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Tx problem on LinkStar

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Ex Member

Jan 23rd, 2008 at 5:53pm  
I've had a couple posts similar to this before, and I appreciate all the responses, but I need to ask for you guys' help once again.

I've had a linkstar terminal up and running on W6 for about a month now, and since the beginning I've seen what I believe are sub-standard transmit levels/upload speeds.  Also, about two weeks ago I began dropping my connection with the satellite quite often, so I engaged my service provider.  They checked the Tx level from the NOC and it read -2, which apparently is very bad.  One of the things they recommended was to re-check/re-align the dish.  I did so, but took it one step further and also replaced my BUC (and LNB just for the hell of it) and my F connectors and re-aligned the dish and ended up with a best QPSKBER of .00003.  I had the ISP get another Tx reading and now its -8, which I guess is better but still not too good.  My first question is: what is an acceptable Tx level?  What should I be trying to acheive?

Here's the next part.  From what I have read on this forum, I need to call the NOC and stand out by the dish making slight adjustments to the polarization, getting their feedback on the Tx level until I get it as good as can be.  The problem is, my ISP is telling me that direct communication to the NOC is not allowed, and I have to go through their online trouble ticket system.  But then it takes them 24 hours to respond to my online questions.  At this rate I'll never be able to troubleshoot this thing, because I'll make an adjustment and have to wait a day to see if it made a difference.  Does this seem right to you?  Please help.
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Eric Johnston
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Reply #1 - Jan 24th, 2008 at 4:41pm  
Receive:  You say you ended up with QPSK BER=0.00003   This is good but you should check that everything about your dish is tight.  Pushing or pulling on the dish should not make much difference.  It is no good if the dish moves in the wind and the performance varies or fails.  Accurate stable pointing to the centre of the beam is essential, since the transmit beam is narrower than the receive beam.

Transmit:  The transmit level setting involves an attenuator adjustment in the modem which alters the transmit level into the cable.   The longer the cable the higher the level needs to be.  The level must not be so high that it causes the BUC to saturate, which means its transistors will be operating at their limits.  You need to operate at least 1 dB below this.   If you operate at too high a level the signal is distorted, interference occurs to adjacent carriers, the quality into the hub decreases and the BUC transistors suffer early failure due to voltage and current spikes.

During adjustment, the hub should start with a low level and increase the level in steps until the correct quality into the hub is achieved.   Once a 1 dB increase no longer causes a 1 dB improvement in the quality the BUC is going into compression and you have reached saturation and must back off a bit.

It is possible to have a bad service because the transmit power into the BUC is too high.

If the BUC is operating just under its saturated power and the signal quality into the hub is not good enough then you may be mispointed, there is water in the feed horn, the dish is distorted, the dish is not large enough, the attempted transmit bit rate is too high, DC supply to the BUC is poor etc.

F connector workmanship is important since the DC power supply to the BUC also goes up the cable.  The inner pin should stick out 2mm beyond the rim of the plug and should insert properly into the socket when screwed in.  Poor pin contact will cause overheated springs, intermittent loss of service and corrosion damage to the internal springs of the F sockets.  A blob of silicone contact grease on the pin does no harm and excludes moisture and oxygen from the microscopic areas where the pin and springs actually touch.  The braid must make good contact also.  If moisture gets in the cable it can ruin several feet of the cable is just a few weeks.  The attenuation increases rapidly as the L band frequencies flow only in the surface of the metal.

The outdoor rotation adjustment of the feed throat is a cross-polarisation alignment.  The hub will command the modem to generate an unmodulated carrier on a carefully selected frequency where it will not cause interference to others.  The CW carrier is then detected by the hub on the cross-pol transponder and you are asked to make small rotation movements of the feed throat to minimise the cross-pol signal.   Setting the polarisation angle involves setting the starting position - either nominal vertical or horizontal and then applying an adjustment angle.   Horizontal polarisation means that the broad faces of the rectangular LNB waveguide are on either side.   A positive polarisation adjustment means turn it clockwise while facing the satellite.

Best regards, Eric.
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