Hi Jim Jarvis, I need to keep the two Jim's identified in the post.
I agree with you about the line loss and what it does to swr readings. I
didn't think that was what you were saying originally. I had perceived your
post as saying the swr would change because of the impedance being different
at each point along the line.
I don't understand what you are saying below that I am ignoring? What does a
"different matching solution" have to do with the swr being different?
If the line has say a 3:1 swr due to mismatch at the antenna and you add or
subtract a small amount of line to make it an exact 1/2 wave length or 1/4
wave length the swr is still going to be 3:1 on the line. (less of course
any additional losses or gains due to cable loss) But the impedance
transformation will change at the transmitter with changes in line length.
The swr of the line is set by the mismatch between the antenna and line.
Nothing you do anywhere on the line will change that. If the line is a
quarter wave, half wave or something else the swr will be the same. Yes you
will see a different swr with more cable loss but what ever you do as far as
impedance matching at the transmit end has no effect on swr.
Changes in line length will give a different impedance transformation as you
> The other thing which you ignore, when you say the vswr is
> constant along the transmission line, is that the matching
> solution is different, at every point along the line, UNLESS
> the antenna presents the coaxial cable's characteristic impedance.
> And the situation is different for every frequency at which the
> line is used. i.e. 8' is a nominal quarter wave @ 30MHz, while
> 33' is the same at 40m. A dead short will look like an open, through
> a quarter wave impedance transformation.
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