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

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.


--
John Passaneau W3JXP
State College Pa

This mail is a natural product. The slight variations in spelling and
grammar enhance its individual character and beauty and in no way are to
be considered flaws or defects.



"Richard Clark" wrote in message
...
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