>From: W8JI@aol.com
To: <towertalk@contesting.com>
>Date: Tue, 7 Jul 1998 13:23:14 EDT
>
>I'd bet Pete has a 50 ohm bridge measuring a 75 ohm line. If
>that is the case, SWR readings will change when line length is
>altered.
>
>Remember, SWR will remain constant ONLY if the meter and line
>have the same design Zo.
>
>It's unlikely a very long feedline has enough common mode
>current left, no matter how much common mode current is
>developed at the antenna, to cause a large impedance shift at
>the feedpoint when the line length is altered far from the
>antenna.
>
>It's even more difficult (if not impossible) to find a meter
>sensitive to common mode current.
>
>73 Tom
Hi Tom,
While its true that the instrument is probably not sensitive to
common mode current, a lot depends on lead dress in a situation
like this. Even if the line is quite long, if it is not
decoupled from the antenna at the feedpoint, and not "made cold"
somewhere ahead of the point where the line length is changing,
changes in the length of the line can and will be reflected as
actual changes in the _system_ feedpoint impedance that the line
is connected across. In this case, the meter will correctly read
feedpoint impedance changes due to line length changes.
Burying the line for a significant distance probably does make
the line sufficiently "cold" for metering in the shack purposes.
73, Eric N7CL
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