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VSAT technology and installation >> Dish pointing and alignment >> C-Band Buc saturate

Message started by Pierre on Jan 17th, 2012 at 7:33am

Title: C-Band Buc saturate
Post by Pierre on Jan 17th, 2012 at 7:33am
Good day all ;

Got a 2.4m dish with the following equipement :

Belcom 8W BUC
Idirect 3000 modem

Pointing to NSS12 in Madagascar .

ISite gives us a 18V signal and get RX and TX lock but we cannot get into the network because the NOK say that the BUC gets saturated . We used a analizer as well but it showed that all levels are 100% . Replaced the BUC and modem but it made no difference .

What other possible issues can there be ? Suggestions are that focal point in not right (how do I determine this) , dish could have a hairline crack ?

Title: Re: C-Band Buc saturate
Post by Oasis Networks on Jan 18th, 2012 at 11:14am
Dear Piere,

It sounds like a link budget issue; either the uplink was not designed properly, or you have some problem on your site.

When NOC says the BUC is saturated, it means that it cannot transmit strong enough for them to receive your transmission good.

Some possible reasons for this:
1. Link budget was not prepared well: Maybe you need stronger BUC or bigger dish. Or maybe you should decrease your data rate on your Tx so the BUC will manage with it.
2. Some problem on your Tx chain that attenuates the signal too much, like Tx cable is too long, or of wrong type, or connector contacts are not good enough.
3. Maybe the focal point is not good enough, in this case you will need to check the ABCD of the dish. Maybe the dish is not pointed good enough. But as you manage to get 18v on isite, and I can imagine you have been playing with the pointing for some time, I would leave this option for second phase.

Anyway - keep insisting with your NOC - if they provided you with the equipment - then they should hold responsibility for it as well and assist you with the troubleshooting! I dont know who is your service provider, but unfortunatelly we have noticed that most of NOC guys lacking the required knowledge, and have no practical experience at all. Maybe you shoud try to escalate it to a senior engineer.

Good luck,

Title: Re: C-Band Buc saturate
Post by Pierre on Jan 18th, 2012 at 4:46pm
Thanks Nimrod , just a few more things ,

I also tried it with very short cables to eliminate cable problems . My service provider did send out their own engineer to site in Madagascar but he was also unable to solve the problem . This use to be a KU band (W2A) setup and was changed to C-band . A conversion kit was supplied to fit onto the dish but I have been told by somebody else that these feeder arms does not always focus the feeder ! How do I calculate the ABCD your talking about ?

Title: Re: C-Band Buc saturate
Post by Eric Johnston on Jan 18th, 2012 at 6:49pm
1.  I have contacted Belcom and they indicate that the BLT-8W BUC takes about 2 amps at 24V.
Their spec sheet says 54 W.  The iDirect 3000 uses an external power supply and I don't know what power supply that you have.  Typically they are good for 5 amps at 24V.  Check your power supply.  Your power supply may be faulty and giving low voltage/current up the BUC cable.  In which case the BUC will saturate at a lower power than its specification says.

2.  Prodelin antennas have ABCD measurements which define the positions of the top and bottom of the feed window, relative to the top and bottom of the dish. Example:

This picture is an example only.  It is a Prodelin Ku band 2.4m antenna.  Note that your dish may have different F/D ratio (e.g. 0.8 instsead of 0.6 as in this example) and you are using a larger diameter C band feed.  The focal point is some distance down inside the feed horn.  In the above example A=55.5 inches. B=59.4 inches.  The distance to the centre of the window is 57.45 inches and the focal length is 58.52 inches.

We really need to know the model number of your dish. Photos of the feed assembly would be helpful. Take a picture accurately from the side view.

3. NSS12 is circular polarisation at C band.  There must be a long polariser tube (3) between the conical feed horn (1) and the OMT junction with waveguide (2).

Inside the polariser tube there will be rows of pins or slots.  These pins or slots must be at 45 deg to the waveguide(s) to make circular polarisation. It is worth checking that you have a polariser and that it is aligned at 45 deg. The image above does not show the location of these pins/slots on the exterior of the tube. There are probably some tick marks on the flange like LHCP -45, Linear 0, and RHCP +45.  Note that at the mid position, 0, when the pins/slots line up with the waveguide the effect of the polariser disappears and the feed becomes a linear polarisation system, which is not what you want.

If your OMT has an N type coax connector (as in the example image above) for the 6 GHz transmit connection then there is the possibility that the cable is faulty and has high attenuation due to moisture/water corrosion. The type of cable is typically Andrew LDF-450 Heliax. This type cable and the connectors are very expensive and take several hours skilled work to terminate!

Best regards, Eric

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