NOC Tech in need of cross-pol instructions

The frequency chosen for the CW test must be one which is unused on both polarisations on the wanted satellite and also unused on adjacent satellites in the orbit.  This is to avoid the CW carrier causing interference with anyone else. Only the satellite operations centre can authorise CW carrier tests.

The large antenna at the teleport hub is where your VSAT transmit co-pol and cross-pol measurements are made.  The hub antenna has two PLL LNBs or LNAs, one for each polarisation.  The wanted co-pol signal is measured, e.g. -10 dBm.  The weak cross pol is also measured, e.g. -43 dBm.  Simultaneous measurements are possible if they have two spectrum analysers but it may be easier to switch between the two receive paths.  Either way, calibration is important.  The hub wants you to adjust your transmit polarisation to minimise the crosspol level so as to obtain a high isolation (43 - 10 = 33 dB isolation in the above example).

It is not easy measuring low levels so, when requested, turn your VSAT feed by a very small amount (e.g 1 deg) and leave it there till the hub is able to measure the new level. It may take them 30 seconds to get a good reading, so be very patient and don't rotate the feed again until the hub say to do so.

The objective it to have at least 30 dB isolation at the beam centre, when the antenna under test is pointed directly at the satellite.  This 30 dB isolation must be maintained when you are mispointed by -1 dB in any direction so the test may be repeated 8 more times, by mispointing the antenna up, up right, right, right down, down, down left, left and up left.  This more complex cross-pol test should be done on larger VSATs, for example 3.5m with subreflector or VSATs with high power BUCs or HPAs.  All the emphasis is on reducing interference to other people's services.

Regarding BUC P-1dB gain compression tests.  The drive level into your BUC must not exceed the P-1dB level otherwise your transmit signal will be distorted, causing high error rate, and, more important due to spectral regrowth, causing interference to adjacent carriers.  Also you BUC output transistors will be damaged.  While transmitting a CW carrier the teleport hub measures your level.  The output power from the modem is increased in 1 dB steps and the level at the hub measured also.  Plot the graph.  As you get near BUC saturation the BUC output level no longer goes up in 1 dB steps. Once the BUC output level is lagging the input by 1 dB you are at the P-1dB gain compression point.  Stop and record the modem output level.  This level must never be exceeded in service.  The level varies from site to site according to the cable length. If you are going to transmit 8-PSK or 16-QAM modulation it is best to keep 3 dB below the P-1dB compression point as even slight distortion of the transmitted signal will increase the error rate.

Hi,
Do you have some pictures from the site?

What SNR are you supposed to get?

Did you check the dish is installed properly (the feed is placed in the exact position in front of the reflector, at the correct angle?).

Did you check the cables maybe?

Did you try to check the signal with a different modem or with a spectrum analyzer?

What BADR satellite?.

If BADR-6, C band at 26deg east, are you using a circular polarisation feed ?  Have you assembled the feed for linear polarisation by mistake ?  The OMT should be at +/-45 deg to the two lines of pins/slots inside the polariser.  45 deg one way will work perfectly.  45 deg the other way not at all.  Half way (i.e. in line and linear pol) will give at least 3 dB degraded and totally degraded if there are cross-pol carriers.

Is the feed in the correct location relative to the dish ?  Are the feed support struts in the right holes ?  Is the front rim of the dish flat ?  Pictures might help.

What kind modem, bit rate, FEC ? Reducing the bit rate  and trying 1/2 rate FEC might get something working.

Best regards, Eric.