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 MIL-HDBK-419A
Both the dc and the ac resistances of a tubular conductor are greater than those of a solid conductor with the
same outside diameter. However, the ac resistance does not increase as much as the dc resistance, and,
therefore, the resistance ratio of a tubular conductor is always less than that of a solid conductor. The ac
resistance for four sizes of copper tubing is given in Figure 5-8, and the resistance ratio for isolated
nonmagnetic tubular conductors of various sizes is given in Figure 5-9. For a given length of conductor, the ac
resistance per unit weight (i.e., per given amount of copper) is less at high frequencies for tubular conductors
than for any other shape.
The self-inductance of a conductor is reduced by the absence of a conductive medium in the center (5-7).
Therefore, the self--inductance of a tubular conductor will be less than that of a solid conductor with the same
diameter. The self-inductance of three representative sizes of copper tubes is given in Figure 5-10.
5.2.4.4 Structural Steel Members. The steel I-beam in the structural framework of a building is another
conductor that is frequently used as a ground conductor. The resistivity of steel is approximately ten times
that of copper; however, the skin depth of steel is greater than 3 times that of copper. This increased skin
depth in steel increases the conducting area for high frequency currents. For example, in comparing a 0.3
meter (12-inch) I-beam with a 4/0 AWG copper cable, the perimeter of the I-beam is about 30 times as great
and with a factor of 3 increase in the skin depth, the conducting area for high frequency currents in the steel
I-beam is close to 90 times larger. This advantage is offset somewhat by the fact that the current tends to
flow in the edges of the I-beam and by the surface roughness. The ac resistance will be increased by a factor of
4 because of this surface roughness and current distribution. Even so, the ac resistance of a 4/0 AWG copper
cable is 4.25 times as great as that of a 0.3 meter (12-inch) I-beam. In addition, the building framework usually
offers many paths in parallel, thus lowering both the ac resistance and the inductance between any two points
(5-8)
5.3 SIGNAL REFERENCE SUBSYSTEM NETWORK CONFIGURATIONS. Within a piece of equipment the
signal reference subsystem may be a sheet of metal which serves as a signal reference plane for some or all of
the circuits in that equipment. Between equipments, where units are distributed throughout the facility, the
signal ground network usually consists of a number of interconnected wires, bars or a grid which serves an
equipotential plane. Whether serving a collection of circuits within an equipment or serving several equipments
within a facility, the signal reference subsystem will be a floating ground, a single-point ground, or a
multiple-point ground known as a multipoint or equipotential plane. Of the aforementioned signal reference
subsystems, the equipotential plane is the optimum ground for communications-electronics facilities. For
existing facilities where the presence of equipment prohibit the installation of an equipotential plane beneath,
on, or in the floor, the plane may be installed overhead and the equipment connected to it. It is desirable, but
not mandatory, to retrofit existing C-E facilities with equipotential planes.
5.3.1 Floating Ground.
A floating ground is illustrated in Figure 5-11. In a facility, this type of signal ground system is electrically
isolated from the building ground and other conductive objects. Hence, noise currents present in the building's
ground system will not be conductively coupled to the signal circuits. The floating ground system concept is
also employed in equipment design to isolate the signal returns from the equipment cabinets and thus prevent
noise currents in the cabinets from coupling directly to the signal circuits.
5-15
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