[Tech-Assist] STL problem now critical

[Burt I. Weiner]

I've been reading this thread for a while and will now add my 2-cents to the pile.  First off, I will make an assumption that in the beginning the path was properly engineered.

I believe you said this is a Moseley 6030 or one of the 6000 series systems.  The AFC does not tell you anything about the incoming signal.  It is an internal measurement of the receiver or transmitter telling you if it's innards are locked.  Again, this will not tell you if the transmitter and receiver disagree as to frequency.  You really should check the transmit frequency against a known counter, service monitor or a spectrum analyzer that has a calibrated frequency counter.  To measure the receiver, you have to measure the L.O. injection.  If the frequency is off in either the transmitter or the receiver you will need to replace the crystals.  The crystals do age and it's not uncommon for them to require changing.  Strange as it may be, the 6000 series radios DO NOT have netting capacitors to trim frequency - you need to replace the crystal/s.

With regards to Mark / Anixter Mark, and whoever the new company is who took over the line, you need to disassemble the feeds and see if they have water in them or have damage from water.  Contrary to what the manufacturer will tell you, I have yet to see one of their feeds that has the "O"-rings greased from the factory.  In fact, on many occasions I've open up new feeds only to find them totally dry.  This has been the case with these feeds for more than twenty years that I personally know of.  Recently I had to disassemble one that had similar symptoms to what you are describing.  What I found was that the end of the rod that connects the Type-N connector had disintegrated from water sitting in the feed at the point were it connects to the back of the Type-N connector.  I had to replace the connector and re-machine the end of the rod to bring it back to life.  In any case, I really suggest that you disassemble the feeds at both ends and completely give them a thorough inspection and properly "grease" the "O"-rings.  They are easy to disassemble.  By the way, if they are properly sealed, it shouldn't make any difference whether the radome is up or down - it didn't leave the factory with a weep hole.  I normally put it on the down side.

Something else I've run into: The polarity of the feed and the dish grid must agree.  I've seen situations where a system polarity has been changed and rather than flipping the dish, only the polarity of the feed was changed.  Sometimes we stare at these things and never see the obvious - at least that's my case!

Make sure the dish is properly aligned and not on a side lobe - both azimuth and elevation.  Have your rigger check to see if the feed is bent.  Falling ice can tweak them and even a small amount can seriously affect the antenna's gain.  Makes sure a tree or building hasn't gone up in the path.  A few years ago I got called to check a system only to find that birds had built a nest on the end of the receive feed.

It never hurts to look at the channel with a spectrum analyzer using the STL's receive antenna.  In the case of a digital signal, an interfering signal can be low enough to not open the squelch or read on the meter, but still strong enough to corrupt data.  In an analog system it can add noise to the signal, especially if you have processing after the STL.  While pagers are not as common as they used to be, look down into the 930 MHz region and see if there is anything there,  Watch it for a while, while listening to your own signal.

Burt