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Yet another military HX thread

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Feb 26th, 2008 at 1:05pm  
Greetings satsig, I wouldn't post if I hadnt exhausted every resource, so here goes.

We have a HX system here on WAFA riding AM22-SEASAT2 53deg. We had configured the initial shot using a horizon meter and peaked up with what we thought was the best config. Achieved 76d/91 signal on the hughes box. Worked great for about 1-2 wks, until degrading during a snow storm. Re-aligned dish in case snow had shifted, no avail.

We were able to get 91 consistently at the hughes box, but transmit kept going out of sync. In addition the suggested 21deg polang brought this number down below 90. Even at our optimal peak angle and the suggested angle, transmit was intermittant.

I would like to verify that we should be using Horizontal transmit/Vertical recieve, meaning the LNB should be vertically oriented.

I would also like to wonder why the suggested polang provided poor performance as opposed to our 'found' polang, and why the transmit would be weak when the recieve is so good. Yes, the mode matched horn is configured correctly.
Any guidance would be appreciated on these strange phenomenae.
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Eric Johnston
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Reply #1 - Feb 26th, 2008 at 2:24pm  
I would be interested to see some photos of your antenna/feed arrangement.  Please send to eric@satsig.net

The LNB position for vertical downlink receive polarisation would be with the LNB at the side, 9 o'clock or 3 o'clock  position, as viewed towards the satellite. Note the broad faces of the rectangular LNB input waveguide will then be on top and underneath.  This is the definition of vertical polarisation.

Using https://www.satsig.net/maps/satellite-dish-pointing-pakistan.htm

The polarisation adjustment required for the AM22 Pakistan/Afghanistan beam coverage is approx +10 to +30 deg clockwise. This means adjust the polarisation by turning the entire dish so many degrees clockwise, while facing towards the satellite.  The big scale on the back of the dish is perfect.  Calculate your particular polarisation adjustment needed, using your location.

I am particularly interested in your feed arrangement, so close up images showing the 505, the single and dual tick marks and the fat side of the feed throat are of interest.  I can say for certain that the fat side of the feed throat must be exactly towards the feed arm and the 505 directly away from the feed arm.

How the OMT/side filter/LNB are attached is another matter.  The orientation of the side filter LNB sets the nominal receive polarisation.   If the filter LNB is up on top that gives horizontal receive polarisation, as used for W3A satellite for Iraq.  With the filter/LNB at either side you are set for vertical receive polarisation, which may be applicable for AM22.

...

The OMT/filter assembly I have here is marked P/N 110196-7 and it is a cross-pol device.  

The Transmit module has its own polarisation.  The unit has an obvious underside and top.   The transmit unit works vertical when its top is uppermost (or underneath).

In the case of AM22 your service may be configured for co-pol operation.  This means that the OMT/filter block may need to be changed for a diplexer/dual filter, which will have different model number.

If your AM22 service is a co-pol operation and you have a (wrong) cross pol assembly then when your receive is perfect your transmit signal should not get to the hub at all.  It should fail completely.

Ask WAFA what kit they have supplied to you and clarify on which polarisations you are supposed to transmit and receive.

Are you supposed to transmit and receive co-pol, i.e. both on the same polarisation ?  
wxw
Best regards, Eric.
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« Last Edit: Feb 26th, 2008 at 3:35pm by Eric Johnston »  
 
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Reply #2 - Feb 27th, 2008 at 11:27am  
This is the first configuration we tried, that got us the highest signal strength (91), and worked quite well for about 2 weeks before degrading. Even though it is, technically, incorrect I believe.
[img]https://www.satsig.net/images/hughes/hx-dish-co-pol-horizontal-30-deg-s.jpg[/img]

This is the current configuration.
[img]https://www.satsig.net/images/hughes/hx-dish-co-pol-vertical-plus30pol-s.jpg[/img]
I acquired a Horizon meter to try and peak it out today. We were experiencing the '15' signal strength phenom on the hughes box, so I decided to get the meter to troubleshoot. Currently the meter is detecting 59db for AM22 no matter what direction the dish is pointed in, which is odd. The LNB is connected to the correct side and etc. The worse omen is that it also returns around 59db for any bird stored in the Horizon meter, which leads me to believe there is an anomaly with the LNB unit. I've used this horizon before and know it to not be defective.

Also

-The RPR states Horizontal Transmit and Vertical Receive.
-This demonstrates cross-pol behaviour
-The 505 is up and the fat side is down as well as the tick marks in the correct alignment.
See below for (poor) top-down photo of feed throat assembly.
[img]https://www.satsig.net/images/hughes/hx-feed-co-pol-diplexer-mode-matched-horn-s.jpg[/img]

Thank you for your help thus far.

