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[TowerTalk] Re: Force12

To: <>
Subject: [TowerTalk] Re: Force12
From: (Tom Rauch)
Date: 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 

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 

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