Pete,
Having worked for 13 years at an RF shielding manufacturer prior to joining
Rockwell Collins, I can tell you in no uncertain terms: NO, a shield box
does NOT need to be grounded to be an RF shield. The Space Shuttle, while
in space flight, would work just fine as a Faraday cage if all doors,
windows, etc, were RF tight.
The chief reason for grounding an RF shielding box (or "enclosure") is so
that filters used on penetrations of the shield have someplace to shunt the
unwanted energy. (Most RF enclosures have power and signal line filters
for passing necessary voltages and controls thru the enclosure wall. Those
filters are typically of pi-network or L-network construction and require a
ground for the shunt capacitors.)
Just remember that to be a shield, ALL 6 sides of the box (or all portions
of a sphere or polygon) must be in place and any apertures must have an
RF-tight seal. No conductors may enter or exit the enclosure without
either being filtered or being contained within an extension of the shield.
(The latter case would represent a coaxial cable attached to an enclosure.
The outer conducter of the coax serves as the extension of the shield
providing, of course, that the cable is attached to a bulkhead connector on
the shield and not passed thru a hole in the shield. Note that only
hardline has a shielding performance which approximates that of a true
shielded enclosure. Ordinary braided shield coax, such as RG-213, falls
about 30 to 40 dB short of solid metal performance.) Any compromise of
these principles results in reduce shielding performance - period.
Conductors are antennas, and that includes circuit board traces which pass
under a shielding box on a pc board.
Magnetic shields have to be specified by application and frequency of the
field. You need something like mu-metal or very thick plate steel to be an
effective magnetic shield at or near DC and for low frequency (power line)
fields. Thinner silicon grained steel works well at the upper audio
frequencies and in the range of IF's and lower RF fields. By the time you
get to a few MHz, copper is just fine.
I don't have a handy dandy reading list available at the moment, but the
tech section of large bookstores and the library of technical scholls and
universities should contain helpful books. You can also check the IEEE
publications on the IEEE website; look under "Electromagetic
Compatability".
73, Dale
WA9ENA
Pete Smith <n4zr@contesting.com>@contesting.com on 11/07/2002 07:59:25 AM
Sent by: rfi-admin@contesting.com
To: rfi@contesting.com
cc:
Subject: [RFI] Shielding
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? What about a shield against
magnetic fields?
Should high-impedance audio circuits always be installed in a metal box to
prevent hum pickup?
Does the material of a shield box make any difference as long as it is a
good conductor?
Should signal grounds in a computer cable be grounded to the shield box at
each end, or deliberately left to float?
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.
Can anyone suggest a good, entry-level discussion of the subject anywhere?
Thanks,
73, Pete N4ZR
73, Pete N4ZR
Sometimes a tower is just a tower
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