SatSig topic:

7.3 m Andrew circular c-band antenna

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Mar 16^th, 2012 at 1:15pm

We have a 7.3 m Andrew anetnna c-band which use to be linear. We refirbished the antenna and replaced the feed with a new circular one. With the new feed we are seeing a lot of cross pol on the rx path. We have tried everything we can think of, but we can not seen to the the cross pol more that 16 db. Any suggestions as to what the problem might be??? We are lost at the moment. We have done the pattern cust etc and they are good!

Mar 16^th, 2012 at 4:12pm

Check with Andrew that the parts you have are correct. The polariser should be designed and made to work over the full 4/6 GHz bands, which is very difficult. The polariser will be a long circular or square tube with two rows of pins or slots along the inside, on opposite sides. These rows need to be at 45 deg to the OMT 4/6 GHz waveguides. The OMT itself should have >30 dB polarisation isolation. A cross-pol isolation as bad as 16 dB is typically due to the feed/OMT itself, not the antenna assembly or sub-reflector alignment.

Water in the feed rings. The surface of the water will be horizontal. If the LNBs are mounted horizontal and vertical the water will have no effect on polarisation isolation. If the LNBs are mounted at 45 deg the water will have an effect on polarisation isolation. Dry the feed horn rings. A similar effect occurs if there are loose nuts/bolts,washers, gaskets/tools left inside.

All the circular or square flanges between the feed horn and the OMT must all be accurately flat to flat. When attaching these flanges tighten the bolts evenly while supporting the weight of the unsupported part. Gaskets must be evenly compressed all around.

If there is plain tube between the feed horn and the polariser this tube must be straight. When assembling, make sure the tube it is not bent due to the weight of the polariser/OMT/LNBs etc.

If you have two receive ports, use two LNBs plus two TRF filters, or one LNB/TRF plus a good 4 GHz dummy load on the unused port. Don't leave the unused RX port with just a short circuit blanking plate over it.

Is the OMT attached to the polariser at exactly 45 deg?. This joint would normally have holes at 45 deg but if this joint is smooth then it must be set to exactly 45 deg.

If the problem is unresolved try turning the feed system and test at different rotational angles. It should make no difference at all but due to the way it may sag and bend slightly you can expect different results.

Best regards, Eric.

Mar 17^th, 2012 at 11:33am

Eric thanks a lot for your reply.
This feed was shipped as one unit (the feed and the 4 port combiner). We have a spare combiner which we also tried and got the same results. There is no water in the feed and all the nuts and bolts are tight. We spoke to Andrew, and they gave us the factory test results for this feed systems. These test results shows >30db isolation, but we are not seeing it!!
The combiner is a unit on its own that bolts onto the feed system. Is there anything ( other parts) between the combiner and the feed that we might not have installed o that was not shipped perhaps. We have an LNB on both RHCP and LHCP ports. We are only using one TX port, but when we did the tests we shut down the SSPB as not to cause any noise floor problems. We do have an additional filter (appolo) on the transmit port. We have installed a lot of Andrew 9.3 m antennas, bu this is the first 7.3 m antenna that we are refurbishing.
We send the pattern cuts to Andrew and they say that they are concerned with the fact that we are only seeing 2 sidelobes and not a lot more. I think this might be due to the LNB`s noise floor that might be slightly higher than their lab test LNB or whatever they are using.
This is what they said "For the patterns that were sent to me I would expect to see more dynamic range and by this I mean more additional sidelobes that you typically would see in 7 or 10 degree pattern. This give us a better view of the entire antenna. Are there any additional patterns that are available to send us?"

They also said the follwing: As we discussed on the phone I would like to see the receive patterns that were taken with this feed.. I will also need the S/N from the feed as this should be indicated on any label and should be 11W471 or 11W472. I need this to look at the factory test data on the feed. Also I would like to do the below recommendation of turning the SSPA off completely and seeing if the performance improves.
I do not quite agree that switching of the SSPB will improve the corss pol!!!
Thanks for your help any further suggestions will be appreciated!!!

Mar 17^th, 2012 at 1:32pm

You can send your sidleobe plots to me eric@satsig.net if you wish.

Best regards, Eric.