Tacos Digest, Vol 102, Issue 8

[wb4jfi at knology.net]

If I remember right, the DC voltage to the coax to the LNB not only powers the LNB, but also selects the receive polarity.  It does something like: below 17VDC chooses clockwise, and above 17VDC (or 15VDC) selects counter-clockwise polarity.  This actual switching voltage and polarities may be backwards.  The satellites alternate polarity on adjacent RF transponders (depending on individual satellite numbering scheme).

If you have more than two receivers, the normal arrangement is to have a separate polarization switch between the receivers and the LNB, this switch also powers both LNBs.  If you had that arrangement, I would consider the switch for failure first.  This switch permanently selects one LNB as CW, and the other as CCW, and then selects the proper polarization signal to feed each individual receiver by that receiver’s voltage.  With only two receivers (I think your arrangement), each receiver can select the proper polarization within the LNB by moving the voltage above or below the switch threshold.  Since each receiver provides its own voltage to it’s port on the LNB, it is unlikely (but not impossible) that both receivers are bad at the same time.

None of this helps you figure out the problem, other than the LNB is probably bad.  But, you could measure the DC voltage on the coax at the LNB, and see if it is there, and if so, if it changes as you change channels.  Keep in mind that most transponders carry more than one channel in the digital world, and you may need to do a lot of channel changing to see the voltage change.  You could also measure the voltage at the back of the receiver, by CAREFULLY using an F tee.  That would not tell you if the  coax is bad, but the coax is probably the least likely to fail in the whole system.

I’m sure there are a LOT of satellite troubleshooting guides and information on the net these days.  It does sound like the LNB is fried.  They are cheap, and pretty standard.  I’ve even seen them come free, as part of a free dish program.  The LNB physical mounting is probably the least universal thing about them.

The above is based on the older dish systems (like DirecTV), without HDTV and multiple satellites/feeds.  But, with the newer systems, most of them just hang multiple LNBs at different “focal points” on the same physical dish, and therefore require additional coax leads.

Good luck.
Terry, WB4JFI


From: rabruner at aol.com 
Sent: Thursday, August 04, 2011 1:00 AM
To: tacos at amrad.org 
Subject: Re: Tacos Digest, Vol 102, Issue 8

The power is multiplexed over the coax from the satellite receiver.  You should be able to measure about +20 volts or so (there is a wide range of voltages used and it's not critical)  at the input F connector on the receiver and again at the antenna end of the coax.  Unlikely the coax has failed.  It's possible you've lost the power from the receiver to the antenna due to a failure in the receiver, but more likely the LNB is toast caused by some nearby lightening surge.  Consumer LNBs are not terribly expensive, you just have to get one with the right IF for your receiver and with the right feed setup -- one bird, two bird, for your system. 
    The LNB contains a local oscillator and mixer that converts the incoming signal from the satellite to an L-band IF frequency at a pretty high level.  The down mixing is what distinguishes the LNB (low noise block converter) from an LNA (Low Noise Amplifier).  LNAs are not commonly found on systems these days as they require that everything is handled on frequency which requires either that the receiver be at the dish, or that you have wave guide all the way to the radio.  LNBS are more less a frequent failure item -- they live in a harsh environment.  The wonder is that they don't fail more often than they do.
Bob Bruner
W9TAJ


                              >> When examining the LNB, I did not see any power wires, just the RF lines.  I have wondered, and do not understand how the LNB gets it's power.  Robert, anything you could say about this would help.


                              Best Wishes
                              Richard Demaret
                              KI4KXJ

                                                                 
                             
                       
                 
           
     




 

 

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