[Tech-Assist] STL problem now critical

[Chip]

From: Jerry Mathis <thebeaver32 at gmail.com>
>BTW, yes, I've connected the dots and am also wondering, since there is a 6' dish on the other end, maybe the two got mixed up when the system was installed? I don't know yet, but I'm sure going to find out. I can't see the STL dish on the receive end very well, even with binoculars, because it's so high up.

Jerry:

A couple of things:

It doesn't matter from a path standpoint which end the larger dish is  
on.  The path loss will be the same.  However, if there is a receive  
interference issue, the larger dish may be needed on that end.  If  
your STL transmitter is interfering with some other system, the larger  
dish MIGHT be needed on that end.  On the other hand, the minor lobes  
will be at different angles, so sometimes a smaller dish will move the  
minor lobes to minimize interference.

I also wouldn't move different sized dishes from one tower to another  
without running tower loading studies.  They may have been chosen  
because of tower loading.  Swapping them may result in a tower  
failure, especially in a time of ice or high winds.

Just putting a directional Wattmeter at the base of the tower is only  
a little useful.  You need to supply the rigger with a similar  
directional Wattmeter and a dummy load.  He needs to put the Wattmeter  
in line with the load, then with the antenna.  This is the only way  
you can check the line and the antenna, short of using a network  
analyzer, which is a good thing to have as well.  With the NA, the  
rigger can adjust the screw(s) on the feed for best match.

Next, the multimeters on STL equipment is kind of a rough guess.  They  
are not precision instruments so I wouldn't get too wrapped up in 300  
versus 934 micro-Volts, plus many times the path isn't as good as  
planned, yet it works.  If it has been working at your 300 indicated,  
and it still indicates 300, that isn't the problem.

You have to find what has changed.  If the received signal strength  
has not changed, it isn't the problem.

Look for new interference sources.  One way I have done this is to  
connect my Icom IC R-7000 receiver to the antennas and see what I  
hear.  Amazingly, in the DC market I can go to nearly any 900 MHz  
paging antenna and pick up an STL on every STL channel.  What is  
amazing is that I can hear transmitters which have dishes pointed away  
from the receive antennas I have used.  The spectrum is thick with  
users, and DC has a number of co-channel users.  Paging antennas are  
vertical, but they pick up those horizontally polarized STLs very  
well.   Those few digital STL users have complicated using the R-7000  
a bit.

Also, check with stations within about 100 miles (considering your 18  
mile path) and see if they have made any changes on co or adjacent  
channels.  You may have someone new sharing your spectrum.

Good luck.

--chip

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