2011 EMC Directory & Design Guide - Interference Technology

S c h o e m a n, Ja k o bu s
shielding / cables & connectors
Figure 7. Braided cable showing different weave parameters.
Coupling of External Fields into Cables
Transmission lines can be excited by electromagnetic fields
where their effect is to induce currents and voltages on the
line and in the load impedances at the ends. There are three
approaches for describing the coupling of an external field
to a line using transmission line theory: the Taylor approach
[10], the Agrawal method [11] and the Rashidi method [12].
Each of these coupling formulations gives the same response
for the transmission line, although there are subtle differences
in these techniques.
In FEKO the coupling of external fields into cables is
considered with the scattered voltage formulation described
by Agrawal. The problem can be considered to be an electromagnetic
scattering process in which the tangential
incident electric field along the conductors can be viewed
as distributed voltage sources exciting the transmission
line (see Figure 6).
Treatment of Cable Shields
In transmission line theory, the conductors in a cable
bundle can be grouped into outer and inner circuits, each
of which is coupled with a mutual conductor called a shield.
The outer and inner circuits are completely separated by
this shield, except that they are connected by current- and
voltage-controlled sources (there should be no other connection
between outer and inner circuits). The shield coupling
parameters defining these controlled sources are termed
transfer impedance Z T
and transfer admittance Y T
, which
may be formally defined as follows
where I s
, V s
and I i
, V i
are the currents and voltages on the
outer shield and inner conductor of the separate circuits.
Both Z T
and Y T
are basically dependent on the geometric
and physical properties of the conductor system and as such
are valid for both solid and braided shields.
For a solid tubular shield, the Schelkunoff model [13] is
typically used
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