Thread: SWR - wtf?
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Old June 29th 05, 11:21 AM
Roy Lewallen
 
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Thanks very much to Owen for pointing out the following errors in my
recent posting:

Roy Lewallen wrote:
. . .
Next experiment: Connect the SWR meter through a *quarter* wavelength of
50 ohm line to a 100 ohm load. Now the impedance looking into the line
is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25
ohms as well as 100, so it still reads 2:1, which is also still the SWR
on the 50 ohm line. You can change the length of the 50 ohm line all you
want and, if it's lossless, the line's actual SWR stays the same -- but
the impedance at the input end of the line changes. For a 100 ohm load,
when the line is any even number of half wavelengths long, the input Z
is 100 ohms. . .


That last sentence should be "For a 100 ohm load, when the line is *any
whole number* of half wavelengths long, the input Z is 100 ohms."

Likewise,

Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm
line to the output of the same 50 ohm SWR meter and hook that to a 50
ohm resistive load. The line's actual SWR is 2:1 and, just like any
lossless line, the SWR stays the same regardless of its length. If the
transmission line is an even number of half wavelengths long we'll have
50 ohms at the input and the SWR meter will read 1:1, since it's a 50
ohm meter and interprets 50 ohms as 1:1.


"an even number of half wavelengths" should be "any whole number of half
wavelengths".

I appreciate the corrections, and encourage anyone who spots errors to
bring them to my attention, or the newsgroup's.

Roy Lewallen, W7EL