EB/NO on 1.5 M Swedish 1.5F/A

It is not normal.

The final optimisation of the polarisation should very slightly improve your wanted receive signal and reduce the interference your transmit signal causes to others significantly.

Possible reasons for 2 dB drop in receive quality:

There is some problem with LNB or BUC cable connectors such that the LNB supply voltage is lowered when the BUC is activated.  Open circuit outer sheath on one cable is a possibility.

The transmit carrier is leaking into the LNB input.  Could be due to poor transmit reject filter.  Are you using unusually high power BUC ?  

The output of the BUC contains spurious transmit signals in the receive band.  Could be due to overdriven BUC causing spectrum spreading and spurii all over the place.

The transmit carrier is on an adjacent transponder frequency slot to wanted receive carrier. The carriers are too close together or the transmit carrier spectrum contains spectral regrowth (BUC overdrive) directly interfering under the wanted receive carrier.

There is some major power supply problem.  When the BUC is transmittting the increased BUC current taken is reducing the main power supply voltages in the modem and it is no longer working properly.  Is the BUC power supply adequately rated ?

Does the 2 dB reduction occur if the tx carrier is turned off or reduced in level by 10 dB?.

Does depowering the BUC make any difference ?

All below here is probably irrelevant..

If you have spectrum analyser you can peak up your receive polarisation by finding some cross pol carrier opposite to a clean part of your wanted polarisation and making careful exact equal measurements either side, say aout 20 deg off, then halfing the angle.

Could there be any loose metal nut, water drops, or other matter (e.g. spider) inside the OMT or feed system ?

If the antenna comprises a single offset reflector does the feed point just a fraction above the centre of the dish?  If there is a subreflector then the mechanical alignment of everthing is very sensitive and critical.

Best regards, Eric.

I don't know your network but was assuming that both the hub site and the remote site might be in the same beam coverage and that the two carriers, one each way, might be on adjacent frequencies in the transponder.

The normal frequency spacing between two adjacent carriers is 1.4 x half the symbol rate of carrier A + 1.4 x half the symbol rate of carrier B.

If you look at the two adjacent carriers there should be a clear null, almost down to the noise floor between the carriers.

If the null between the carriers is only 3 to 8 dB deep I would say it looks like the carriers are too close together.  If you lease the whole spectrum then you can decide your own spacing between your carriers, trading your adjacent carrier interference for bit rate.

If a BUC is overdriven, e.g. trying to transmit 3 watts with a 2 watt BUC, the effect is spectral regrowth, as shown in the image below:

The carrier on the left (overdriven BUC) interferes with the carrier on the right. If the carrier on the right were much narrower, while remaining close up alongside the interfering carrier the interference effect would be worse.  Note that operating a BUC with overdirve may destroy the output transistors - permanently !

If you are interested to see interference (if any) turn off the wanted carrier and look to see that there is a clean flat noise floor underneath.  If you can see what looks like distinct humpy carriers underneath your carrier then the interference may be cross-pol, intermodulation or adjacent satellite.

Best regards, Eric.

Good point Greg.   The cross pol performance of the antenna may differ significantly from tx to rx.  So optimising it for both simultaneously, is simply not possible.

I've just had a look at some Swedish pictures and they show dual reflector Gregorian designs. This design should, in theory, give excellent cross-pol results, but it is extremely sensitive to slight relative misalignment of the three parts (dish, subreflector, feed).  

Best regards, Eric.