Vic Rosenthal wrote:
>Jon, if you were right, SWR meters would be useless, since readings would
>change
>as cable lengths changed! You may be misled by the fact that in practical
>systems, common-mode ('antenna') currents on the outside of the coax often
>disrupt SWR readings.
>
Obviously, Vic, you've missed the whole discussion I have participated
in. If my transmission line impedance is the same as the impedance that
I am operating in, then you move along a constant VSWR circle as you
change the length of transmission line. Yes, the SWR remains constant.
But in the case that Rich is using here, our transmission line impedance
is 93 Ohms and is not the same as the impedance of our "system" which is
50 Ohms. I define the system impedance to be 50 Ohms because our exciter
and SWR meter are based on 50 Ohms. If we define 93 Ohms to be the
center of our Smith Chart, there is a constant VSWR cicle. However if
you define 50 Ohms to be the center of the Smith Chart, your rotation
around the 93 Ohm point is no longer centered on 50 Ohms! So you cannot
say that the VSWR is constant. You've changed transmission line
impedances!
In a 50 Ohm normalized Smith Chart, the 93 Ohm point is at the 1.8
constant impedance circle.
The 50 Ohms of our load impedance is now at the center of the Smith Chart
(certainly you'd agree that if I hooked our 50 Ohm load directly to the
50 Ohm VSWR meter, I would read an SWR of 1:1). Now as we add length of
93 Ohm transmission line to that 50 Ohm load, we would move on a circle
centered at 1.8 on the Smith Chart. At 1/4 wavelength, we'd be at 3.6 on
the real axis or 180+j0 Ohms. At a 1/2 wavelength we'd be back at our 50
Ohm point in the center of the Smith Chart. Now, using the VSWR scale on
the bottom of the Smith Chart, what does a reading in the center of the
Smith Chart give you for VSWR: 1:1!!!!!
QED!
Vic, if your theory was correct, then how do 1/4 wave transformers work?
I have two omni directional antennas stacked for 6 meter operation at my
station. Coming off of each is 12 feet of RG-11 (75 Ohm) coax. These
are brought together and fed with a T-connector. At that common
feedpoint, my VSWR is 1:1.
How does it happen if your theory is correct?
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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