[EMAIL PROTECTED] wrote:
> Why so much loss? I have a 2 port combiner I built that has about 1.5 dB
> loss. Are your TXs close-spaced in frequency?
>
> Bob NO6B
Ed covered it... it's the type of combiner it is. :-)
145.145, 145.385, 146.67, and 146.94 were the original frequencies in use.
The bottom two are ours. 145.145 is the machine blowing PA's. VSWR at
the input to the first isolator looks better at our frequency than it
does up at 147, actually.
Someone tested the ports with one of those MFJ analyzers... not the best
test gear, but someone had one handy and everyone happened to be up
there working on things at the same time, so it was a quick-and-dirty
test to see how things looked in general.
145.385 is our port still on the first "half" of the combiner, but not
currently in use. 146.67 and 146.94 are on the other half of the combiner.
The combiner system was spec'ed by a previous site manager and required
by the site. Then there's the usual three or four changes of ownership
and the new owners saying, "What combiner?" when you talk to them... the
usual craziness at a site that changed ownership hands a whole bunch of
times.
We installed the pass cavity to help out the 146.67 and we've notified
their lead tech that we noticed we would desense their repeater from
time to time when they were experimenting with a different receive
antenna off the combiner system. On the combiner, we never bother them
that I know of.
We set up the can for 1dB insertion loss and that resulted on that
particular cavity we had available to be about 10dB down at their input
frequency (146.07) on the service monitor/tracking generator. (Their
input frequency is 0.925 MHz up from our output frequency, obviously.)
Using a Bird looking into the can, I remember the VSWR numbers there
were "reasonable" too. I don't have the numbers handy, but seems like
the PA was puttering along at about 80W, with less than 1W reflected
there at that point.
No signs of trouble until a month or so after that visit in the fall.
Then the rapid decline of the PA (the last one burnt up the Wilkinson
divider resistor... the "usual" M2 failure mode), and loss of power.
One person locally who likes to measure things off-air just to see
"relative" signal strength said his measurements from his home on his
service monitor from a "known" antenna (yes, we all know that's not a
100% perfect measurement, but it's nice to know if gross changes happen)
pretty much conclusively showed that the PA is completely gone at this
point, and we're not getting any more than exciter power to the antenna
now... if that.
Thoughts on all that... 80W is probably pushing the M2 PA just a bit too
hard at that high altitude (11,440' MSL), but it would have been awfully
cold up there throughout the failure timeframe. There's probably no way
it was ever more than 50F and probably more like 30F the entire time.
However, the repeater had been VERY busy up until the failure, and was
probably keyed 8-10 hours or more a day in 2 hour blocks... it's a
ragchew machine and with the high coverage, gets used a lot.
Big hot/cold cycles when it would go quiet, I'm sure. (Yeah, we know
about the other "usual" failure between the final board and the low-pass
filter board...)
The site's great... but on VHF... it's painful. Two crispy-critter VHF
PA's later, and a question that the repeater that was handling 145.385
might have been unhappy there before we pulled it down, too.... and
we're all scratching our heads a bit wondering why the heck they're
blowing up.
It honestly could be that we've just been exceedingly unlucky this
repeater "season"...(when you know you're snowed out much of the year,
you think in terms of "seasonal" repeater maintenance!).
We'll know more when we can go measure the various test points for weird
return loss numbers or changes, and get a visual look at that PA, but
the other systems are apparently happy and healthy up there right now,
so it's not likely it is antenna or feedline or anything like that at
this point.
Quite the mystery, but right now I'm inclined to find a PA that's happy
at around 50W output, and try again. Without a Z-matcher or anything
other than turning the power down by a considerable margin (while still
pushing the PA hard enough to keep it from getting dirty, but
considerably lower than its rated output).
I guess other useful info might be that it's a MASTR II power supply...
Not much in the way of big concerns that we have dirty power or
anything, but we're at least going to check that out too...
Quite the intriguing mystery, Dr. Watson...
Indeed Mr. Holmes. Indeed.
And the new UHF goes up in the Spring -- which should be a heck of a lot
nicer to maintain and deal with, and will act as a backup in case this
VHF peters out again...
Nate WY0X
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
