On Wed, 11 Nov 2009 16:30:03 -0500, Joe Subich, W4TV wrote:
>You see
>a change in resonance because of the coupling via the coax
>shield to one side of the driven element. That effect is very
>difficult to predict due to the unknown length of feedline and
>its routing (capacitance between shield and tower/mast).
There is a toroidal "balun" inside the drive motor enclosure for my
3-el SteppIR. This won't do much on 160M, but you can add a coaxial
common mode wound on ferrite cores at the antenna feedpoint. If you
tune the choke to the operating frequency of the SteppIR, it will
tend to reduce receive noise. If you add a choke tuned to 160M, it
will isolate the driven element from the feedline on 160M, so you
won't see changes in the loading on 160 as you change the SteppIR
driven element. Of course, if you WANT that change, don't use the
160M choke. :)
73,
Jim K9YC
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160 meters is a serious band, it should be treated with respect. - TF4M
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