The following tools are available to the engineer for reducing magnetic
- Reduce pickup loop areas
- Increase cable separation
- Orient loop perpendicular to source
- Transpose loops
- Use shielding on cables
- Use high permeability materials on or around the cables
1. Pickup Loop Area Reduction
Reducing the physical area enclosed by the pickup loop will reduce coupling
significantly. The loop area can be made effectively smaller by running the
signal and its return wire adjacent or parallel. Conversely, when the structure
is used for a return wire, a large pickup loop area is established. The use of
the chassis or ground as a return increases the loop pickup area and greatly
increases the cable's susceptibility to interference.
2. Cable & Wire Separation
The most obvious means to reduce cable coupling is to increase the distance
between the source and victim. Figure 4-6 shows the reduction in induced
voltage when separation is increased, while maintaining all other factors as
3. Perpendicular Loop Orientation
Coupling between the source and pickup loop is reduced to a minimum when the
conductors are oriented in perpendicular planes. Crossing cables at right
angles, however, is not always feasible and other means must be used to reduce
coupling when parallel runs are required by aircraft and missile
4. Loop Transposition
The transposition of a wire and its return conductor is an old technique
originating in telephone line construction. Transposition is accomplished by
alternately placing the signal and its return close to the source wire. This
results in the cancellation of interference.
Twisted pours are an example of the transposition of loops. The use of
twisted pairs often will minimize a difficult interference problem . Recent
developments include twisted pairs of special configuration that yield an
extremely tight and uniform twist.
5. Balanced Circuits
Another method by which interference voltage and current may be canceled out
of the susceptible wiring is the use of balanced lines fed by balanced circuits.
The signal wire and its return are maintained at the signal potential with
respect to each other and are equally balanced between the ground potential. The
coupled interference voltage appears equally on both wires. 180 degrees out of
phase. thus canceling the interfering voltage. This interference cancellation is
effective if a perfect balance condition is maintained and interference source
does not drive the circuit into the nonlinear response region. Two-wire shielded
cable can be used to reduce the magnitude of this type of interference
6. Shielding on Cables
Two basic principles may be used to provide magnetic shielding:
a) magnetic flux from the source may be absorbed from the pickup loop in high
permeability materials; or
b) the magnetic flux will not be reflected away from a pickup loop by high
Section 5 (paragraph 5.1) addresses cable shielding techniques in
The technique of surrounding sensitive wires by other wires within the same
cable provides a degree of shielding if the surrounding wires are near ground
potential. This process can be carried into the plug and connector region where
it will assist in reducing stray coupling.
7. High Permeability Materials
High permeability materials restrict the leakage flux path from magnetic
components such as transformers, and may be used to enclose susceptible cables.
High permeability tape has been used successfully to contain the magnetic
interference fields from long lengths of power transmission lines. To be most
effective, a high permeability shield must present closed path for magnetic flux
with a minimum of reluctance in the magnetic circuit High permeability materials
needed for magnetic shielding have the disadvantage of being heavy and easily
damaged. Therefore, their use shared he limited.
Most of the standard shields found on shielded cables provide no significant
protection against magnetic coupling at low frequencies.