The whole idea of a handheld instrument is
that you can climb the tower and measure directly
at the antenna. If you want to stand on the
ground, and measure through a line, you
can use laboratory test equipment, which
is generally much more accurate.
If you insist on measuring through a line,
you should make calibration measurements
of a short and open at the far end, at
each frequency. If you are not certain of
the Zo of the line, you should also make
a measurement of a 50 ohm resistor at the
far end. You might then be able to mathematically
back out the effect of the line. (As Tom
mentioned, the fact that R!=G on the line
means the Zo is complex, so the math may
be nontrivial!) Laboratory network
analyzers may be able to do this
automatically.
You can either measure the line on the ground once,
and hope it doesn't change, or you can mount a
relay box at the antenna that allows you to
connect open, short and load calibrations remotely,
and do an insitu calibration.
You might also be able to calibrate out the
balun effects.
All in all, it sure sounds easier to climb
the tower, especially since you need to do
that to change out the wires.
One additional comment re: wire beams. The
insulation on the wire not only changes the
resonant frequency, but also the mutual
coupling. This plan only compensates for
the former.
Rick Karlquist N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com
>
>
> The length of feedline from the antenna to the measurement device
> will affect the measured resonant frequency, so you also
> must be sure
> the feedline is an exact multiple of 1/4 wl to determine EXACT
> resonant frequency.
>
> If it is not an exact multiple of 1/4wl, the line will shift the
> reactance off center or even create a "false' indication of antenna
> zero reactance. That's because the line will have some SWR, and the
> SWR will cause reactance shift in the line. An odd- 1/4 wl
> line will
> invert the sign of the reactance, but will not alter it's absolute
> value assuming the line is lossless.
>
> The next problem you might have is line loss. Feedlines do not have
> evenly distributed losses between conductor series resistance and
> dielectric losses. The conductor loss dominates the system. The end
> result of using a lossy feedline is it adds reactance to
> the system,
> skewing the results.
>
> You are trying to measure one of the most difficult things
> to measure
> with near-perfect accuracy. You will probably learn more about how
> difficult measurements like this actually are than you will
> about the
> insulation. Be careful the measurement method you use and equipment
> errors don't determine the results!
>
> 73, Tom W8JI
> W8JI@contesting.com
>
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