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Shielding Question

I'm not sure if this is the right place to ask, but here goes:

Are there any good references to the effects of shielding effectiveness
as related to shield coverage? I want to make a shield for HF
frequencies, and the most convenient method would be to use wire cloth
on one side, with the other 5 sides of the box made of solid aluminum.

- Mike KB3EIA -

On Thu, 12 Feb 2004 14:57:43 -0500, Mike Coslo wrote:

I'm not sure if this is the right place to ask, but here goes:

Are there any good references to the effects of shielding effectiveness
as related to shield coverage? I want to make a shield for HF
frequencies, and the most convenient method would be to use wire cloth
on one side, with the other 5 sides of the box made of solid aluminum.

- Mike KB3EIA -

Hi Mike,

That is called a Faraday Shield (more or less). You need only
determine the penetration depth of the materials used and the
frequency you want to shield. Handbooks (for RF Engineer) usually
have such tables by material.

Oh, all bets are off if you penetrate the shield for any reason (like
for a wire lead, or a viewing port, or....). The wails of grief and
shock at poor effectiveness are numerous from those who expect a 100%
cure from a 99% solution (-20dB generally isn't all that good).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Thu, 12 Feb 2004 14:57:43 -0500, Mike Coslo wrote:

I'm not sure if this is the right place to ask, but here goes:

Are there any good references to the effects of shielding effectiveness
as related to shield coverage? I want to make a shield for HF
frequencies, and the most convenient method would be to use wire cloth
on one side, with the other 5 sides of the box made of solid aluminum.

- Mike KB3EIA -

Hi Mike,

That is called a Faraday Shield (more or less). You need only
determine the penetration depth of the materials used and the
frequency you want to shield. Handbooks (for RF Engineer) usually
have such tables by material.

Oh, all bets are off if you penetrate the shield for any reason (like
for a wire lead, or a viewing port, or....). The wails of grief and
shock at poor effectiveness are numerous from those who expect a 100%
cure from a 99% solution (-20dB generally isn't all that good).

73's
Richard Clark, KB7QHC

Remember to use "hardware cloth" for the screen. This material is galvanized
steel, and the galvanizing "solders" every crossover point. !/4" mesh is
fine for HF. Your problems will be the penetrations, as Mike described.
Also, you will need to bond the cloth to the aluminum. Soldering would be
best, but that would only work for a copper or tin-plated steel chassis. You
will have to clamp the cloth to the chassis, possibly using lengths of bar
stock with screws every inch or so. Don't forget to polish the aluminum area
that contacts the cloth just before assembly. Get it down to shiny aluminum;
aluminum oxide is an insulator, and you need good conductivity across the
joint.

Ed

Mike, KB3EIA wrote:
"Are there any good references to the effects of shielding effectiveness
as related to shield coverage?"

Yes. As Richard Clark noted, it is related to penetration depth of the
materials and that is related to "skin effect".

The best exposition of skin effect in my opinion is given in
"Radio-Electronic Transmission Fundamentals", a text by B. Whitfield
Griffith,Jr.

Originally published in 1962, it`s return is a reprint by Noble
Publishing Corp., 630 Pinnacle Court, Norcross, GA 30071; tel
770-449-6774; fax 770-448-2839; orders@ noblepub.com; www.noblepub.com.

Original publisher was McGraw-Hill.

The new Second Edition is a 648-page hardcover book which was reviewed
in the "New Products" section of the Jan. 2001 QST on page 48.

Griffith graduated from MIT, was a licensed radio amateur, senior member
of the IRE, Texas Society of Professional Engineers, National Society of
Professional Engineers, and ARRL.

In the original edition, "Skin Effect" begins on page 232. "Radiation
and its Control" begins on page 243. You don`t need calculus to
understand Griffith, but calculus never hurt anybody.

The reprint is well worth the $75 asked in 2001. In fact, it would be
worth searching elsewhere if the 2nd edition is now out of print. This
book is outstanding! ISBN 1-884932-13-4: order number NP-34. You won`t
find clearer or more concise.

Best regards, Richard Harrison, KB5WZI

Ed Price wrote:
"Remember to use hardware cloth for the screen."

In my experience, the screened room was constructed of copper insect
wire screen. All seams and joints were soldered.

As in pre-fab rooms, the room was double screened. Both sides of the
wooden studs were screened to make a copper box inside a copper box.

Every penetration was carefully screened and bypassed. The door was
shielded and had low resistance contacts all around.

We were able to accurately make very sensitive measurements in our
screened room which was sited in the center of a very high power
shortwave broadcast plant with many transmitters, usually simultaneously
operating in at least 3 bands. One band predicted as the optimum working
band for the path and, the same program simultaneously transmitted on
the next higher and next lower bands, just in case. We had several
languages and target areas at nearly all times during all 24-hour days.

