On Fri, 16 Nov 2007 00:17:55 -0500, Peter Dougherty wrote:
>When I put my
>MFJ-259 on the coax at the shack end of the antenna I'm seeing pretty
>close to 50 ohms over a relatively wide range
The feedline is doing two things to you. First, it is transforming the
impedance by virtue of its length. If the line had NO loss, the SWR would
remain constant, but the measured values of R and X will change depending
on the length. Study the section in the ARRL Antenna Book about the Smith
Chart to get a mental handle on this.
Second, any transmission line reduces the SWR by virtue of its loss -- the
more loss, the closer the SWR falls back to 1:1.
To get an accurate measurement of the feedpoint impedance, you need to
either measure AT the feedpoint, or use a Smith Chart to transform the
impedance back to the feedpoint. To do that, you also need to know the
length of the line.
73,
Jim K9YC
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