows the height of the rib tips. Typically 12 measurements around the rim.  Should be all level. Actual heights, once dish is tilted to operational elevation, vary a few mm up and down, with the top and bottom coming up a bit, and sides going backwards.
It is not helped if the four sub reflector supports form an X. Putting stresses in the struts is then no use as the attachment points are all neutral, zero error points.

Cure is to adjust all ribs at the operating angle (very difficult to operate the theodolite and to make the adjustments) or return the dish to zenith and pre-adjust all the ribs the wrong way.

Do a crossed strings type test at the operational angle and then, with the dish vertical, make all the adjustments, gently, based on calculated estimates.  All assumes you have rib adjusters in the first place !!

Best regards, Eric
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