Types & Applications
Served shields are spiral-wound
groups of small-gauge wire strands surrounding the insulation
of the conductor(s). They are easy to unwind and terminate,
but are prone to be relatively inductive because they
are coiled around the cable. They are the most flexible
of shielded cables, and are often used in audio applications.
Served shields are usually soldered, or crimped to a
lug or termination post.
Braided shields are woven over
and under one another to form a tight but flexible cylinder
of wires. This may result in the need to unbraid or
loosen the weave in order to terminate it, though it
lends itself to easy coaxial connector termination,
and preserves the shield all the way to the connector
body. 95% coverage is not unusual on high-quality cables.
Most commonly, braids are formed of
groups of small-gauge wires known as “carriers.”
These are laid side-by-side, a ribbon-like multi-path
conductor. Braids can also be a “strip braid,”
using solid ribbons of conducting material, providing
a more uniform inner surface to a coaxial conductor.
This is an advantage at very high frequencies, and if
it is combined with other shield designs, forms a very
effective EMI barrier.
Braided coax shields are usually
terminated in the field by crimping or clamping, and
are occasionally soldered, or are terminated with a
heat-shrink shield pigtail. This latter approach is
common in aircraft wiring harnesses. Braided non-coax
shields can be soldered if the conductors within are
dressed to exit the shield through an opened space in
the braid, or can be terminated with a heat-shrink shield
pigtail.
Foil shields consist of a metallized
flexible plastic (Mylar, polyimide, etc.) wrapper, spiraled
around the conductor(s). The metallized layer is very
thin — on the order of .0003 inch. Foil coverage
can be effectively 100%, although its resistance is
far greater than the other shield described here, and
thus its ability to shunt noise is limited. For this
reason, EMI protection is best if foil shields are used
in combination with braided (better conductors) shields.
Foil shields, since they are usually
aluminum, are necessarily crimped, though some are combined
with a "drain wire", which makes contact with
the foil and may be soldered.
Solid shielding comprises a
metal tube, rigid or semi-rigid — usually copper
or aluminum — surrounding the dielectric and center
conductor. Semi-rigid coaxial cables of this design
can be formed by hand, though tube bending tools are
recommended, especially for smooth tubing. (Larger cables
may incorporate a corrugated-like tubing.) Coverage
is 100% and resistance is low. There is no better shield.
Common applications of solid-shielded
types include short, fixed coax jumpers inside an instrument,
or ground-based antenna feeds.
Solid shields are usually soldered
or clamped. Soldering aluminum solid shields is impractical.
Shield Coverage depends upon design
and level of quality — and ranks from poor to
perfect. Not every application justifies the costly
pursuit of perfection. The degree of need depends on
the frequencies of concern and the noise susceptibility
(or signal strength, for shield containment roles) of
the circuit.
Shield Termination should not be taken
lightly, since more problems crop up at connectors than
any other part of a cable.
In all cases, shield integrity is best
if the shield is intact (no broken strands or flaking
foil) and prepared for maximum contact with the connector.
This includes cleanliness. The connector manufacturer's
instructions deserve serious attention.
Shield Effectiveness
Regardless of its construction, a shield
will be only as immune to induced noise as its effectiveness
provides. This means that the shield will "intercept"
and/or deflect magnetic or electrical fields which interfere
with the signal needed. Frequency, amplitude and physical
spacing are factors. Management of the problems of noise
— induced, or as a source — includes shielding.
Shielding effectiveness is the ratio
of incident wave (source) field strength to the allowable
field strength. It is customarily expressed in dB.
Shields can function as reflectors
or absorbers (shunting to ground) of radiated electrical
or magnetic fields. Since in avionic systems we commonly
concern ourselves with rampant RF, we'll focus on properties
of shields which are most effective at high frequencies.
Reflection of unwanted signals can
be likened to a mirror. The surface of the shield is
the operative element, and in truth, it is the conductivity
of this surface — the skin — which plays
the most important role in high frequency applications.
This is one excellent reason for the silver plating
on the shielding wires used in high-performance cables.
Beneath the reflective surface, the
conductivity of copper lends itself well to absorption,
draining the interfering signal to ground. While copper
is not as effective as steel in absorption of frequencies
below 1GHz, it is a more effective shield, overall,
if both absorption and reflection are taken into account.
As the shield is designed, it is not
necessarily true that layer upon layer of shields are
any more appropriate than simple well-made shields may
be. Sometimes, however, layering can block openings
(interstices — the intersections of braid elements),
important because even a pinhole is a window to noise
at high frequencies.
Shields are beneficial in containing
interference as well as protecting from it, serving
to reduce the effects of noise which might be induced
in neighboring cables, or bundles of wires.
Testing for leakage in coaxial cable
shields tells the story of how to cope with the RF "traffic
jam" that is now more invasive than ever anticipated.
It is a given that 100% shielding,
such as provided by rigid or semi-rigid cables, is an
ideal. What is less evident is that MIL-C-17 coaxial
cables are far off the ideal. Perhaps this is why PIC's
multi-layer low-loss cables are increasingly accepted
— for perhaps other reasons, too, such as loss
or weight — but bring the substantially improved
leakage factors along as well.
Network analyzer testing of PIC coax
designs — incorporating the silver-coated inner
strip braid, the metallized polyimide 100% wrap, and
the tight wire outer braid — show performance
that approaches that of semi-rigid cables. Comparison
testing shows leakage on the order of 55-75dB for RG142,
and 85-90dB for PIC S44193. Semi-rigid RG402 is 110dB.
(These are all cables whose other characteristics are
roughly comparable.)
To summarize, shielding usually
just works. But if EMI problems crop up — and
they will when you least expect them — it is always
important to consider the quality of the cable as a
first line of defense.
What's the importance of all
this? In the complex RF environment of an ever-growing
rat’s nest of signals, often bundled together
and “sharing” noise, better shielding improves
signal integrity and system reliability.
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