Best regards, Richard Harrison, KB5WZI

Ed Price wrote:

Remember to use "hardware cloth" for the screen. This material is galvanized
steel, and the galvanizing "solders" every crossover point. !/4" mesh is
fine for HF. Your problems will be the penetrations, as Mike described.
Also, you will need to bond the cloth to the aluminum. Soldering would be
best, but that would only work for a copper or tin-plated steel chassis. You
will have to clamp the cloth to the chassis, possibly using lengths of bar
stock with screws every inch or so. Don't forget to polish the aluminum area
that contacts the cloth just before assembly. Get it down to shiny aluminum;
aluminum oxide is an insulator, and you need good conductivity across the
joint.

Good point and thanks Ed! I was thinking of hardware cloth, and this
would give me a reason not to switch from it.

There will ba an access port in the shield, but I can design a recessed
portion of the shield to keep it continuous.

- Mike KB3EIA

Richard Harrison wrote:
Mike, KB3EIA wrote:
"Are there any good references to the effects of shielding effectiveness
as related to shield coverage?"

Yes. As Richard Clark noted, it is related to penetration depth of the
materials and that is related to "skin effect".

The best exposition of skin effect in my opinion is given in
"Radio-Electronic Transmission Fundamentals", a text by B. Whitfield
Griffith,Jr.

Originally published in 1962, it`s return is a reprint by Noble
Publishing Corp., 630 Pinnacle Court, Norcross, GA 30071; tel
770-449-6774; fax 770-448-2839; orders@ noblepub.com; www.noblepub.com.

Original publisher was McGraw-Hill.

The new Second Edition is a 648-page hardcover book which was reviewed
in the "New Products" section of the Jan. 2001 QST on page 48.

Griffith graduated from MIT, was a licensed radio amateur, senior member
of the IRE, Texas Society of Professional Engineers, National Society of
Professional Engineers, and ARRL.

In the original edition, "Skin Effect" begins on page 232. "Radiation
and its Control" begins on page 243. You don`t need calculus to
understand Griffith, but calculus never hurt anybody.

The reprint is well worth the $75 asked in 2001. In fact, it would be
worth searching elsewhere if the 2nd edition is now out of print. This
book is outstanding! ISBN 1-884932-13-4: order number NP-34. You won`t
find clearer or more concise.

Best regards, Richard Harrison, KB5WZI

Thanks very much, Richard and Ed, I'm doing a search at our University
Lib right now.

- Mike KB3EIA -

On Fri, 13 Feb 2004 13:38:42 -0500, Mike Coslo wrote:

Thanks very much, Richard and Ed, I'm doing a search at our University
Lib right now.

- Mike KB3EIA -

Hi Mike,

Add:
"Grounding And Shielding Techniques in Instrumentation,"
Ralph Morrison

"Noise Reduction Techniques in Electronic Systems,"
Henry W. Ott

Two slim volumes with more practical information than a thousand EE
books.

73's
Richard Clark, KB7QHC

On Fri, 13 Feb 2004 13:38:42 -0500, Mike Coslo wrote:

Look here too:

http://www.ce-mag.com/99ARG/Bjorklof137.html

"John Passaneau" wrote in message
...
Hi Mike:

I have those books if you can't find them. Many shield rooms also have a
shield of soft iron as well as the copper. The soft iron shields against
magnetic fields.

The traditional modular shielded enclosure has become standardized to a
design of 4' x 8' x 3/4" panels, held with a "hat & flat" clamping rail
system. Each panel is a sandwich of a sheet of galvanized steel, plywood and
another layer of galvanized steel. The steel is often about 24 gauge. This
is a trade-off between weight, manufacturing ease, and durability. The
galvanizing ensures good conductivity at the joints. The steel does give you
a bit of LF H-field SE, but the real reason for the steel is that it's just
a whole lot cheaper that copper screen, and lasts better, since the SE
doesn't degrade and it isn't nearly so vulnerable to puncture.

If you want serious H-field shielding, you need to go to thick steel
(welded) chambers, or employ exotic alloys (expensive, and physically &
magnetically vulnerable).

Ed
wb6wsn

"Richard Harrison" wrote in message
...
Ed Price wrote:
"Remember to use hardware cloth for the screen."

In my experience, the screened room was constructed of copper insect
wire screen. All seams and joints were soldered.

As in pre-fab rooms, the room was double screened. Both sides of the
wooden studs were screened to make a copper box inside a copper box.

