On Oct 5, 2009, at 10:22 PM, Thomas Oliver wrote: > My thoughts exactly! He should be losing around %35 through a six > cavity > duplexer, my 4 can duplexer loses %29. > > Tom, do you have a toggle switch in the PTT line and a local speaker > you > can listen to the received signal with? This is a must on repeaters > to be > able to listen to a noisy signal and be able to turn off the > transmitter to > check for a difference in signal quality.
Note for Tom, this is also needed if you ever have something like a Sinaddr or other way to measure 12 dB SINAD, to get an accurate base reading of how well/badly your receiver performs, and to see if it changes once the transmitter is turned on... or if you ever want maximum performance of your receiver by adding a pre-amp on the RX side. You have to measure it to know if it's doing some good... or not. > I changed to a hotter receiver one time that was almost totally un > usable > due to being noise even on a strong local signal. The problem was > traced to > the transmitter seeing reflected power looking into the duplexer. I > added > short jumpers until the reflected power went away then made up a new > jumper > with that new length. > It made all the difference in the world to the new receiver. The old > receiver did not mind at all the extra noise that was being > generated due > to reflected power. Professional/Commercial repeater operators use Isolators for this problem/reason. > My G-7 was tuned to have absolutely no reflected power at my TX > Freq. and > joints were double hose clamped and gooped with silicone so as to > keep the > moisture and corrosion out. It worked for several years until it was > blown > away by a direct lightning strike. all that was left of the vertical > part > was the one inch wire from the center of the N connector. Every fiberglass vertical deployed out here in the 2nd highest lightning strike prone State of the Union, dies this death. They make pretty white confetti all over the top of the mountain and fit nicely into a kitchen trash bag, no matter how big they once were, when hit by lightning. Blowing snow will make 'em crackle like a son-of-a-gun on receive too... and adding Kevin's favorite, those "static buster" things often makes that problem worse in our dry air out here. Folded metal dipoles (preferably the type with all parts welded together, no screws, no moving parts, period...) are the only way to fly... once the budget allows. I cringe when I hear folks are shooting sheet metal screws into multi- part antennas... if those are installed at high-RF sites along-side high-power broadcast, your antenna is very likely to become a source of Passive Intermod once something rusts or via dissimilar metals touching. Slather on the NoOX, and/or use stainless steel screws, please. Even then... PIM is evil evil stuff to find and fix. You'll spend hours on a tower with a non-conductive stick (broom-stick works well) banging on other people's antennas to find the culprit, and then you'll want to strangle whoever hung that crappy thing. Of course, if the repeater's at a site with no high-RF nearby to "light up" those "rusty joint" problems... fine. But verticals you can grab and bend over a few inches, let 'em go, and her bang, bang, bang, going on inside... really have no place on a busy commercial tower in the near-field of lots of transmitters... been there, done that. -- Nate Duehr, WY0X n...@natetech.com
