Hey Tom,
I think we agree on about 99% of what has been discussed. Please
see my comments below:
W8JI wrote:
>
> 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.
>
Not sure what you mean by "across and through" vectors.
> 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.
>
Yes, in the case of the Force 12 Antenna with the 50 ohm feedline,
I would agree 100%.
> 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.
>
Yes and No. If we put the 50 ohm VSWR meter in the 75 ohm line purely
for the sake of academic discussion, then it would be somewhat foolish.
On the other hand, my 50 ohm equipment really doesn't care what the
75 ohm meter reads or what is the actual ratio of voltage maxima to voltage
minima. It cares how close the line's input impedance is relative to 50 ohms.
My 50 ohm meter will tell me that. Thus I would argue that there is value in
installing the 50 ohm meter in the 75 ohm line, because it will help me
determine where to cut the coax for best match to my 50 ohm equipment.
That was the whole point of my example of the 75 ohm line with the 50 ohm
meter, and I think the point of the reference to the Leeson article.
Unfortunately, neither of these examples have any bearing on the Force 12
situation as that is as you pointed out, a 50 ohm line in a 50 ohm system
(I thought you were making a general statement about line loss and VSWR
bandwidth rather than the more specific statement where the line Zo and the
the system impedance are equal).
> SWR by *definition* is the ratio of amplitude of a standing wave at
> a node compared to the amplitude at an anti-node. >
Yes, I agree with your definition of SWR. If I probe the RF voltage along
the length of a mismatched transmission line, the VSWR is simply
the ratio of the maximum voltage to the minimum voltage. The minima and
maxima would be 1/4 quarter wavelength apart. If the line were lossy,
I would find that the ratio of the minima and maxima would decrease as
I moved toward the generator. On a lossless line, the ratio will remain
constant.
>Impedance is not in the definition.
Yes, but the relationship between the line Zo and the load impedance are the
parameters which wholey determine the value of the standing wave ratio
(at least in the lossless case).
> 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 de Mike, W4EF.........
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