At 4/13/2007 11:47 AM, you wrote: >In a message dated 4/13/2007 10:26:37 A.M. Pacific Daylight Time, >[EMAIL PROTECTED] writes: >2 problems: one is the LMR400. If it's brand new & the connectors are >correct (made especially for LMR400 & properly installed), it should be OK >for now but expect problems later. The other, probably more severe problem >is the Cushcraft antenna. Unless I'm mistaken, that model hasn't been >produced in several years, so the phasing harness is automatically several >years old. The braided shield of the phasing harness is not made with >silver-plated coax, so it will generate duplex noise. Replacing the >antenna is the only good solution. If that simply can't be done, you might >be able to cure the problem by stacking several pass cavity filters on the >TX to strip off as much noise off of your TX as possible. When there's no >TX phase noise, there's no noise to convert back to the input to cause the >scratchy noise you hear. However, IMD from other sources may still be a >problem. > >Bob NO6B > >Bob > >I'm a little confused by this analysis. If the copper phasing harness >oxidizes and starts generating noise, I was under the impression that the >noise point is for all practical purposes a low level broadband noise >generator. If it is broadbanded, how can additional filtering on on the TX >side dissipate what is essentially low level on-channel receiver noise. >What am I mising here? > >Thanks > >Bruce K7IJ
The oxidized braid doesn't actually generate broadband noise on it's own. What it does is act as a nonlinear mixing element to mix the TX carrier with its own phase noise. Obviously there's nothing you can do about the TX carrier itself, but you can filter the adjacent noise at the TX output to a certain degree with pass filtering, thus reducing the amount of noise available for conversion to the RX input. I tried adding a pass filter on a system that had only a notch duplexer. This system had no desense most of the time but occasionally it would get severe intermittent, scratchy desense due to the loose hardware around the antenna (it's mounted on a crank-up tower). Adding the pass cavity to the TX eliminated about 95% of the problem. Bob NO6B
