Hi Mike,
> > 1.) The SWR on the 75ohm line remains constant with length of the
> > 75ohm 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 antinode. 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
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