One minor change to Mr. Barnes' input needs to be made in paragraph 3. Per my recollection from many years ago, if the mesh dimensions are small relative to a wavelength, the mesh will indeed look like a solid sheet, but the "effective thickness" of that sheet would be as if the wire metal was melted down and made into a sheet of constant thickness over the area covered by the mesh. So if the wire mesh had an average open are of 90%, then the "effective thickness" for SE purposes would be only 10% that of the wire in the mesh. Ken Javor
Phone: (256) 650-5261 > From: John Barnes <[email protected]> > Reply-To: <[email protected]> > Date: Sat, 15 Mar 2008 13:46:02 -0400 > To: Derek Walton <[email protected]>, <[email protected]> > Subject: Re: SE of mesh > > 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 - 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
