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Re: Topband: Common mode choke 160m inverted L

To: ON4GPE <>
Subject: Re: Topband: Common mode choke 160m inverted L
From: Guy Olinger K2AV <>
Date: Sat, 23 Dec 2017 15:08:47 -0500
List-post: <>
Hi Gert,

What you are suggesting, *** at 1.8 MHz ***, even with 34 of those cores on
a 1 meter length coax would have only a few hundred ohms blocking, not
nearly enough blocking to do anything serious. It might somewhat knock down
back-feed of noise from the house running out to the antenna, but you could
always do much better. If you are trying to keep the transmitted RF at your
antenna feedpoint from using your feedline as its primary counterpoise,
it's far short of what's needed.

For one of my chokes I have six separated turns of RG400 wound through a
stack of five FT240 form #31 toroidal cores taped together. That's to keep
1.8 MHz from using a run of feed coax as a very lossy untended parasitic
element.  The  RG400 is a much smaller outside diameter than 393, but RG400
is rated at 7 kW. It is not necessary to use RG 393. If you're worried
about the beads heating up and destroying the coax, you're already designed
wrong. A well designed choke at ham power levels will not heat up,
especially not to the point of destroying ordinary coax.

I chose the RG400 because of 7 kW, silvered fine copper wire in shield and
center conductor, Teflon dielectric and jacket, it's small, it was designed
to be bent (center conductor is stranded, for near right angle turns in
aircraft wiring harnesses, it won't cut through the dielectric at a bend),
and it is still being manufactured new.  RG400 lasts forever, so these
longish surplus jumpers with connectors on eBay will always do the job for
shack wiring, especially the ones with a male BNC on one end which saves me
the hated job of affixing BNC connectors.

You need to read and digest K9YC's work:

It's quite more general in scope than just RFI. It's just that RFI is
usually making a ham station unworkable, blowing up contests and missing
that once in a lifetime DXpedition. It's a hair-on-fire emergency that can
finally force a ham to address common mode issues. So Jim stuck that in the

Best of luck on your project and a merry holiday season.

73, Guy K2AV

On Sat, Dec 23, 2017 at 1:27 PM, ON4GPE <> wrote:

> Hi All,
> I would like to build a good high power common mode choke for 160m.
> I have a piece of RG393/U ptfe 10mm coax. (1 meter)
> I was thinking of using this and make me a "W2DU style" choke.
> Searching for the ferrite cable cores, i found on the Mouser website 2
> types which would fit:
> Fair-rite 31 material :
> Laird  LF material :
> Laird got a higher impedance at 1.8 Mhz.
> I think a lot of ham's on this topband reflector got more experience, so
> what's your advice ?
> Thanks, Gert  ON4GPE
> _________________
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