Free-Space Coupling

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MIL-HDBK-419A
The signal reference plane is another potential coupling path for unwanted signals between equipments and/or
circuits. Since practical signal reference planes do not exhibit a zero impedance, any currents flowing in a
signal reference plane will produce potential differences (voltages) between various points on the reference
plane. Interfacing circuits referenced to these various points can experience conductively coupled interference
in the manner illustrated by Figure 6-5. The signal current
flowing in Circuit 1 of Figure 6-5 returns to its
source through signal reference impedance
producing a voltage drop
in the reference plane. The
impedance
is common to Circuit 2, hence
appears in Circuit 2 as a voltage in series with the desired
signal voltage source,
This undesired source produces an interference voltage,
Circuit 2; similarly, the desired current,
in Circuit 2 will produce interference in Circuit 1.
In a facility, the conductive coupling of interference through the signal reference plane of interfaced equipment
can occur in a manner similar to that described above for internal  If Circuit 1 in Figure 6-5
represents two interfaced equipments and if Circuit 2 represents a different pair of interfaced equipments, then
a current flowing in either circuit can produce interference in the other circuit as described. Even if the pairs
of equipments do not use the signal reference plane as the signal return, the signal reference plane can still be
the cause of coupling between equipments. Figure 6-6 illustrates the effect of a stray current,
flowing in
the reference (or ground) plane.
may be the result of the direct coupling of another pair of equipments to
the signal reference plane, or it may be the result of free-space coupling to the signal reference plane. In
either case,
produces a voltage EN in the reference plane impedance,
This voltage produces a current in
the interconnecting loop which in turn develops a voltage across
in Equipment B. Thus, it is evident that
interference can conductively couple via the signal reference plane to all circuits and equipments connected
across the non-zero impedance elements of that reference plane.
6.2.2 Free-Space Coupling.
Free-space coupling is the transfer of electromagnetic energy between two or more circuits not directly
interconnected with a conductor.  Depending on the distance between the circuits, the coupling is usually
defined as either near-field or far-field. Near-field coupling can be subdivided into inductive and capacitive
coupling, according to the nature of the electromagnetic field. In inductive coupling, a magnetic field linking
the susceptible device or circuit is set up by the interference source or circuit. Capacitive coupling is produced
by an electric field between the interference source and the susceptible circuit.
Radiation of energy by electromagnetic waves is the principle coupling mechanism in far-field coupling. The
term "radiated coupling" is sometimes used to describe both near-field (inductive and capacitive) coupling and
far-field coupling. However, radiated coupling is generally accepted to mean the transfer of energy from a
source to a susceptible circuit by means of electromagnetic waves propagating through space according to the
laws of wave propagation.
6.2.2.1 Near-Field Coupling.
When two or more wires or other conductors are located near each other, currents and voltages on one wire will
be inductively and capacitively coupled to the other wires. The wire acting as the interference source for this
near-field coupling may be any conductor such as a high level signal line, an ac power line, a control line, or
even a lightning down conductor.  The currents or voltages induced into the other wires can further be
conductively coupled into susceptible circuits.
6-6