The shielding effectiveness (SE) equation that you described,
SE(dB) = 36 - 20*Log(Zt)
where Zt = frequency dependent transfer function
was derived in a paper written by Albert R. Martin in EMC Technology, July
1982. Some of his work was based on an earlier paper by Merewether and
Ezell, published in 1976. [Note: see list of references below].
This basic equation is a very simplified version of the original complex
model that was developed, and in the simplication process several
assumptions were made. Therefore, it is recommended that the equation be
used with caution, or at least have a good understanding of the impact of
the assumptions. [Note: you are asking excellent questions about the
equation and it's associated model from which it was developed].
There are numerous published papers on the general subject of transfer
impedance, but there are few that specifically address the correlation of
shielding effectiveness and transfer impedance. Some others who have
developed similar models for SE vs Zt are listed below, and there are
probably a few more not mentioned here.
REFERENCES: SE vs Zt
---------------------
A. Martin, "Introduction to Surface Transfer Impedance",
EMC Technology, July 1982, p44-52
Merewether, David E, and T. Frank Ezell, "The Effect of
Mutual Inductance and Mutual Capacitance on the Transient
Response of Braided-Shielded Coaxial Cables", IEEE
Transactions on EMC, Vol EMC-18 No. 1 (1976), p15-20
Mills, Jeffery P., "Electromagnetic Interference Reduction
in Electronic Systems", PTR Prentice-Hall, Inc., Englewood
Cliffs, NJ (1993) Chap 4: Shielding of Cables
(ISBN 0-13-463902-02)
Mardiguian, Michel, "Controlling Radiated Emissions by
Design", Van Nostrand Reinhold, New York, NY (1992)
Sect 11.5.5: Specifying Zt from SE (ISBN 0-442-00949-6)
Lastly, there is a recent book dedicated to cable shielding that contains
numerous detailed discussions on transfer impedance: Tsaliovich, Anatoly,
"Cable Shielding for Electromagnetic Compatibility", Van Nostrand
Reinhold, New York, NY (1995) 469 pages
(ISBN: none)
Hope this is helpful.
Sincerely,
Manny Barron
EMC Staff Engineer
Tandem Computers
------------ ORIGINAL ATTACHMENT --------
SENT 10-02-97 FROM SMTPGATE ([email protected])
The equation for cable shielding effectiveness
SE [dB] = 36 - 20 log(Zt)
where Zt is the total frequency dependent transfer impedance has
been around for a number of years.
Does anyone out there have a good derivation or reliabe source for
it?
Or for that matter what are the limitations of the equation?
MIL-C-24640 specs Zt as 700 milliohms per meter which implies a
90 meter cable, which is not unheard of, has zero dB of shielding.
(Does a 100-meter cable have gain?)
Jim Sketoe
Boeing Company - St. Louis
[email protected]
