The amount of the variation in the current that
directly affects variation in the magnetic field
surrounding the conductor depends upon the nature of
the current. When the conductor is a power lead to an
electric motor, all the frequencies and amplitudes
associated with broadband interference are present in
the magnetic field. When the lead is an ac power lead,
a strong sinusoidal magnetic field is present. When
the lead is carrying switched or pulsed currents,
extremely complex broadband variations are present.
As the magnetic field cuts across a neighboring
conductor, a voltage replica of its variation is induced
into the neighboring wire. This causes a current to
flow in the neighboring wire. When the neighboring
wire leads to a sensitive point in a susceptible
receiver, serious interference with that receivers
operation can result.
Similarly, a wire carrying a
steady, pure dc current of high value sets up a
magnetic field capable of affecting the operation of
equipment whose operation is based upon the earths
Shielding a conductor against magnetic induction
is both difficult and impractical.
shielding materials have little or no effect upon a
magnetic field. Magnetic shielding that is effective at
low frequencies is prohibitively heavy and bulky.
In aircraft wiring, the effect of induction fields
should be minimized. This can be done by use of the
proper spacing and coupling angle between wires.
The degree of magnetic coupling diminishes rapidly
with distance. Interference coupling is least when the
space between active and passive leads is at a
maximum, and when the angle between the leads
approaches a right angle.
Capacitive (electric) fields are voltage fields.
Their effects depend upon the amount of capacitance
existing between exposed portions of the noisy circuit
and the noise-free circuit. The power transfer
capabilities are directly proportional to frequency.
Thus, high-frequency components are more easily
coupled to other circuits. Capacitive coupling is
relatively easy to shield out by placing a grounded
conducting surface between the interfering source and
the susceptible conductor.
COUPLING BY RADIATION
Almost any wire in an aircraft system can, at
some particular frequency, begin to act like an
antenna through a portion of its length. Inside an
airframe, however, this occurs only at very high
frequencies. At high frequencies, all internal leads
are generally well shielded against pickup of
moderate levels of radiated energy. Perhaps the only
cases of true inside-the-aircraft radiation at HF and
below occur in connection with unshielded or
inadequately shielded transmitter antenna leads.
Some examples of interference coupling involve
more than one of the types (conduction, induction, or
radiation) just discussed. When more than one
coupling occurs simultaneously, corrective actions,
such as bonding, shielding, or filtering, used to
correct one type of coupling can increase the coupling
capabilities of another type of coupling. The result
may be an increase in the transfer of interference. For
example, an unbended, unfiltered dc motor can
transfer interference to a sensitive element by
conduction, inductive coupling, capacitive coupling,
and by radiation. Some frequencies are transmitted
predominately by one form of coupling and some
frequencies by others. At still other frequencies, all
methods of transmission are equally effective. On the
motor used in the example above, bonding almost
always eliminates radiation from the motor shell. It
also increases the intensity in one of the other
methods of transmission, usually by conduction. The
external placement of a low-pass filter or a capacitor
usually reduces the intensity of conducted
interference. At the same time, it may increase the
radiation and induction fields. This occurs because
the filter appears to interference voltages to be a
low-impedance path across the line. Relatively high
interference currents then flow in the loop formed
between the source and the filter. For complex
coupling problems, multiple solutions may be
required to prevent the interference.
RADIO INTERFERENCE REDUCTION
Learning Objective: Recognize various
methods and components used to reduce
radio interference caused by electrical noise.
Radio interference reduction at the source maybe
accomplished to varying degrees by one or more of