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 MIL-HDBK-419A
allow more air to flow with less pressure drop for the same diameter opening,
b.
cannot be damaged as easily as the mesh screen and are therefore more reliable, and
c.
are less subject to deterioration by oxidation and exposure.
d.
All non-solid shielding materials, such as perforated metal, fine mesh copper screening, and metal honeycomb,
present an impedance to air flow. Metal honeycomb is the best of these materials because it enables very high
electric field attenuations to be obtained through the microwave band with negligible drops in air pressure (see
Volume II). However, honeycomb has the disadvantages of occupying greater volume and costing more than
screening or perforated metal. Further, it is often difficult to install honeycomb paneling because flush
mounting is required. Thus, screening and perforated sheet stock sometimes find application for purely physical
design reasons, although honeycomb panels can achieve attenuations greater than 100 dB for frequencies below
10 MHz.
The waveguide attenuator is also of considerable value where control shafts must extend through an enclosure.
By making use of an insulated control shaft passing through the waveguide attenuator, the control function can
be accomplished with little likelihood of radiation. However, where a metallic control shaft is required, it must
be grounded to the case by a close-fitting gasket or metallic fingers.
Fuseholders, phone jacks, panel connectors not in use, and other receptacles can be fitted with a metallic cap
that provides an electrically continuous cover and maintains case integrity.
The waveguide attenuator approach may also be considered where holes must be drilled in the enclosure. If the
metal thickness is sufficient to provide a "tunnel" with adequate length, a waveguide attenuator is effectively
produced. For example, a metal wall 0.5 cm (3/16-inch) thick would permit a 0.16 cm (1/16-inch) hole to be
used without excessive leakage. This technique definitely should be considered where it is necessary to confine
extremely intense interference sources.
8.5.3.2 Screen and Conducting Glass.
Often it is necessary to provide rf shielding over pilot lights, meter faces, strip chart recorders, oscilloscopes,
or similar devices that must be observed by the equipment user. The alternatives available include:
Use of a waveguide attenuator,
a.
Use of screening material,
b.
Providing a shield behind the assembly of concern, and filtering all leads to the assembly, or
c.
Use of conducting glass.
d.
A waveguide attenuator is a practical approach for rf shielding of lamps. The technique has the advantage of
not introducing light transmission loss. However, it is not particularly suitable for most meter openings or
larger aperture; because of the space requirements involved.
8-52
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