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[TowerTalk] Common-mode current on feedline

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Subject: [TowerTalk] Common-mode current on feedline
From: (Guy Olinger, K2AV)
Date: Thu Jun 26 09:54:56 2003
Would note here that Chuck was measuring current key down, which is a
different measurement than what ON4UN was making.

Even if Chuck's central piece of coax was isolated by 100 db from the
coax either side, the transmitting antenna is able to induce some
current in the central piece just because it's lying on the ground in
the vicinity and it's a conductor.

The intent of the chokes is to keep the central conductor from BEING
IN SIGNIFICANT PLAY, either by effecting the pattern or local noise
pickup by DIRECT UNIMPEDED connection to the antenna, or along the
shield into the shack and pickup by miscellaneous devices, not to
mention RF in the shack.

The effect of the central piece of coax between the chokes is just
another piece of wire.

>>>> As a bench mark, take an unconnected piece of coax about the same
length, lay it on the ground, and make the current measurement key
down. You may be surprised how much current is induced on it. For
evaluation purposes that amount of current will need to be factored

As an aside, the "choking" ability of various devices seems to be
overstated and there is little around about measurements in actual
field situations.

73, Guy.

----- Original Message ----- 
From: "Pete Smith" <>
To: "Chuck Counselman" <>
Cc: <>
Sent: Thursday, June 26, 2003 7:27 AM
Subject: Re: [TowerTalk] Common-mode current on feedline

> At 01:03 AM 6/26/03 -0400, you wrote:
> >I agree.  Because the impedance of a common-mode choke approaches
zero as
> >the frequency approaches zero, it tends to be difficult to achieve
> >effective common-mode choking at low frequencies.  Because the
> >lowest-frequency band that I use is 80 meters, I placed two
> >chokes 70 feet apart -- about one-quarter wavelength for this
band -- on
> >my coaxial line.  In effect, the current node at one choke is
> >(potentially) to a current maximum at the other choke, which
maximizes the
> >effect of the other choke.  This arrangement is probably
responsible for
> >the superior, rather than inferior as one might expect, common-mode
> >isolation that I measured on 80 meters.  On the
> >upper bands, the spacing of these two chokes is the worst-possible,
> >course.  But another trick, described below, is effective at higher
> >frequencies. (Did you notice that my best common-mode isolation
occurs at
> >the highest and lowest frequencies?)
> This is intriguing.  I note that in his book (page 7-19) ON4UN
> using two 100-bead chokes located at least 5 meters from a Beverage
> feedpoint, with the shield of the coax grounded between the two
chokes.  He
> asserts that with 1500 ohms of impedance in each choke (at 1.8 MHz)
he gets
> minimum 70 dB suppression of common mode currents on the outside of
> feedline.  Since this methods doesn't seem to be terribly dependent
on the
> point on the feedline where it is installed, I wonder if you think
it would
> provide good common-mode suppression on the higher bands as well,
for use
> on a dual-purpose coax feedline?
> 73, Pete N4ZR
> The World HF Contest Station Database was updated 17 June 03.
> Are you current?
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