Topband: Common mode chokes

[Pete Smith]

Inteesting.  Are your assumptions changed by the presence of the 
amplifier at the base of the whip, which is designed to match to 
75-ohm cable?  I will give the beadless version a try.

73, Pete N4ZR

   At 09:24 PM 7/4/2009, DAVID CUTHBERT wrote:
>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 
><<mailto:n4zr at contesting.com>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>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>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
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>
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