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Re: [RFI] Earth isolation

To: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>, <rfi@contesting.com>
Subject: Re: [RFI] Earth isolation
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Wed, 8 Sep 2004 18:20:58 -0400
List-post: <mailto:rfi@contesting.com>
> What you say is true, Jim... but having been picked up by
> antennas, that RF current also needs somewhere to flow
*to*. Whatever
> mother nature hasn't enlisted as an "antenna", she'll try
to enlist as a
> "ground" or "counterpoise" instead.

Hi Ian,

I think I know what you meant, but I'm not sure I agree with
that picture. RF that is accidentally picked up doesn't need
to flow anywhere. It isn't a bucket of something we have to
pour out somewhere.

As worded above, it reminds me of the popular (but
incorrect) idea that low-pass filters shunt unwanted
harmonics to the filter case, and the case needs to be
grounded to get rid of the harmonics. People think because a
wire is connected to what they call "ground", it magically
isn't an antenna. There are even those goofy "coaxial
grounds" or "shielded grounds" around.

> There are some situations, particularly involving end-fed
wires and
> inverted-L antennas, where large ground return currents
are unavoidable.
> These return currents will divide among all possible
> conductors, according to the series impedance that they
see along each
> pathway. And if even a small percentage gets into the
wrong places -
> especially the mains wiring - it can cause significant

The proper solution is to isolate the common mode RF ground
path via the outside of the feedline before reaching the
rig. It's better (and safer, and easier) to not allow the RF
into the shack.

> I had that problem with the low-band antennas here. There
was no
> possibility to change either the antennas or the feedline
runs; both
> were in the only places they could be. Enough RF current
was flowing
> back through the shack, and out again into the mains, to
cause RFI to
> the neighbors. The cure was to place an RF block in the
shack mains
> feed, using a three-wire filter which includes a
ground-line choke. The
> unwanted RF current coming into the shack is greatly
reduced, because it
> no longer has a low-impedance exit path into the mains.

It seems to me a feedline shield choke would be safer and
much more effective. In particular when a person does not
now how to build a decent and safe power mains choke.

Too many people think baluns are not necessary on balanced
antennas like dipoles. Too many people think unbalanced
antennas like verticals or inverted L's are perfectly
unbalanced, and don't need baluns.

If you are going to go through the difficulty of blocking
the mains for RF, you probably need to just over a couple
feet further and stop the equipment from being part of the
antenna's ground. I can't think of a single good reason to
allow the RF into the shack only to stop it at the wall
outlet. It not only is less safe, it is more complicated to
stop it on the mains.

73 Tom

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