Topband: CAT6 Feed Line

[Guy Olinger K2AV]

Hi Mikek,

The trick is to remember that just one common mode block rarely does the
trick. Usually if it's bad the first one will make a difference. The second
maybe not at all. What happens with the second depends entirely on where it
is with respect to the first block.

The trick is to understand what happened when the first common mode block
was installed at the feed (isolation transformer in your case). That became
a high impedance, high voltage point or voltage node in the common mode
standing wave on the feedline, like the ends of the wire in a dipole. If at
1/8 or 1/4 wave away from the feed, there is no additional block, that
point with its common impedance to the ground, will become a low voltage or
current node in the common mode standing wave on the feedline. The way to
really get to the common mode, is 1/8 to 1/4 wavelength from the the
voltage is to put in another common mode block. Now BOTH ends of the common
mode between the two will want to be a voltage node. But since neither end
can pass much current now, NEITHER end can be a voltage node because
neither opposite end can be a current node.

You often hear the answer that the second common mode block made all the
difference. That's because it takes TWO blocks to insure that neither end
can pass current, so neither end has an opposite current node so it can be
a voltage node. Two blocks at 1/8 to 1/4 severely restricts the ability of
the blocked stretched conductor to carry current.

Some people will say to quickly ground coax shield away from an isolation
transformer. The problem with this is that it sinks common noise from the
house into the ground conductivity around the RX antenna.  Ground it for
electrical purposes where it goes into/comes out of the house. Then add a
common mode block on the antenna side of the grounding. You don't want
common mode current to have an easy current path to ground anywhere.

Hope that helped a bit.

73, Guy K2AV

On Tue, May 19, 2020 at 10:08 AM Mikek <amdx at knology.net> wrote:

> Now that I have the CAT6 feed line ingress mostly tamed.
> (Installing the isolation transformer did that.)
> What methods can I try to reduce the ingress even further?
>
>   I have tried coiling the CAT6, 15 turns in 8" coil. That increased
> the ingress. I have a huge Iron core toroid, 15 turns on that also
> increased ingress. (I know wrong material. but it was there)
>   I tried a much smaller CMC, it didn't help or hurt.
>
> So, What methods can I try to reduce the ingress even further?
>
>                                Mikek KF4ITA
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