SatSig topic:

Interference on adjacent satellite

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Jun 11^th, 2010 at 8:04am

Hi
Is it possible to have a C-band 3.8M dish to transmit to two satellites. one satellite is at 62° and the other receiving the interference is 64.15°
We have tried re-peaking but interference still remains.
could we be having some issues with the antenna itself?

Jun 11^th, 2010 at 9:01am

Where is the dish located?

To make sure, the problem is that when you transmit it cause interferences to other satellite, or signals from the other satellite interfere with your receive?

What is the model of the dish and how old is it?

Jun 11^th, 2010 at 10:45am

It is possible that you have a bad sidelobe pointed at the adjacent satellite. You may also have low on-axis transmit gain meaning your transmit power (BUC watts) is much higher than it needs to be.

You need to plot the antenna radiation pattern, which should look something like this:

It is most likely that your actual patterns are greatly different, with serious imbalance between the sidelobes on either side and with the main beam much broader and lower in gain than it should be.

Possible reasons are panel distortion/mis-alignment and feed location and orientation wrong, and subreflector also if you have one.

Get hold of the manufacturers installation/alignment instructions and mechanically set up your dish using physical measurements.
Check that the rim of the dish, at its operational elevation angle, is flat. See: Satellite dish panel adjustment.
If your dish was supposed to have special spacers inserted between the steel backing structue and the dish then putting the correct spacers in the right places is really important. Wrong or missing spacers or wrong positions can result in the dish being 10 dB low in gain !

Check the feed is central and pointed towards the centre of the dish (circular dish) or very slightly above centre in case of an offset dish. If you have a short f/d circular front fed dish the location of the feed is extremely critical. If you have a subreflector then its position and orientation is critical also. See: Sub reflector adjustment. Long f/d offset dishes are more tolerant of feed location errors.

This Prodelin 3.8m ABCD measurements link may be useful if your antenna is the same as that referred to.

Make receive sidelobe patterns yourself using a CW satellite beacon. You need a PLL LNB, spectrum analyser with 1kHz resolution bandwiodth and a plotter.
Repeat the sidelobe measurements and adjustments till you have a clean pattern with equal height (+/- 0.1 dB is worth trying for) first sidelobes and deep nulls. This could take anything from a few hours to as long a week to complete. See the sidelobe pattern image above.

It is quite common for multi panel dishes to have an initial performance 3 to 6 dB lower than designed, equivalent to a dish half or quarter the area, so don't be suprised if you get a big improvement.
Measuring the relative G/T before and after gives you an idea of the gain improvement (measure beacon level versus noise level off satellite). Looking at the receive sidelobe patterns will indicate the reduction in receive interference from adjacent satellites.

It is important get the receive patterns right first as this saves a vast amount of time and money getting the transmit patterns tested and adjusted till they are acceptable.

Get your transmit sidelobe pattern measured with the assistance of the satellite operator. You pay for this. They will assign you a test frequency where you will not interfere with adjacent satellites, or will limit the angular range of the testing. Transmit a CW carrier and drive the antenna steadily +/- several deg both upwards and left to right across the satellite while the ESVA test earth station plots your elevation and azimuth sidelobe patterns.

Review these patterns and make adjustments. Repeat the process till your antenna is correctly set up. Hopefully your antenna will pass on the first test but more likely you will have to make adjustments. It may take a few hours if you have prepared your dish yourself first.

Best regards, Eric.

Jun 11^th, 2010 at 12:57pm

Location of the dish (Prodelin) is in Kenya and its about 5 years old. I am transmitting to the 62° satellite but 64.15° satellite is pickig up my signal. (at least enough to cause interference). I don't seem to have RX problems because the Eb/No and BER on the modem look good and I don't have obvious call setup or call drop issues.

Jun 12^th, 2010 at 12:03pm

Hi,

According to my calculations, to look on IS-902 from Nairobi will require elevation of around 60.5 degrees while IS-906 is at around 58 degrees. Azimuth is also almost the same. Both using circular polarity for C-band.

I agree with Eric that you should take special care for the geometry and panels alignment of the dish. I would also recommend to make sure the feed support tube is installed correctly, i.e. the side support tube are connected to the LAST hole of the main support tube and not to the one before the last, as many installers do with Prodelin dishes.

Also, I may suggest to consider replacing the feed system:
Over the years Prodelin has designed several models of th 3.8m dish, and they have modified the feed geometry (that is the angle the feed is looking toward the reflector). If the feed is tilted in the wrong angle, that could cause other satelliets pick up your signal.
Plus, the 3.8m prodelin circular feed has well-known problem with the cross pol isolations, this could cause you some more problems in the future.

Last but not least, make sure your pointing is excellent, that the mast is completly vertical and not tiltet. That you are trying to point transmitting a CW, and modifying the azimuth and elevation very gently, while someone at the teleport guides your steps (i.e. pointing by Tx and not by Rx).

If still you can't manage, we have very excellent engineers in Nairobi, and we can try as well.

Kind Regards,
Nimrod

Jun 15^th, 2010 at 5:29am

Thanks guys for your advice. be back with results.

Jun 16^th, 2010 at 5:44am

Adjust your sub-reflector according to antenna manual it's distance make big difference in RF propagation from antenna.