Here is a link to a web site with a table of data for solenoid chokes.
Unfortunately the impedances measured are for a choke alone and not for one
installed where there is coupling to the coax on each side, the antenna,
the tower and other things. Obviously if you were lucky enough to have the
choke resonant with all these outside sources of capacitance it would be
very effective . I have actually used a variable capacitor across an
inductor made of coax feedline to stop RF traveling on the outside braid
in a project at my work place.
It is much more effective to make a choke using type 31 ferrite as
described on K9YC's webpage. I have made and measured a number of chokes of
that type using my signal generator, a test jig, and a wide band scope with
similar results to his. Jims technique of operating the ferrite on
frequencies where the impedance is largely resistive is a very good idea.
You can buy the ferrite cores at Mosely or other suppliers for a few bucks
and wind your own. If you would rather buy one already made and in a
weather tight box try Balun Designs. I have made a number of chokes like
their 1116d myself and they are very effective from 160-10m
On Sat, Oct 20, 2012 at 8:56 AM, Jim Lux <email@example.com> wrote:
> On 10/20/12 8:47 AM, Cqtestk4xs@aol.com wrote:
>> Along with that line of thinking...using RG58 on a 4 inch core. About
>> many turns? My thinking is around 25 or so.
> why not 4 or 5 turns on a 2.4" ferrite/powdered metal core? 4" PVC core
> is a foot per turn, 25 turns is 25 feet of coax (which I grant you probably
> already have), but say it's 0.25/ft. that's 5-6 bucks worth of coax
> against a $5 ferrite core, which will be more broadband, higher choking
> impedance, physically smaller, etc. Air core chokes don't have a lot of
> resistive impedance: you're depending on their inductive reactance, and
> that's subject to cancellation from parasitic C, etc.
> You'd have to check K9YCs writeup to see what an appropriate material for
> the core would be. #31 is the sort of general go-to material for ham band
> use, but there might be something better for lower frequencies.
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