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Several questions about NSS7 and AM22

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Jan 21st, 2007 at 10:38am  
Hi .. this is my first time on this forum ... I need help so bad because I am having problems andIi will try not to be so annoying Wink

My first problem is with NSS7 .. My location is in Libya, Center of North Africa ..My problem is with the Isolation Test I always got low Isolation even when I am sure that my Anntenna is pointed well .. I am use a downlink 11572 and 11577 for pointing and also i descovered 11626 11618 all of them I got the best thing I could do .. lately I notice that my problem sometimes is with cables and connectors I am using ..

So if any one has a tip/trick about how to make a good connectors and about how to know the quailty of the cable ??

In my installation I am always using typically 1.2m Andrew dish and 3 watt BUC but the NOC is tell me to use 1.8m but if I use 1.8m it won't be cost effective for the clients ..

The other question is about AM22 .. I am always having a problem with the rejector filter on the feed horn .. Because when I connect both the TX/RX cables I have a low downstream ..and when I remove the TX cable the downstream signal goes up .. the NOC told me that to change the  feedhorn and I did change it for 5 sites up to now ...

So if anybody knows what cause this problem or how to fix it ?? ..  Becasue this is the only problem I am having with the AM22

Thanks a lot and excuse me for my long topic and poor english ..

Best regards
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Eric Johnston
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Reply #1 - Jan 21st, 2007 at 11:26am  
If the receive quality improves when the TX cable is disconnected then bad connections may be the problem.   The DC power current for the BUC may be coming back on the outer of the LNB cable, for example.   You need to make sure that the inner and outer of all four F type connectors are perfect.   The inner pin or end of the centre wire should stick out 2mm beyond the plug rim.   When you push it in and screw together make sure the centre pin actually goes fully into the socket hole.  There is a risk of the centre pin being pushed back into the cable.  Perhaps smooth the end of the pin and twist the cable/pin gently as you insert it to make sure the pin is captured by the springs inside the socket.
The inside contact springs are vulnerable to overheating if the sping pressure is low, so a smear of silicone grease on the pin does no harm.

The outer braid connecton if the F types must be good.
The braid wires may be wrapped around.

Sealing tape to keep out moisture is essential and silicone grease helps also by excluding oxygen from the contact between the wire and metal parts.

If the receive quality is low when the transmitter actually operates (i.e. when a burst is transmitted there is a reduction of Eb/No or a burst of errors on the receive side) then there are two possible problems:
1. It is the same phenonena as above.  The BUC takes more amps current when it transmits a burst and the extra return current amps on the outer of the LNB cable alters the DC supply to the LNB.  Check the connectors.
2. You have either no transmit reject filter in front of the LNB (or, less likely, the wrong type transmit reject filter.)  Some of the transmit power is getting into the LNB input and overloading it.  Install the correct transmit reject filter in front of the LNB.

Poor cross-polarisation

Two possibilites:
Adjustment...
You have not rotated the feed to the correct angle.  The cross polarisation null is very narrow and you need to turn the feed and set the angle to about 1 deg accuracy.  It is difficult for the NOC hub to make the measurement and it may take them 15 - 20 seconds to make one measurement and get a reading like -24 dB   You then need to make a tiny alteration and then wait till the next measurement is completed,  say -20dB.  In this case you have turned it the wrong way so need to go back.   Make single small movements with long pauses in between for the NOC to react.  Since we are dealing with such tiny rotation movements and the scale (if any) probaly has inadequate detail markings this adjustment process requires much patience.  With a narrow feed throat tube, the rotational movements required are in the order of 0.5 mm at a time.  Aim for the middle of a sharp -30 dB null.  

One idea is to set the polarisation by calculation and use an accurate inclinometer rested sideways across the BUC or LNB.

Wrong assembly causing cross-pol problems...
There was a case recently where a transmit reject filter had been assembled with the 90 deg H plane waveguide bend between the OMT and the filter.   For some strange reason this caused poor cross-pol performance. See here for the correct assembly for this partiular feed assembly. 
...

Always follow the manufacturers design exactly.  If the above does not refer to your particular hardware ignore this picture.

Dish size:  If you really need a 1.8m dish to receive the outlink in Libya and are using a 1.2m dish then you will be operating in clear sky conditions with negligible link budget margin.  Expect the service to fail during even slight rain, either rain at your site or rain at the hub teleport site.  You are supposed to operate with a big margin in clear sky conditions so that the service will continute to operate when there is rain at either the teleport hub or at your site.  If your transmit signal is not getting through to the hub try halving your transmit bit rate if possible.

Best regards, Eric.
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Reply #2 - Jan 22nd, 2007 at 12:52pm  
thank you Mr Eric for ur full reply .. i am realy satisfied with these info..

but it seems that i didnt explian well ..

i am dealling with Netdish as my service provider
and the NOC told me that the problem is from the OMT and that LNB filter is nt working fine .. so when i change it everything working fine ...

what i am asking for is what cause the filter to be harmed .. cuz there was a site which was working fine and suddenly it got harmed i tested it and there was nothing .. no miost no dust nothing..

the other thing is what is meant by the Isolation and does the cables and connectors effect it ..??
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Eric Johnston
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Reply #3 - Jan 22nd, 2007 at 6:19pm  
If the transmit reject filter is undamaged, and has no moisture, loose nut/screw etc inside it may be not faulty at all but rather that the feed rotation was not set accurately enough before.  It is quite possible that a badly adjusted site will transmit cross-pol interference for some time till the NOC investigate and turn it off or request you to give it attention.

If you have changed the filter you have probably since readjusted the polarisation angle, with better results.

...
Cross-pol isolation varies from 0 dB to about 35 dB.  If you can't get it better than say 28 dB then you have a faulty feed assembly, poor design antenna or incorrect assembly.

Best regards, Eric.
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Reply #4 - Jan 22nd, 2007 at 7:05pm  
I am bit puzzled:

It happens sometimes that the bandreject filter is faulty, which causes the exact phenomena, i.e, receiving ok, but when transmitting the EbNo droppes. this is as a result of too much noise from the BUC reaching the LNB. However, those are quite rare cases. I think I had only two or maybe three times I had a faulty band reject filter. Now you are telling about five!
On the other hand, I can't understand how polarity issues could cause those phenomena, unless you are trying to transmit on the same frequency as the downlink (i.e., you are blocking yourself, maybe you should calculate the frequencies again, but this is not exactly a polarity issue).
I also dont have any experience with BUC DC power affecting the LNB, but Eric explains very well how it could be, in case of isolation problems. However, in that case, I don't think you could get a good lock on the downlink signal on the first place.

My suggestion is to double check the frequency conversion you are doing when transmitting. maybe you are simply transmitting on the same KU frequency as your downlink and blocking yourself.
If this is not the case and it looks more like band reject issue, try to check if all those OMT or filters are from the same batch..

just dont forget to put a hat on your head and to drink lot of water over there......
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