Every penetration was carefully screened and bypassed. The door was
shielded and had low resistance contacts all around.

We were able to accurately make very sensitive measurements in our
screened room which was sited in the center of a very high power
shortwave broadcast plant with many transmitters, usually simultaneously
operating in at least 3 bands. One band predicted as the optimum working
band for the path and, the same program simultaneously transmitted on
the next higher and next lower bands, just in case. We had several
languages and target areas at nearly all times during all 24-hour days.

Best regards, Richard Harrison, KB5WZI

The Lindgren "double-shielded" copper screen rooms may have had some
theoretical SE advantage over double-skin panels where the inner & outer
skins were joined at every clamp rail. OTOH, whatever advantage the design
might have had was, in practice, negated by the nearly doubling of
fasteners. The wood frames in those copper mesh screen rooms were notorious
for changing dimension due to atmospheric humidity, resulting in a
"breathing" effect that worked to loosen all those fasteners. A periodic
re-torque of all fasteners was advisable.

Since you went to the extra effort to solder all your seams (the floor seams
too g), you created a considerably better than average screen room. My
comment about hardware cloth being soldered at each crossover was meant to
point out that the copper screen isn't bonded that way. (For one thing, the
small wire spacing would create enough surface tension in molten solder or
zinc to completely fill the holes, so it would be rather hard to create
wire-to-wire crossover bonds without resulting in a solid sheet.)

In my experience, new screen rooms, with bright & shiny copper screen, can
yield an 80 dB SE or better. But, after a couple of years exposure to
atmospheric oxygen, the copper forms a decent layer of copper oxide
(remember, the wire cross-overs are not gas tight), and the SE degrades by
20 to 30 dB. (This is independent of the clamped or soldered panel seams.) I
haven't found any practical way to reverse this degradation.

If you have a very-used screen room (like one that's been stored on pallets
for a few [dozen?] years), you will see that the screen is very oxidized.
The best you can do when assembling this old room is to scrupulously buff
the panel clamping areas until the copper shines again. (This is delicate,
get too aggressive, and you will tear the screening!) I once tried to merge
two mid-size, old double-shielded screen rooms, by soldering the merge line
with a patch of new copper screen. This was horribly labor intensive, as
cleaning the old copper screen well enough to flow the solder was really
tough. Trying to solder over your head is also an experience to avoid at all
costs.

Also, about the "door with low resistance contacts." The copper screen rooms
usually are not strong and rigid enough to properly maintain perfect
alignment of the heavy door leaf. You need to periodically check the contact
action of the (typically) beryllium copper fingerstock. Look for shiny rub
marks on the mating frame surface. Very well designed modular rooms will use
a heavy, solid frame to carry the door leaf. Another factor is the heavy
mechanical loading of the door and frame compressing the long perimeter of
fingerstock. Unless well-built, both leaf and frame can bend under load,
giving you a poor low-impedance joint. Knife-edge doors are less of a
problem with compressive forces, but then, they need even more precise
alignment registration to work well.

Getting back to the OP, I thought that he was asking about a shield over
equipment, not a room volume. For HF, the 1/4" cloth will work as well as
copper screening, and will allow much better air circulation. As I pointed
out earlier, his biggest problem will be bonding the cloth to his heavy
aluminum plate chassis.

Ed
wb6wsn

"Mike Coslo" wrote in message
...
Ed Price wrote:

Remember to use "hardware cloth" for the screen. This material is
galvanized
steel, and the galvanizing "solders" every crossover point. !/4" mesh is
fine for HF. Your problems will be the penetrations, as Mike described.
Also, you will need to bond the cloth to the aluminum. Soldering would
be
best, but that would only work for a copper or tin-plated steel chassis.
You
will have to clamp the cloth to the chassis, possibly using lengths of
bar
stock with screws every inch or so. Don't forget to polish the aluminum
area
that contacts the cloth just before assembly. Get it down to shiny
aluminum;
aluminum oxide is an insulator, and you need good conductivity across
the
joint.

Good point and thanks Ed! I was thinking of hardware cloth, and this
would give me a reason not to switch from it.

There will ba an access port in the shield, but I can design a recessed
portion of the shield to keep it continuous.

- Mike KB3EIA

Remember to never rely on the hinge of a port cover to provide a bond path.
Best practice is to use the hinge mechanically only, and have the leaf come
down on a full perimeter of a compressible conductive gasket. Beryllium
copper fingerstock, mating against a strip of copper, is best for high usage
ports. If you don't operate the port very much, you can use a folded-over
length of knitted wire mesh facing onto the hardware cloth. Just remember to
ensure that the gasket is compressed all along its length.

Ed
wb6wsn