pol/isolation problem

I've searched the forum and found some good information about my problem regarding other people's systems which were having problems regarding pol/isolation. It sounds like there's some extremely knowledgeable people here. Can anyone help me with this?

I'm located in Venezuela using service from a company called Bantel. I believe the underlying provider is actually Hughesnet. We're using an HN7000 modem. We are on the Hispasat Amazonas II satellite.

This service doesn't allow the end user to run the pol/isolation test from the terminal instead I have to contact Bantel and they have the test run. During that time the modem has a red status light and the transmitter code is 2 and it says the transmitter is in a test mode. Bantel tells me the results show numbers such as 30 and 6. The 30 number should be high and the 6 number should be adjusted to be as low as possible, and should be below 4. We have spent days trying to get the pol/isolation better and it can't be done.

The system is running well and our receive signal is 93 but we just can't get the pol/isolation right. It's not good enough that Bantel is comfortable with the numbers, but after hours of alignment of the polarization and tweaking the antenna alignment they give up.

One thing is we are not using the dish they supplied, instead we're using a bigger dish from a defunct satellite telephone system. It's a very professional and heavy duty Channel Master dish. The mount for the original outdoor equipment on the arm of the dish would not accept our satellite internet outdoor equipment so I adapted it, taking care to put the satellite internet outdoor equipment in the same position and plane as the original outdoor equipment was. During the pol/isolation alignment I even tried moving the alignment of the outdoor equipment which brought no improvement.

It sounds like the only possible causes for not being able to achieve good pol/isolation would be obstructions (none) or a defective BUC and/or feed horn, water in the feed horn/wave guide or a problem with the outdoor equipment not being perfectly aligned with the dish (bent arm etc).

We have the dish Bantel supplied with the system still in the box.

What would folks suggest? What would you suspect is the problem.
Could it be a defective BUC (a mechanical or electrical failure)? To me it seems like it couldn't be the BUC because the radio waves coming out of the BUC can only be one polarity in order for the waves to travel through the waveguide. I don't believe arbitrarily polarized radio waves could travel through the rectangular waveguide. If it was a round waveguide then yes. The outdoor equipment is in perfect condition, no cracks, no water in the waveguide etc. Could the problem be the antenna we're using?

Any suggestions or information would be greatly appreciated. Thanks! Greg

Thanks so much for your replies! I'm also leaning towards the antenna being the problem (even though I'm confident I have the equipment mounted very accurately) so I'm going to be switching to the antenna which came with the system shortly. I'll post the results here. Thanks!

Greg

Quote:


While Bantel have your transmitter ON in CW test mode, you need to be under the feed arm, reaching up to turn the feed and radio system following their verbal instructions. Bear in mind that it may take them 30 seconds to make one cross-pol measurement. The adjustments you make should be extremely small, amounts of feed rotation of 1 deg or less, i.e approx 1mm at 6cm radius from the feed centreline. Don't move it while they are measuring! The deep cross-pol null is very narrow and requires significant skill and patience to achieve the centre.

Trying to set the polarisation using receive measurements only, without the assistance of the hub is difficult without a precision spectrum analyser. The only method that might work is to turn the feed till you get a significant degradation, say from 94 down to 60. Measure the 60 value accurately for a minute or two and mark the angle carefully. Then turn the feed to the other side and adjust till you get exactly the same 60, averaged over one or two minutes. Mark the angle, calculate the half way angle and set it exactly. Repeat this whole measurement procedure several times and average the results, if you have any doubts.

If you are not using a type-approved antenna design note the critical factors outlined by Greg. The antenna must be in focus, i.e. distance from feed to dish correct (along the feed support arm in the case of offset fed antennas). The feed must also point at the centre of the dish (axisymetric dishes) and maybe 1 or 2cm above the center for offset dishes. The circular or square waveguide joint between the OMT and the feed horn should be flat to flat. If there is a rubber gasket, the waveguide screws must be tightened evenly. The rings in the feed horn should be dry and with no loose screws etc anywhere inside. Check that the main reflector is not distorted. Do the dual tight fishing line test. One up down the front and one sideways across the middle. In most dish designs the rim should be flat and the strings should just touch. Correcting such distortion can make several dB difference in gain.

Best regards, Eric.

Everyone thanks for the comments. I once again have learned a lot from the helpful and knowledgeable folks on a forum.

An update. I installed the provider's dish and alignment was fast and simple. I tweaked up the antenna for the highest signal, and with three turns of the polarization we achieved a cross pol of 30 and they said "lock it down". The system is running better than before.

The previous antenna was left over from a now defunct satellite telephone system. The antenna is a very large and professional Channel Master. I tried our satellite internet provider's outdoor electronics with the feed horn from the old sat tel system so I know the feed horn was right. Tried using the feed horn supplied by the sat internet company and it made no difference. I know I had the outdoor electronics/feed horn very close to where it should have been if not perfectly where it should have been. But what ever, we just couldn't get the cross pol right. Not only that the response from turning the polarization adjustment wasn't responsive. Across a wide range they'd say there was no change (still bad) and finally when it was really far one way or the other it would get worse. With the right antenna the response was immediate and it took just three attempts to get it right. Since we have no other comms here they would have to test our cross pol, bring us back on line, tell me what to do, I'd confirm the movement, and they'd test again. It was a slow process. But now finally it's really good.

Thanks to all who commented! You guys are angels.

Greg