Topband: Common mode chokes

[DAVID CUTHBERT]

Very good. Another experiment would be to ground the coax shield 20' from
the antenna as you did but with no ferrite beads. I suspect you will see
similar results.

Yes you have a 108' dipole comprised of an 8' whip and 100' of coax shield.
The 8' whip presents an impedance of 3000 ohms and it would take many, many
beads build a balun having an impedance much greater than this.

     Dave  WX7G

On Sat, Jul 4, 2009 at 8:16 AM, Pete Smith <n4zr at contesting.com> wrote:

> I thought readers here might be interested in some crude experiments
> I have been doing here with a Clifton Labs active antenna and various
> common mode choking schemes.  It's conventional wisdom that small
> antennas like this need common mode chokes to keep feedline pickup
> from diminishing directivity or increasing the overall noise
> floor.  What I didn't realize was how much difference it could
> make.  My noise floor dropped about 20 dB by using the
> center-grounded type of bead choke instead of the same number of
> beads, in the same place on the coax, without the center tap.
>
> You can take a look at my results at
> http://wiki.contesting.com/index.php/Common_mode_chokes, which is an
> Open Notebook on the Contesting Compendium.  The Open Notebooks idea
> is based on a concept called "Open Wetware"
> <http://openwetware.org/wiki/Main_Page> for sharing scientific
> research in progress. At K3NA's urging, we've incorporated something
> similar in the Compendium as a place for contesters to share their
> projects and experiments while they are in progress, in the hope that
> others will see, comment, and improve.
>
> Take a look, and tell me what you think!
>
> 73, Pete N4ZR
> New Articles Daily - the Contesting Compendium at
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>
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