I really appreciate all of the comments on my duplexer mystery.
I made sure everything was tuned properly and took it back to my
friends home location. We put it back on the air and experienced <2db
of signal desense with an output of 15 watts. That still doesn't
duplicate the results of the dummy load test, but that's a far cry
from the 30db I was experiencing here at my home shop. Some of the
things I did learn from the experience was that:
1. Though an antenna may only have a 1.5 SWR doesn't mean that the
antenna is a 50 ohm match. Using my MFJ ("Mighty Fine Junk" which, by
the way work pretty darned good, but not in a high RF environment)
antenna analyzer, the antenna that gave me the most difficulty
presented an 80 ohm load. The SWR was 1.5. A discone antenna worked
the best and it presented a 55 ohm load with a 1.2 SWR.
2. Double shielded coax is a must in repeater operation. I
experienced this first hand. I had one short jumper I thought was
double shielded and was not which caused an intermittent such that one
time it worked into a dummy load and the next it went flaky. Moving
the coax with the repeater transmitter keyed revealed the culprit. Use
hardline or double shielded coax. Hardline to the antenna is very
important.
3. Bench testing duplexers into a dummy load may not duplicate the
results experienced with the antenna.
4. In theory isn't necessarily the same as reality. I suppose if you
know all the variables the problem can be calculated and identified,
but there are a lot of unknown variables when working with RF.
5. The environment your repeater is in can cause you to loose your
hair. Yes it is related to the rf generated, no not because of the
health effects but because it makes you tear your hear out trying to
identify a problem.
6. I learned about whiskers in GE Mastr II receivers... I experienced
their effects and how to fix the problem, though maybe for only a year
or two, but I learned to to disassemble and retune the receiver.
7. Your experienced Techie RF friends are a great help. I have three
good friends that gave me a lot of ideas and helped me trouble shoot
this problem. The folks here on Repeater-Builder gave me some good
tips which pointed in the right direction as well, for that I thank
you all.
Respectfully,
Bliss
