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Re: [RFI] Earth isolation

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Earth isolation
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Wed, 08 Sep 2004 23:12:41 -0500
List-post: <mailto:rfi@contesting.com>
On Wed, 8 Sep 2004 18:20:58 -0400, Tom Rauch wrote:

>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.

That is not the model I am thinking of. Rather, it is of a cable acting 
as a short long-wire antenna, and there is a current path for it, just 
as with any antenna. The current path for many ( most) long-wire 
antennas is to "ground," and in this case it is through the equipment 
to which it is connected. 

So here is this receiving antenna wired to a circuit board trace and 
from there to the chassis, and the chassis goes to "ground" via 
whatever path we provide for it. The inductance of the circuit board 
trace is an impedance in that path that is frequency dependent, and 
there will be IZ drop across it due to the receiving antenna current. 
The key to keeping this part of the RF out of the box is to prevent 
that current from flowing inside the box. 

What part of that do you have trouble with? Would it help you think 
about it if the receive antenna (mic cable) was much longer?  The 
only thing that changes is the magnitude of the current induced in 
the antenna, and thus the magnitude of the voltage across the 
impedance of the circuit trace. 

In this example, I'm not suggesting anything "magic" about ground. 
But there is a fundamental difference in the circuit topology -- when 
you connect the shield properly (directly to the chassis, rather than 
to the circuit board), the RF current doesn't flow through an 
impedance that causes it to be coupled into the circuitry. Another 
way to understand this is to realize that the circuit trace and the path 
to "ground" is part of the counterpoise for the receiving antenna 
(mic cable). As with any such antenna, it will be subject to boundary 
conditions -- zero current and maximum voltage at the open end, 
maximum current and a voltage minimum at the other end. 

Jim Brown K9YC

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