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frequency (f+c,). At frequencies which are below the cutoff frequency the
waveguides attenuate the wave energy. For circular waveguide metal piping,
the lowest cutoff frequency is:
f+c, = [6929/D] MHz
EQUATION:
(15)
where
D
=
waveguide diameter in inches.
The attenuation for frequencies below the cutoff frequency is given by:
SE = [31.9L/D [1  (f/f+c,).2].0.5] dB
EQUATION:
(16)
where
L
=
length in inches
f
=
frequency
f+c, =
cutoff frequency
For rectangular ducting with width (a) and height (b) and with width greater
than height, the cutoff frequency is:
f+c, = (5906/a) MHz
EQUATION:
(17)
where
a
=
longest dimension in inches.
The attenuation for the rectangular duct for frequencies below the cutoff
frequency is given by:
[1  (f/f+c,).2].0.5)
EQUATION:
SE = (27.3L/a
dB
(18)
When the expression under the radical for the square of the frequency divided
by the cutoff frequency is less than 0.1 (when f is less than 1/3 f+c,) it can
be neglected, and the expressions for SE become:
EQUATION:
SE = (31.9 L/D)
dB
(19)
for circular waveguides and
EQUATION:
SE = (27.3 L/a)
dB
(20)
for rectangular waveguides.
2.6.2
Door Penetrations. Doors are required for personnel and equipment
access in the shielded enclosure. They are the largest openings in the
shielded enclosure, and are the most easily damaged through daily use. They
are the weakest link in the overall shielding system. The doors must be
constructed of shielding material in such a way as to provide SE equal to or
greater than the remainder of the enclosure. There are several basic types of
door closures currently used in shielding construction, several of which are
seen in Figure 1. These include the pneumatic bladder type, the compressed
rows of berylliumcopper fingerstock types, the knife edge guillotine type,
and the magnetic strip type door. The fingerstrips are also used in the knife
edge type doors, but do not receive the same compression and wiping forces
that fingerstrips around the door periphery which are compressed between
overlapping door and frame surfaces do, but they do have an added labyrinth
presented to induced currents, penetrating from the outside to inside surfaces
11


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