At 9/4/2008 07:35, you wrote: > > >There have been example cases where unwanted product generation > > >has been "fixed" by replacing portions of the antenna system > > >coaxial cables with a less or lower Q cable. Some transmit > > >antenna combiner low-level generation issues have been addressed > > >with lower-Q coax jumpers. > > > Not really a fix. "Lower Q" in transmissionlinespeak is "lossy". > >If the (relative higher) Q of a coaxial transmission (in this >example) line contributes toward unwanted interactions and/or >product generation... > >Using a lower Q cable like RG-214 with more loss/ft... compare >that loss value against rigid or higher Q lines (at UHF), its >sometimes a much desired pad effect well worth the trade.
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. The one place a pad (or lossy coax) can go without affecting system performance is between the preamp & RX, 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. > 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. 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. Granted, depending on the level of system performance needed vs. quality of equipment involved it may not be worth the effort to properly remedy the interference. My point is that if one is trying to fully maximize system performance (maximum power output, maximum possible effective sensitivity), padding the TX or RX is not the answer to an interference problem. > > Using a lossy cable to fix some interaction between, say a > > TX & duplexer and/or antenna is IMO a band-aid solution. > >Mil-Spec (quality) RG-214 is the flexible cable used by >many/most companies making antenna combiner systems and >duplexers.... not the RG-400 (RG-58 sized) type Teflon cables. >I don't believe they feel using RG-214 coax is a band-aid >solution... Nor do I The short lengths involved do not introduce significant loss, & 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. Of course, those cables are more expensive & would drive the unit cost up as well. However, I believe there are several on this list that have in fact replaced their duplexer & interconnect cables with Superflex. > > Particularly at low-level sites, I find I need all the > > performance I can get & have very little margin for any > > additional loss in either the TX or RX path. > >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
