Derek,
One paper that comes to mind is:
Cowdell, Robert B., "Simplified Shielding for Perforated Shields," 1968
IEEE Electromagnetic Compatibility Symposium Record, Seattle, WA, July
23-25, 1968, 308-316.
One way to think out the problem would be as:
1. An antenna emits an electromagnetic wave with a wave impedance Zw,
with Zw between the driving circuit's impedance and 377 ohms (the
characteristic impedance of vacuum and air).
2. As the electromagnetic wave propagates away from the antenna, the
wave impedance Zw = E(r)/H(r) decreases/increases with distance
until it reaches 377 ohms, then it stabilizes there.
3. If the wavelength of the electromagnetic wave is much greater (say
at least 10-20 times) the spacing between wires in the wire mesh, we
can treat the wire mesh as a solid sheet of thickness d = diameter
of the wires, with an impedance Zs, where 1 > Zs > impedance of a
solid sheet of the wire's metal, of thickness d.
4. We get a reflection off the front of the shield due to the impedance
mismatch between the impinging wave's wave impedance and the shield
impedance (this is probably the major factor in the shielding
effectiveness) of (4*|Zs||Zw|)/((|Zs|+|Zw|)^2), where |x| is the
magnitude of x.
5. We get a slight attentuation of the electromagnetic wave as it
propagates through the wire mesh.
6. We get another reflection from the shield-to-air impedance mismatch;
the electromagnetic wave bounces back and forth inside the wire mesh
until all of the energy has been propagated or turned to heat-- so
approximately half of the energy that entered the wire mesh
manages to leak through it, and the other half is reflected back
toward the antenna (maybe to cause problems in other directions).
Referring to Appendix K in my book, Robust Electronic Design Reference
Book, Volume II Appendices, in spherical coordinates:
* The electric field of a small dipole has 1/r, 1/r^2, and 1/r^3 terms.
* The magnetic field of a small dipole has 1/r and 1/r^2 terms.
Looking at Figure K-5, the wave impedance depends on the angle between
the axis of the dipole and the direction of interest. But in general,
the wave impedance Zw drops at 1/r until it approaches 377 ohms, in
which vicinity it may show a little undershoot or overshoot, then
settles at 377 ohms.
In the same appendix, again in spherical coordinates:
* The electric field of a small loop has 1/r and 1/r^2 terms.
* The magnetic field of a small loop has 1/r, 1/r^2, and 1/r^3 terms.
Looking at Figure K-8, the wave impedance depends on the angle through
the axis of the loop (a line through the center of the loop, and
perpendicular to the plane of the loop) and the direction of interest.
This time the wave impedance Zs increases at r until it approaches 377
ohms, in which vicinity it may show a little undershoot or overshoot,
then settles at 377 ohms.
To summarize, in the near field, at distances r < wavelength/(2*pi), the
wave impedance of an emitted signal is very sensitive to distance r.
Thus the reflection off the front of a shield will change significantly
with distance r, and thus so will the the shielding effectiveness of the
shield.
Enjoy!
John Barnes KS4GL, PE, NCE, NCT, ESDC Eng, ESDC Tech, PSE, SM IEEE
dBi Corporation
http://www.dbicorporation.com/
-
This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list. Website: http://www.ieee-pses.org/
To post a message to the list, send your e-mail to [email protected]
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas [email protected]
Mike Cantwell [email protected]
For policy questions, send mail to:
Jim Bacher: [email protected]
David Heald: [email protected]
All emc-pstc postings are archived and searchable on the web at:
http://www.ieeecommunities.org/emc-pstc
