At 5/10/2008 21:38, you wrote:
>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.
Yup, all makes sense. The 2 meter repeater I recently built & put on the
air here with 10 dB of desense is down to ~2 dB of desense at the site, &
that's probably due to the antenna being fed with RG-8 (I pointed that out
to the owner the second I found out, but he had already installed it; we
already have plans to rip it out & replace with 100% 1/2" hardline & RG-214
from the duplexer T on out).
> 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
Not sure what you're getting at, but there is a direct correlation between
VSWR & impedance. It's possible to have several different complex
impedances that all result in the same VSWR, but if the impedance is 50 +
j0, the VSWR by definition is 1:1.
>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.
Your bad jumper was probably copper braided. The double shielding isn't
that important, but since single-shielded silver braided coax is pretty
rare, just making sure all duplexed lines are hardline or silver braided is
good enough.
>3. Bench testing duplexers into a dummy load may not duplicate the
>results experienced with the antenna.
Yes, but at least you know the duplexers are performing to specs, & hence
probably not the problem.
>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.
Very true, especially on 2 meters where the short 600 kHz spacing lends
itself to all kinds of mixes within consumer electronics devices.
>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.
When I run into them, I just screw all the helicals all the way down & then
retune. I figure if they took 30+ years to grow long enough to short
something out, it'll take another 30 years for them to grow back again. If
I'm still using this nearly obsolete equipment after another 30 years
something's wrong :O
Bob NO6B
