Low-frequency RF Coupling To Unconventional (Fat Unbalanced) Dipoles Page: 4 of 44
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The report explains radio frequency (RF) coupling to
unconventional dipole antennas. Normal dipoles have thin equal
length arms that operate at maximum efficiency around
resonance frequencies. In some applications like high-explosive
(HE) safety analysis, structures similar to dipoles with "fat"
unequal length arms must be evaluated for indirect-lightning
effects. An example is shown in Figure 1.1 where a metal drum-
shaped container with HE forms one arm and the detonator cable
acts as the other. Even if the HE is in a facility converted into a
"Faraday cage", a lightning strike to the facility could still produce
electric fields inside [1.1 - Clancy]. The detonator cable
concentrates the electric field and carries the energy into the
detonator, potentially creating a hazard. This electromagnetic
(EM) field coupling of lightning energy is the indirect effect of a
Figure 1.1. A lightning strike will
create electric fields in a "Faraday
In practice, "Faraday cages" are formed by the rebar of the cage".
concrete facilities. The individual rebar rods in the roof, walls
and floor are normally electrically connected because of the construction technique of using
metal wire to tie the pieces together. There are two additional requirements for a good cage. (1)
The roof-wall joint and the wall-floor joint must be electrically attached. (2) All metallic
penetrations into the facility must also be electrically connected to the rebar. In this report, it is
assumed that these conditions have been met, and there is no arcing in the facility structure.
Many types of detonators have metal "cups" that contain the explosives and thin electrical
initiating wires, called bridge wires mounted between two pins. The pins are connected to the
detonator cable. The area of concern is between the pins supporting the bridge wire and the
metal cup forming the outside of the detonator. Detonator cables
usually have two wires, and in this example, both wires generated the Field
same voltage at the detonator bridge wire. This is called the common- Vdetonator
mode voltage. (See Figure 1.2.) The explosive component inside a -
detonator is relatively sensitive, and any electrical arc is a concern. In a
safety analysis, the pin-to-cup voltage, i.e., detonator voltage, must be
calculated to decide if an arc will form. If the electric field is known, the
voltage between any two points is simply the integral of the field
along a line between the points. Eq. 1.1. For simplicity, it is Figure 1.2. The detona
assumed that the electric field and dipole elements are aligned, voltage can be determine
Calculating the induced detonator voltage is more complex the integral of the elect
because of the field concentration caused by metal components, field in the gap.
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Ong, M M; Brown, C G; Perkins, M P; Speer, R D & Javedani, J B. Low-frequency RF Coupling To Unconventional (Fat Unbalanced) Dipoles, report, December 7, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc834261/m1/4/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.