Topband: "Center-tapped" common mode choke

[DAVID CUTHBERT]

With this arrangement - beads,GND, beads - the 20' of coax between the whip
and the GND connection continues to operate with the whip as one half of a
dipole. The whip presents a high impedance to the 20' length of coax - and
the chokes on that side will not be effective until the impedance is tens of
thousands of ohms. If enough beads are used so that they are effective they
not only reduce the signal picked up by the shield but by the whip. The whip
needs an impedance to work against that is approximately equal to or lower
than its own (3 k ohms).

The GND at the 20' point and the beads between the GND and the far end of
the coax do form series and shunt elements making a filter for signal
current induced onto the coax shield. So, the GND connection is essential
and the beads on the far side are helpful. Let's say the GND looks look 100
ohms at 1.8 MHz. And the shield looks like a 100 ohm source as 1.8 MHz. The
GND connection has reduced the shield pickup by 6 dB. Now place beads on the
shield for an impedance of 500 ohms and the shield pick up is reduced by 16
dB.

   Dave WX7G




On Sat, Jul 4, 2009 at 11:49 AM, Pete Smith <n4zr at contesting.com> wrote:

> Apologies for being less than clear - the choke that gave the much
> better results was built on the inspiration of ON4UN's low-band DXing
> book, 4th edition, figures 7-88.  A length of RG-59 coal is used,
> with two stacks of 50 beads each, and a ground between the two
> stacks, using a single short ground rod.  There is no shield ground
> at the antenna, and the distance between the ground rod and the
> antenna input is on the order of 20 feet.
>
> 73, Pete N4ZR
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