Posted by
Kok Chen on
Nov 05, 2009; 1:51am
URL: http://elecraft.85.s1.nabble.com/K3-SWR-Accuracy-reprise-tp3943810p3949666.html
On Nov 4, 2009, at 5:05 PM, Steve Ellington wrote:
> It's called a transmission line transformer and is very common.
Yes, we all know about them. Just walk 180 degrees on a constant SWR
circle on the Smith Chart, with the transmission line impedance at the
center of the Smith Chart (or use 1/4 wavelength in the Telegrapher's
Equation).
But this is what you'd stated (I am not changing a single word):
> 4. Example: A full wave dipole center fed with 50 ohm coax.
You can use a 600 ohm transmission line to transform a high impedance
to get a reasonably close match to 50 ohms because the impedance at
the center of that dipole is *not* infinite but some large number
(W8JI has good estimates in the Zepp article on his web site).
But you cannot transform anything other than a 50 ohm feed point into
a 50 ohm termination by using a 50 ohm transmission line. (Unless the
line is infinitely lossy.)
It should be obvious from the Smith Chart -- constant SWR circles
won't hit 50+j0 unless the SWR circle itself has 0 radius (i.e., SWR =
1.0:1)
73
Chen, W7AY
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