Here is a close-up shot of the feed horn assembly.
[img]https://www.satsig.net/images/hughes/hx-feed-co-pol-horizontal-polarisation-s.jpg[/img]
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« Last Edit: Feb 27th, 2008 at 11:09pm by Admin1 »  
 
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Eric Johnston
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Reply #3 - Feb 27th, 2008 at 4:58pm  
You say "The RPR states Horizontal Transmit and Vertical Receive."  I have spoken with the operator who says that this is in error.  You should be transmitting and receiving, both on Horizontal polarisation.

You have the correct co-pol feed assembly with a pair of diplexer filters.  Your transmit and receive will be the same polarisations.

I've put my explanations of your images below:

...
Image above shows co-pol arrangement with nominal horizontal polarisation for transmit and nominal horizontal polarisation for receive.
A polarisation adjustment of approx -30 deg has been applied by turning the entire dish anti-clockwise, while facing towards the satellite. There is a big scale on the back of the dish to set this adjustment.


...
Image above shows co-pol arrangement with nominal vertical polarisation for transmit and nominal vertical polarisation for receive.
A polarisation adjustment of approx +30 deg has been applied by turning the entire dish clockwise, while facing towards the satellite. There is a big scale on the back of the dish to set this adjustment.


...
Image above shows co-pol arrangement with nominal horizontal polarisation for transmit and nominal horizontal polarisation for receive.


...
Image above shows the mode matched horn at the top.  This is correctly oriented.  The 505 is away from the support arm.  The fat lump (not visible and underneath the throat) is towards the feed arm. The single tick mark on the tapered spacer tube is away from the feed arm.  These positions apply regardless of the nominal polarisation settings.  Polarisation is achieved by altering the joint marked. As shown the diplexer (two filters) is vertical for both transmit and receive.

My recommendations:

The 505 is away from the arm, fat bit towards the arm, single tick mark away from the arm. (This is always the case)

The LNB needs to be directly away from the feed arm for Horizontal polarisation.  Looking at your pictures you need to undo the six tiny screws between the taper adaptor tube and the diplexer filter and rejoin with the LNB upwards, same as the 505.  The BUC will be on its side.

The entire dish should be turned for the polarisation adjustment.  Positive (+) angles mean clockwise while facing the satellite.  You need to determine the polarisation adjustment angle using your lat long and AM22 orbit position.  See  https://www.satsig.net/maps/satellite-dish-pointing-pakistan.htm  Set the pol adjustment angle exactly using the large circular scale.
wxw
Best regards, Eric.
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« Last Edit: Feb 27th, 2008 at 11:25pm by Eric Johnston »  
 
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Reply #4 - Feb 27th, 2008 at 5:49pm  
I realize there are security implications, but it would be interesting to know the frequency range of that antenna in the background. It looks like a YAGI, some of which can work the same frequency as your IFL (cable freq). You should be concerned if the YAGI is radiating anywhere between 900 and 1500 MHz

//greg//

I have obscured the dipole antenna in all the images above - forum admin.
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« Last Edit: Feb 27th, 2008 at 11:18pm by Admin1 »  

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Reply #5 - Feb 27th, 2008 at 9:27pm  
Dear Sir

This is BW I agree with USN retired get the dish right away from that transmitter yagi it will definitley affect the the signals , you need to move it at least 15 meters away or more if the cable permits ,this is a co polar installation so recieve and transmit are in the same plane , please contact directly us or Wafa although we are the HX noc our contact is found on the BW web site www.bentleywalker.com or contact me directly anthony@bentleywalker.com
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Reply #6 - Feb 28th, 2008 at 5:50am  
Well that certainly made sense. I was trying to figure out why the transmit module seemed to be oriented in the same polarization as the arm. As for the yagi, thank you for obscuring it although it was a non-tactical system and has been out of use for a bit now. We simply can't remove it though due to the fact that it isn't ours.

This, however, doesn't explain the phenomenon we're seeing with the LNB side spewing 59db to all the birds on the horizon box. I can't decide why it would do this across every bird except for a defect in the LNB module itself. I will go back there with the horizon today and re-arrange all the feed elements to the correct configurations, but I can't imagine that would cause it. Perhaps there's a setting in the horizon meter thats off?

Thank you so much for your guidance so far, it confirmed my suspicions and makes life much easier.

So to wrap up:
-What could cause the extraneous reading from the horizon
-Will I need to re-commission the box now with the horiz/horiz setting?
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A.Walker
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Reply #7 - Feb 28th, 2008 at 1:02pm  
Dear Sir

For Kabul region the position of the feed/LNB is anticlockwise 33 degrees , hope this helps
Best Regards,
Anthony Walker
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Eric Johnston
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Reply #8 - Feb 28th, 2008 at 1:53pm  
Regarding the Horizon meter.

