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[TowerTalk] 75ohm cable

 To: [TowerTalk] 75ohm cable "Jim Thomson" Tue, 20 Jul 2010 01:52:20 -0700 mailto:towertalk@contesting.com>
 ```Date: Mon, 19 Jul 2010 18:26:27 -0700 From: "Rick Karlquist" Subject: Re: [TowerTalk] 75ohm cable Gary Schafer wrote: > You can have up to a 2:1 mismatch (SWR) at the transmitter using 75 ohm > line > if the antenna is a flat 50 ohms. If your 75 ohm line happens to be a > multiple of a quarter wave length that will transform the 50 ohm load to > 100 > ohms seen at the transmitter end. > > 73 > Gary K4FMX Actually, the transmitter mismatch will be 2.25:1 (112.5 ohms). Additionally, if the "50 ohm" antenna is actually 33.3 ohms, it would be a 1.5:1 VSWR with 50 ohm coax but 169 ohms with 75 ohm coax. Your 50 ohm transmitter (or tuner) would effectively be working into a 3.4:1 VSWR. This may or may not be OK. Rick N6RK ## agreed. And if the "50 ohm" ant is actually less than 33 ohms, it gets even worse. ## Run YC-156 power into high swr's = asking for trbl..even with the mating vac caps tuner. Everything from arced connector's to melted hot spots on the coax. If the broadband Array solutions 50:75 xfmr's are used at each end, then you won't have any problems. With 9 x HF bands, and say just one remote switch box used... on the end of 75 ohm coax... your chances of the 75 ohm coax being a multiple of a quarter wave long are very good. I operate at the upper/lower band edges [and in between], so the chances of a multiple of a qtr wavelength is pretty good. ## In Rick's example above, the swr at the feed point is really 1.5:1 but the TX 'see's 3.4:1 . Who needs that? Later... Jim VE7RF _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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