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## [TowerTalk] Re: Force12

 To: [TowerTalk] Re: Force12 W8JI@contesting.com (Tom Rauch) Thu, 11 Oct 2001 12:24:25 -0400
 ```Hi Mike, > > 1.) The SWR on the 75-ohm line remains constant with length of the > > 75-ohm line. > > > Yes, but only if you are measuring VSWR referenced against 75 ohms. If > your are in a 50 ohm system, then the VSWR measured (relative to 50 > ohms) will change as a function distance along the 75 ohm line VSWR is always properly measured or referenced to the surge impedance of a line! SWR is, by definition, the ratio of across and through vectors along a distance of line. even if we agree to go outside standard protocol and measures SWR referenced to 50 ohms on a 75 ohm line, it most certainly does not apply at all to what is being discussed. In the case being discussed, a 50 ohm line is the ONLY line impedance between the antenna and the source, so it is clearly a 50 ohm system. > > If the line is 75 ohms, SWR on the line will be 75/35 or 2.14 : 1. > > > Yes, of course, but switch over to a 50 ohm reference and then > mover your 50 ohm VSWR meter along the length of the line. > At 1/4 wavelength from the 35 ohm load, the 50 ohm VSWR will > be 160/50 = 3.2:1. Move another 1/4 wavelength (total 1/2 wavelength) > from the load and the 50 ohm VSWR will be 50/35 = 1:43:1 VSWR. The SWR on the 75 ohm line is ALWAYS the same except for loss. Of course we could introduce a measurement error, by using a meter or system set to look for 50 ohms. In that case it would look like the SWR was changing although it was not really changing. SWR by *definition* is the ratio of amplitude of a standing wave at a node compared to the amplitude at an anti-node. Impedance is not in the definition. None of that matters, even if we simply toss out or misuse the definition of SWR or use incorrect measurement techniques, because this is a 50 ohm system throughout. 73, Tom W8JI W8JI@contesting.com List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems, Trylon Titan towers, coax, hardline and more. Also check out our self supporting towers up to 100 feet for under \$1500!! http://www.anwireless.com ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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