Let me explain how I think it works.  Initially is operates as a simple, but sensitive, broadband power meter.  If you disconnect the LNB is gives a low reading.  If you connect the LNB and turn the DC power on then the reading increases.  If you point the LNB directly up at the cold sky (~3 deg K) then the reading corresponds to the noise generated by the LNB input transistor (~59 deg K).  If you point the LNB at your warm hand or the warm ground the noise generated corresponds to the  input transistor plus the warm ground (59 + 273 = 332 deg K).

If you point the antenna at the sky and swing it across any satellite, or the sun, the Horizon will give increasing readings.

The Horizon meter also acts as a satellite TV receiver.  If it is tuned correctly it will lock onto a satellite TV carrier and display FOUND.   The display then changes to show signal quality.   The satellite recognition process will only work if you have the correct LNB, the correct polarisation and the correct, matching, programming.

If the Horizon shows a steady reading all of the time I would disconnect the LNB and see if the reading goes to zero.  Then reconnect and see if the reading really is fixed or if it increases slightly when pointed at my hand or if it varies slightly with different 13/19 volts. If anomalous I would look for connector short or open circuit, or less likely, high power interference from nearby 900 - 2050 MHz transmitter etc.  A very short cable from LNB to the Horizon meter will give a high reading.

Regarding re-commissioning.

If the service was working normally, and the outlink carrier has not been changed in frequency or symbol rate in the meantime, I would not expect recommissioning to be necessary.  

When repointing the dish I would prefer to connect the LNB alone at first until pointing is perfect.  This avoids the hub attempting upgrades, restarts etc until the dish pointing is all completed.  Once the BUC is powered ON and communications with the hub starts up the unit is best left alone and not interrupted during its initial boot up and updating of its software and configuration.

Best regards, Eric.
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Reply #9 - Feb 28th, 2008 at 2:47pm  
Well, went out again with the new information in hand (-33 polang and horiz/horiz orientation) and a horizon meter. We swung around and to the previous location we had been looking for seasat and had no success. Could not achieve a signal of more than 63 on the horizon, that was the peak after vertical/horizontal adjustment using my method (which works quite well).
The odd thing was, when we switched the horizon to look for W3A, we were able to get over 70db on the meter after a quick peak-up. I'm thinking there might be something wrong with the LNB hardware itself,  but the fact that we are able to get good gain off of W3A seems to obscure the issue even further.
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Reply #10 - Feb 28th, 2008 at 3:14pm  
A.Walker wrote on Feb 28th, 2008 at 1:02pm:
For Kabul region the position of the feed/LNB is anticlockwise 33 degrees
I'm not sure I should tread in this water, but I question that statement. The OP doesn't specifically state his location - which I'm sure is for security reasons. But odds are that he's somewhere east of the AM22 sub-satellite point (53E). So why would the POL orientation be counter-clockwise? In my part of the world, if you are east of the SSP, the POL angle is positive (clockwise) when facing the satellite. If you are west of the SSP, the POL angle is negative (counterclockwise). Or does BW reference CW/CCW when facing the dish?

Even if the OP dish is coincidentally located vic Kabul, I make that POL angle to be about +22.

//greg//
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Reply #11 - Feb 28th, 2008 at 3:22pm  
I agree with Greg.  -33 deg is in error by 55 deg relative to +22 deg, so you would be lucky to get the wanted signal from AM22.  It would require there to be no traffic on the opposite polarisation and the wanted signal would be 5 dB low.  The opposite polarisation traffic, if present, would be higher than the wanted signal so no hope at all...

The entire dish should be turned for the polarisation adjustment.  Positive (+) angles mean clockwise while facing in the direction towards the satellite.  You need to determine the polarisation adjustment angle using your lat long and AM22 orbit position.  See  https://www.satsig.net/maps/satellite-dish-pointing-pakistan.htm  Set the pol adjustment angle exactly using the large circular scale.

I agree the angle should be in the range about +12 to +30 deg for the Afganistan/Pakistan area, but since the exact angle is important, the customer should do it themselves privately with their own known lat/long position.

The co-pol feed is intended for use with this AM22 service.

The service on W3A needs a cross pol feed. In Afghanistan, the W3A 7 East beam coverage is poor, the signal is low level and also low elevation angle (20 - 10 deg) so I would not recommend attempting to use it. 

Best regards, Eric
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Reply #12 - Feb 29th, 2008 at 10:36am  
The -33 was given to us by our agent in Kabul . I will check mystelf and ask him to send a digital photo of an actual operational HX system with a good lock
=))
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