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When
electronic equipment intended to handle certain precise levels of input, whether
logic or continuous signal, picks up undesired inputs at the operating,
triggering or higher levels, a dysfunction occurs. The sources of EMI/RFI
include conducted interference via wire, cable, and/or induced voltage and
current attributable to electromagnetic fields that couple energy into the
calibrated circuits. Sometimes the undesired source is obvious and can be
subjected to line filtering or shielding suitable to the frequency and intensity
encountered. However, unexpected and unpredicted sources and combinations may
not be analyzed so easily. The earth’s magnetic field, of course, is pervasive but not always taken into
consideration. Other unwanted fields including electromagnetic pulses of wide
dynamic range can be caused by local severe thunderstorms and improperly
grounded power cable systems acting as antennas for switching transients on the
power lines, or for the low-frequency power currents. In aircraft, for example,
instruments are closely packaged due to limited space. The radar tube’s
performance can be visibly distorted by nearby tachometers which may radiate a
rotating magnetic field. The radar display is subject to some position shift
each time the aircraft changes direction or attitude relative to the earth’s
field. A magnetic (i.e. permeable) shield enclosure minimizes these effects as
well as supporting and positioning the tube.
Clear, sharp CRT readouts are vital in many applications.
Yet, without magnetic
shielding at the tube neck, this cannot be optimally achieved. In electron
microscopes, a magnetic shield around the vertical column prevents resolution
deterioration caused by beam scattering, bending or displacement from normal
optimum focus position. A sharp, clear focus is thus achieved, permitting full
magnification.
Magnetic shielding is indispensable for providing an economical, repeatable
controlled magnetic environment for determining response characteristics,
sensitivity and orientation direction of magnetic sensor devices used for
signature recognition, proximity sensing, etc. in a wide variety of industrial,
military and commercial security applications.
Complex, high resolution video recorded head assemblies must be shielded from a
wide range of magnetic field interferences that may prevent full operational
capability in recording/playback applications in TV studio/mobile, closed
circuit, professional home and other video display systems.
Some comparatively new hazards to optimum electronic equipment functioning are
still largely unrecognized, such as the low ceilings in modern concrete
structures reinforced with steel beams. The metal in the ceilings is much closer
to sensitive equipment than was the case in older, higher ceilinged buildings.
The resulting magnetic disturbance is substantially greater than 150 Gamma/cm, a
typical magnetic field gradient in older reinforced concrete industrial
buildings.
There are also hazards when analogue or digitized data on magnetic tape or
cassettes is stored or transported. The fidelity of vital recorded information
may be distorted or even partially erased by unforeseen external magnetic
fields, or by carelessness of unheeding or uninformed personnel, or by
deliberate vandalism with powerful permanent magnets. Tape data protectors
provide needed shielding
against such hazards. The protectors are used by all branches of the armed
forces, NASA, other governmental organizations and many private firms.
Once the offending field source is identified, one practical approach in
determining needed shielding is to order a small quantity of heat treated
ready-to-use magnetic shielding foil from a shielding manufacturer. It is
available for immediate delivery in various convenient widths, lengths and
shielding strengths for high or low permeability requirements with a range of
electrical conductivities. Foil is easily, quickly cut with ordinary scissors
and hand shaped to the desired outline. It is ideal for R/D, hard-to-get-at
places, or for small quantity or extremely compact applications. Many shielding
problems thus can be solved quickly.
After hand shaping around the component to be shielded, the foil can be held in
place with simple adhesive tape. Thickness and number of layers can be
determined by ordinary trial and error procedure, or a formula to follow may be
requested from the manufacturer. Begin by using a single layer and then adding
layers until the desired shielding effect is achieved.
When using multiple layers in steady fields and at low frequencies, the low
permeability layer should be closest to the field source. This tends to increase
the flux density shielding capabilities. The low permeability layer diverts the
major portion of the field, permitting the high permeability layer or layers to
operate in a lower reluctance mode.
If you need relatively few shields or are
experimenting, foil is the swift, economical solution.
Once foil shielding is functioning satisfactorily in either experimental or
production applications, it is time to evaluate the economics. The cost of foil
versus prefabricated shields for that particular application should be compared.
A prefabricated shield is less costly in larger quantities and for certain
complex applications.
For designing and manufacturing prefabricated magnetic shields in-house, sheet
stock may be ordered. Sheet stock may be formed by bending, stamping, drawing,
finishing, etc. on ordinary sheet metal equipment and finished by plating, MIL
spec painting, etc. For optimum magnetic shielding characteristics, shields must
be heat treated after all forming, welding and machining operations.
Your supplier will guide you in the use of the various available states of heat
treatment, such as the one which permits ease of forming (mill annealed) or the
treatment which assures the maximum mechanically stable permeability or the
absolute maximum permeability (which is not necessarily stable mechanically or
thermally in some high nickel alloys).
High electrical conductivity and high magnetic permeability both contribute to
the effectiveness of thin foils in fast-rising pulse shielding by reducing the
skin depth. Distinctions have lately been made between the case where the foil
thickness exceeds the skin depth and where it is greater. This type of shielding
against pulse-type interference achieves the highest order of shielding
effectiveness generally obtained by any means. Attenuations between 300 dB and
1000 dB are not unusual. |
