On Thu, 07 Nov 2002 08:59:25 -0500, Pete Smith wrote:
>After 48 years as a ham, there are still many big holes in my technical
>understanding of things. One of the biggest is shielding -- elementary
>questions like:
>
>Does a shield box for RF need to be grounded?
No, if you're talking about an electromagnetic field. However,
what's inside the box needs connections to the outside world, and
those may need shields which may need to be grounded. If you're
talking about shielding against an electric field, grounding is
usually a good idea. Exceptions would be pretty rare.
>What about a shield against
>magnetic fields?
Iron or special alloys such as monel will shield against magnetic
fields. No grounding needed.
>Should high-impedance audio circuits always be installed in a metal box to
>prevent hum pickup?
Only if there are 60 Hz wires nearby. If everything is run from
DC, there can not be any "hum" pickup.
>Does the material of a shield box make any difference as long as it is a
>good conductor?
For low power circuits, it doesn't matter much except maybe at
extremely high frequencies where the skin effect concentrates the
RF current in a very thin layer. For high power circuits, the
loss may be too high unless a very good conductor like silver is
used.
>Should signal grounds in a computer cable be grounded to the shield box at
>each end, or deliberately left to float?
It depends. If both ends are grounded, you may have created a
small "loop" antenna which may be a good picker-upper for RF. If
one end is ungrounded the loop is broken, but you may now have a
voltage difference between the two chassis. This may or may not
be a problem depending on the associated circuits. It all
depends on where the unwanted currents are going to flow. If you
leave one end ungrounded, it's generally a good idea to run a
separate ground wire to help eliminate any voltage difference.
>I have looked for a coherent discussion of shielding against various types
>of signal ingress, without success. There are passing references in the
>Handbook under various other headings, but typically they seem to assume
>knowledge I just don't have.
It's partly science and partly art. What works in one case might
not work in another. There are always LOTS of variables.
Too bad we can't see RF. Would make things lots easier. :-)
73, Bill W7TI
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