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Re: Prodelin feed assembly isolation problem

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Eric Johnston
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Mar 23rd, 2009 at 11:59am  
Send the pictures to me eric@satsig.net and I will add them to your message above.  

Please take several pictures of the feed assembly both close in and further away.

Please also take a picture of the complete antenna from a side view with the camera exactly in line with front side edges of the dish.  From the image it is possible to measure the position of the feed relative to the dish.  The Prodelin ABCD measurements may be checked here: http://www.gdsatcom.com/technicaldocuments.php#feed Click to get the relevent TDNNNN drawing with ABCD measurements.

I know it is laughable to use the word "delicate" in respect of heavy 3.8m antenna parts, but it is really is critical that the assembly is done "delicately" and in exact compliance with the instructions regarding the precision spacer  parts on the rear support frame.
See: http://www.gdsatcom.com/Prodelin/Technical_Docs/assembly_manuals/4096-535J.pdf
The panels must all match up perfectly with no stress or bending at all..  Easier said than done, but it must be ...

I know of one 3.8m instance where they left out the spacers or put them in the wrong places and the result was very low gain - never got as far as cross-pol testing.  There were actually 1/8 inch V gaps in the panel joints which had been filled with mastic to hide them !  Simply loosening everything, cleaning out the cracks, putting in the frame spacers correct and de-stressing it all and gently and gradually retightening fixed it perfectly - no permanent damage.  At least 6 months wasted however plus multiple visits to site.

Circular pol feed assembly manual http://www.gdsatcom.com/Prodelin/Technical_Docs/assembly_manuals/4096-452.pdf

Best regards, Eric.
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« Last Edit: May 27th, 2009 at 11:15am by Eric Johnston »  
 
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ChrisRust
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Reply #1 - May 8th, 2009 at 12:14pm  
Hi, I am Chris from GDSatcom in Europe. We are the manufacturer of Prodelin antennas. I am sure I can help you with that one.

Firts, pls check if the antenna was ordered in the F1 Intelsat type approved configuration offering 27 dB, if you ordered the standard version, you get exactly  17.7 dB isolation.

Should you need any further information, pls send a message to prodelineurope@gdsatcom.com

BTW: There are no issues with the new Prodelin design (which is actually Model 1385 not 1383)

Thanks,

Chris
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Eric Johnston
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Reply #2 - May 26th, 2009 at 7:21pm  
I'm not quite sure what has happened to the original problem message above.  It is sometimes visible, sometimes not.  This is what is said:
Quote:
I am installing what seems as a newly designed Prodelin 1383 model antenna. It is TX/RX with a circular polarizer. I lined up with Intelsat CMC but could not get the isolation past 17 dBs from the RHCP to the LHCP. Intelsat claims it seems to be a problem with the new prodelin design of the 3.8 meters antenna. More specifically they cited the focal point. I wanted to know if anybody else has had this problem and how did they resolve it. Intelsat won't let me access the satellite without a minimum isolation of 27 dB and more important I don't seem to get the proper C/N out of the incoming signal. I wanted to add a picture of the feed assembly but don't see how to do that in here. Any help will be greatly appreciated.


Anyway. I have heard from the person with the problem and it has now been resolved.

"It was indeed a problem with the feed.  It was not the F1 approved type and it did not pass Intelsat's isolation requirement.  Once I switched for an F1 approved feed it passed with flying colors. Thanks again."

So, if you want acceptable crosspol performance with a circular polarisation Prodelin C band system make sure you order the F1 approved feed assembly and associated bracket and feed support arm.

A further point has been brought to my attention.  When assembling and fitting the feed support bracket be aware that the weight of the feed tends to make it sag down with the result that the feed horn points towards the upper half of the dish. This is bad.  Make sure the feed horn points at the centre of the dish (A very tiny fraction above the centre should be the aim point as this more equalises the power levels at the upper and lower dish edges, which are at different distances from the feed).

Best regards, Eric.
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Eric Johnston
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Reply #3 - May 27th, 2009 at 11:06am  
Here are some images of the good Prodelin F1 feed system:

...
Earlier Prodelin F1 feed with white powdercoat finish.

...
Current Prodelin F1 feed with chromate finish.

...
Prodelin F1 feed scematic.

Do not use the "Standard Circular polarisation feed", which has a square polariser tube, for a transmit/receive antenna that is to work with satellites that use dual polarisation operation.

Best regards, Eric.
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