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 EQUATION:
f
=
v/[lambda]
(1)
where
f
=
v
=
velocity of propagation
[lambda]
=
wave length
In free space the velocity (v) is equal to the velocity of light (c), where
c = 3 x 10.8- meters per second. In a dielectric material such as that
found in many coaxial transmission cables, the velocity (v) is equal to the
velocity of light (c) divided by the square root of the relative permittivity
also called the dielectric constant (k+r,).
2.4.1
Antenna Emissions. The EM wave is generated by means of alternating
current or voltage sources driving radiating antennas, which typically consist
of metal conductors formed in loops, or linear lengths of conductors such as
rod antennae placed above a reflecting plane, or dipoles. It should also
include pairs of parallel conductors forming transmission lines. Leaks in the
surface of shielded enclosures such as holes, cracks, poor seam closures, or
untreated metallic penetrations can act as radiation source loops, dipoles, or
transmission lines to transfer electromagnetic waves from one side of
electromagnetic shielding to the other, when excited by alternating currents
and voltages. The result of exciting loop and dipole antennae, and
transmission lines is a combination of both radiated and nonradiated
electromagnetic fields surrounding the source antennae. Close to loops and
dipoles in a region known as the near field, the nonradiated portion of the
electric and magnetic fields are very strong and fall off inversely as the
cube of the distance from the antennae. The electric dipole provides a strong
electric field close to the antenna and the magnetic loop provides a strong
magnetic field. The near field extends to a distance (r) in meters equal to
the wavelength in meters divided by 2[pi], i.e. r = [lambda]/2]. Beyond
the near field is a transition region where nonradiated fields are diminishing
and the radiated fields are more significant which extends to about 1.6 times
the wavelength, in meters. Beyond the transition region is the far field
where the radiated waves are plane waves, i.e. the electric and magnetic field
vectors are at right angles to each other and to the direction of wave
propagation. Here the wave impedance (Z) is that of free space (377 ohms).
2.4.2
Shielding Effectiveness Equation. Shielding Effectiveness (SE) is
defined as 10 times the log to the base 10 of the ratio of the incident
electromagnetic power (P+1,) without the shielding, to the transmitted power
(P+2,) with the shielding in place, expressed in decibels (dB) or:
SE = 10 log (P+1,/P+2,)
EQUATION:
dB
(2)
where
P+1,
=
incident electromagnetic power
P+2,
=
transmitted power with shielding in place (in dB)
Since the power can be expressed in terms of wave impedance (Z) and either
electric or magnetic fields (E or H), the expression for shielding
effectiveness can be further expressed as:
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