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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