[Elecraft] K3 SWR Accuracy - reprise

[Kok Chen]

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