At 9/5/2008 08:46, you wrote: > > [EMAIL PROTECTED] wrote: > > I disagree. If your TX can't handle the near pure reactance > > of the duplexer at the reject frequency, the proper remedy > > is an isolator. Now there's some low Q. If the RX can't > > handle it, replace the preamp with one that is unconditionally > > stable. > >You appear to assume all problems are directly related to or >generated by/in the Tx or Rx radio equipment hardware.
Not necessarily. However, even if they are externally generated, lowering your TX power or reducing RX sensitivity is a compromise solution that fails to address the true source of the problem. I guess my biggest problem with this thread is the terminology being used. Most people here don't know what "low Q" coax is, & might assume it's generally a good thing. As I mentioned earlier, what you call "low Q coax" is just higher loss coax. Nothing more. There are places where its use is preferred, but why not call it what it is: higher loss coax. > > The one place a pad (or lossy coax) can go without affecting > > system performance is between the preamp & RX, > >My turn... "I disagree" but you want to label standard >coax as excessively lossy when in fact my point is to use >a lower Q cable where the loss is an acceptable and reasonable >amount. We may disagree somewhat in the amount of loss that we consider acceptable. My context is UHF, where the antenna noise temperature is low & foliage loss is high. At 2 & especially 6 meters where the noise is much higher, some loss in front of a low noise RX is quite acceptable. > > but even then the solution affording the greatest dynamic > > range is the one where the preamp has only enough gain so > > as to make the noise contribution from the RX insignificant. > > The GaAsFET preamps I use have 16 dB of gain, just enough > > to satisfy the above criteria on stock GE UHF RXs without > > sacrificing dynamic range. > >Kind of off topic but whatever floats the boat further >down the river... Your above flippant comment suggests that my above statement is opinion. It is not; it is fact. > > > I'd trade away a pesky grunge - gremlin or glitch problem > > > for less than a dB additional loss most any day of the week. > > > > One could achieve the same result by turning up the noise > > squelch threshold. > >You appear to be totally missing the main point. > > > Whether you hide the real source of the problem by doing > > this or throwing a pad in front of the RX or TX, you are > > avoiding the actual source of the problem. > >What if the source of a grunge problem was directly related >to or supported by unwanted energy in a high Q network/cable >path that otherwise might not even be there when using >slightly lower Q cable like RG-214? especially when the >problem/unwanted energy comes from external sources... Let's shoot down the possibilities in this example: RX overload: filter out the strong signal or increase RX dynamic range mix in TX: add isolator and possibly BPF external source: locate & fix it! As I said before, if you can't do the above then yes adding attenuation in the appropriate path may fix the problem, but accept the fact you're probably degrading system performance. All I'm asking is don't hide that fact behind buzzwords like "low Q coax" - sounds like something Micro$oft would trademark ;) >If you have a pesky grunge problem... the proper use of lower >Q coax versus say... hard-line and some higher-end coaxial >lines MIGHT HELP suppress unwanted energy within a system, >which could make the difference of a potential undesired mix >even happening. If by using lower Q hardware you don't have a >question of interference then wouldn't have to come up with >an answer to... Again, I translate the above to "my site has too much low-level crud to be able to operate a state-of-the-art repeater without diminishing some aspect of its operation", or possibly "I have an IMD problem that isn't worth the trouble to track down & I don't mind losing a db or two of sensitivity in order to fix it quickly & inexpensively". Nothing wrong with any of this, but IMO it needs to be stated clearly. > > & I believe RG-400 is lossier (hence "lower Q") than > > RG-214. Perhaps you meant to say hardline, superflex or > > some other lower loss cable. > >The manufactures data sheet tells the spec. @ 445 MHz, per 100': RG-214: 4.57 dB, RG-400: 9.52 dB. Thank you, Times Microwave. > > >You've never had a gremlin or grunge problem at a low-level site? > > > > None that had to be solved by adding attenuation on the RX or TX. > > Bob NO6B > >Never even changed out a problematic run of LMR-400 yet? The only place I'd ever use LMR-400 is for portable repeater installations, where hardline is impractical (too much flexing) & the length is too long for RG-214 due to loss. I do find that if I use RG-214 for the 1st 10 to 20 ft. of the run I have no duplexing problems. This is due to avoiding having significant RF current flowing down the outside of the LMR-400 between the braid & foil shield, where the point-contact diodes form to cause the IMD that cable is well known for, & of course properly installed connectors at both ends that don't leak any RF current onto the outside of the foil shield, especially at the repeater end of the line. Bob NO6B
