10.3.1.3 Vertical Structures. The HEMP interacts with vertical structures, such as radio towers, waveguides,
and cables to overhead antennas, and downleads from power and communication lines in much the same manner
as it interacts with horizontal lines, except that it is the vertical component of the electric field that drives the
vertical structures. The current induced in a downlead from an overhead power line is shown in Figure 10-6.
Because the line is short and the angle of incidence is only
little bow-wave effect is observable. The peak
current is also limited by the line height in this example; for taller structures, the leading edge of the current
wave will continue to rise as the integral of the incident wave. The current will increase with structure height
for structures up to a few hundred feet high.
Short-Circuit Current Induced at the Base of a Vertical Riser by a
Vertically Polarized Incident Wave
10.3.2 HEMP Interaction with Local Structure.
10.3.2.1 Shields. The HEMP fields incident on a closed shield induce surface currents and charge
displacements on the outer surfaces of the shield. If the shield is continuous metal (i.e., it has no opening or
discontinuities in its surface) and about 1 mm thick, the voltage induced in circuits inside the shield by the
HEMP will be very small. Table 10-1 shows the voltage induced in the largest single-turn loop that can be
placed inside a spherical shield of 10 meters radius by a zero-rise-time 50 kV/m incident exponential pulse
having a 250 ns decay time-constant (10-7). Note that even for a shield as thin as 0.2 mm, the induced voltage
is less than 1 V; shields made of workable thicknesses of common metals do not allow significant HEMP
interaction with internal circuits. Possible exceptions to this conclusion are those shields that are long and of
small cross section, such as the shields on intrasite cables.