[Top] [All Lists]

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: Thu, 9 Sep 2004 09:27:50 -0400
List-post: <mailto:rfi@contesting.com>
> Sorry, Tom, but it is precisely that!  Kirchhoff's Law
> rigorously inside the shack: whatever RF current comes in,
must have
> some path out again. If it didn't have, it wouldn't come

The system is not a constant current source and a constant
impedance system that provides a fixed amount of current
that has to flow somewhere else.

The system is full of unintentional networks and
transmission lines that have standing waves or transform
impedances, the source is certainly not constant current. If
we have unwanted current someplace the same amount of
current does NOT have to flow someplace else if or when we
stop it.

Take for example an Inverted L. I can have a path through
the radio into the house wiring that provides part of the
ground. Since that path has mismatched terminations, it has
standing waves. The current in the shack can be many times
more or less the current change at the base of the antenna
responsible for that unwanted current.

There are many common examples where that concept fails.
Base impedance of vertical antennas and efficiency is one
(people think base R somehow tells them system ground loss),
low pass filters are another (people think the shunted or
open current has to "go someplace"). 4 square feed systems
would quit working if that were true.

> (There are some exceptions, particularly on the higher
bands where
> wiring lengths inside the shack are a significant fraction
of a
> wavelength; but they don't overturn the basic principle.
Particularly on
> the low bands, I've measured a good balance between
incoming and
> outgoing currents.)

Other than a case of one or two small radios sitting on a
small desk, there is so much stray capacitance and so many
other things "hanging" off the equipment I can't even
imagine how isolating a power line common mode current
strong enough to cause RFI to neighbors is a good idea.

The real cure is to stop it at the entrance to the shack,
not the power line exit!!!

> In some situations (which may not apply to you personally,
Tom, but they
> do apply to some others) it may be necessary to choke both
the entry
> path *and* the exit path to get a complete solution.

Power mains chokes, especially those in the neutral or
safety ground path, should be an absolute last resort if
indeed they are ever used. It is much better to find and fix
the real problem(s).

73 Tom

RFI mailing list

<Prev in Thread] Current Thread [Next in Thread>