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pol/isolation problem

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takoateli
Ex Member


Oct 10th, 2010 at 12:29pm  
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
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USN - Retired
Ex Member


Reply #1 - Oct 10th, 2010 at 5:14pm  
Well, my response here will be identical to that I provided over at DSLR. That is:
Photos would help confirm this. But my first diagnosis is that your dish configuration is at fault. The HN7000S modem statistical analysis that produces the red/yellow/green status. But the baseline for this analysis is based upon use with a limited number of outdoor equipment configurations. It's pretty clear that your dish configuration doesn't fall into this category. Let's start with the "large" dish. By design, the bigger the diameter - the higher the gain. This is what is producing what ordinarily would be an excellent receive signal level (SQF) for a "regular" sized dish. The fact that your oversized dish is producing a large SQF number is then misleading. Usable, but misleading from the aspect of troubleshooting.

Again I'm diagnosing in the blind here, but - next - I suspect you've mounted the TRIA (Transmit/Receive Integrated Assembly) incorrectly on the feedarm. The transmit signal is probably bouncing off the dish at an inaccurate angle to reach the satellite accurately. That factor alone is what produces good transmitter isolation numbers (the 30 and the 6) as viewed by the NOC. The large SQF number is just confusing the issue.

Best solution is simply to procure/install outdoor equipment that's actually compatible with the HN7000S. Short of that I suggest you ignore the SQF when conducting POL adjustments with the NOC. Concentrate on following their telephone advice in obtaining the best transmit isolation numbers possible at their end. This is an adjustment at your end that MUST be made in real time with an operator at the NOC. Once your transmit status is "green", the receive status will almost certainly remain "green". Maybe not a 93, but almost surely at an acceptable operational level.


//greg//
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takoateli
Ex Member


Reply #2 - Oct 11th, 2010 at 11:25am  
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
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USN - Retired
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Reply #3 - Oct 11th, 2010 at 12:09pm  
Well as I understand it, you have a large old dish and feedarm of unspecified pedigree - upon which you've mounted a HN7000S compatible feed (TRIA). When folks attempt this, the majority problem does in fact turn out to be positioning the feedhorn to obtain the correct focal distance and angle. When placed inaccurately, the transmit signal will hit the parabolic reflector in the wrong place.

The size of the transmitter and a lack of auto-tracking make antenna pointing more critical to the transmitter (BUC) than to the receiver (LNB). Due to the larger than necessary diameter, sufficient signal is reflected into the feedhorn to give false confidence in the pointing angles. But not enough of the transmit signal ends up making it to the satellite - accurately.

Your provider supplied outdoor equipment should resolve this mismatch.

//greg//
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Eric Johnston
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Reply #4 - Oct 11th, 2010 at 11:01pm  
Quote:
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.


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.
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« Last Edit: Oct 12th, 2010 at 9:00am by Admin1 »  
 
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satbaja
Ex Member


Reply #5 - Oct 16th, 2010 at 3:23am  
There are three problems to address.
Your numbers are 30 (co pol or good signal) and 6 (cross pol or interference).

The isolation is the difference between the two, or 24 dB.
First, the cross pol will be higher with a larger dish. If the ISP thinks you have a smaller dish they need to update the dish size, maybe this will allow them to accept a higher cross pol figure.

Next, the dish is may not approved for your two way satellite internet system. The approval means the dish shape has been tested for use with your satellite.

Finally, the feed horn may be meant for a certain dish make and size. If you mismatch the feed horn or misplace the feed horn out of the optimal focal point, you will have poor isolation figures and high cross pol.

You should try the original equipment they sent you. If the dish you have isn't large enough consider purchasing a dish with feed horn approved for your satellite and VSAT system.
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takoateli
Ex Member


Reply #6 - Oct 16th, 2010 at 4:35pm  
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
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USN - Retired
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Reply #7 - Oct 16th, 2010 at 9:26pm  
No angels, we're just lucky to have a few guys around here that have been around the block a few times. Glad it worked out forya

//greg//
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