Additional door bonding may be incorporated with either woven Mumetal gasketing (for very low frequencies),
or flexible microwave absorber (for very high frequencies).
To attenuate signals below 50 MHz, waveguide hallways can be used (8-19). The cutoff frequency is
proportional to the largest lateral dimension of the hallway; therefore, a tradeoff is generally necessary
between hallway size and required attenuation. As shown in Section 126.96.36.199, the amount of attenuation of
frequencies below cutoff is a function of hallway length. The waveguide hallway may be constructed of 20
gauge, or thicker, low carbon steel supported by any structurally sound, but nonconductive material.
In all types of door design intended for use at frequencies above a few hundred megahertz, it is desirable to
avoid metallic penetration of the door. A special locking catch has been designed which enables full retention
of the door leaf and release of the latch from both sides of the door without the need for any metallic
penetration of the shield. This lack of metallic penetration is important since even with the most adequate
bonding any operating shaft severely increases the risk of shield degradation at frequencies where the shaft's
length becomes resonant. It is also important to ensure that even insulating penetrations through the shield
which pass through waveguide-below-cutoff tubes are correctly designed. Although the cutoff frequency of a
waveguide in air can be easily calculated, the inclusion of insulating material of high dielectric constant in the
waveguide considerably reduces the cutoff frequency.
A further requirement for shielded enclosures is adequate ventilation. Honeycomb structures provide a
virtually unimpeded passage for air flow and are normally incorporated in ventilation ducts, ventilation
openings, and fans or air conditioner systems.
It is essential to avoid signal penetration via power and signal wiring. This demands that filters achieving
adequate insertion loss are installed in all incoming cables; it is fairly normal to have three-phase power
circuits and several hundred signal lines going into a large enclosure. It is essential that the filters provide the
specified attenuation under full-load conditions at all frequencies. Unless the filter attenuation is maintained
at all frequencies and load currents, the overall shield attenuation will be degraded by the signal penetration via
the filters. Shield penetrations may also be provided for air, gas, and water lines; these can be achieved either
by the use of waveguide-below-cutoff tubes carrying insulating piping or by welding metal pipework to the
shield. It is essential that all input circuits and penetrations occur in a localized area.
It is necessary that the shield be grounded adequately for safety purposes. Although an external ground
connection has no effect on the equipment placed within an ideal shield since the shield itself forms its own
private world, an external ground is essential to prevent the enclosure from reaching dangerous potentials
relative to its surroundings.
8.9.2 Custom Built Rooms.
In spite of the wide range of use of demountable modular enclosures, a considerable demand exists for
specialized custom built shielded areas. These are employed either where the insertion loss requirements are
markedly different from those obtainable from modular rooms or where the area to be enclosed is exceptionally
large and economy dictates that some other design be adopted. Many forms of construction are used and these
include enclosures made from woven copper or steel mesh, from pierced and expanded metal, from aluminum or
copper foils, from high permeability materials such as Mumetal, and from all-welded steel sheet.