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Either way, the goal of using these cables is twofold: to
prevent (or at least reduce) signals inside the cable from
coupling to other circuits or cables; and to prevent stray
electromagnetic fields from creating voltages and currents
in the conductors inside the shield. So far, so good.
In most of these cables — particularly those intended
for use with audio or RF below 1 MHz — if you strip back
the shield, you’ll see that the conductors are twisted
together. (The number of times per inch or cm that the
wires are twisted together is called the pitch.) This is done
for a couple of reasons. First, twisting the conductors keeps
them pressed together so there is a minimum of area
between them. This helps reduce the voltage induced in
the conductors by an external field (see the sidebar).
Second, it maintains electrical balance of the
conductors so they all look about the same to an external
field. This keeps signals that may be coupled to the cable
on all conductors (common mode signals) from being
converted to signals that are not the same on different
conductors (differential mode signals), and so interfere with
the desired signals. Figure 1 shows examples of common
and differential mode signals on different types of cables.
So, doesn’t the shield keep all fields out of the cable?
Not really. At very low frequencies — such as AC power
at 50/60 Hz — the magnetic fields from a transformer or
power line go right through nearly all shields because the
material is not thick enough. (See the sidebar on shielding
effectiveness.) Even a high quality coaxial cable shield is
not enough. Just feed some low-level audio through a piece
of coax laying on a power transformer to hear the induced
hum. As the frequency gets higher, the shielding becomes
more effective, as long as it is connected properly.
The other common type of shielded cable is coaxial
cable with a single center conductor surrounded by an
outer conductor (shield), separated by a plastic or air
dielectric. Current in coax flows on the outside of the
center conductor and on the inside of the shield which is
either a tightly woven braid of fine wires, a solid tube of
metal (foil or thicker), or a combination of the two. Coax
is very effective at shielding the signal flowing inside the
cable from outside fields because of its excellent symmetry
(which causes the effect of the fields to cancel) and
because of the skin effect (see the sidebar) that isolates
common mode current on the outside of the shield from
the signal inside.
The higher the frequency of signals used by a circuit,
the more important it is to use a metallic enclosure.
Assuming wires and cables going in and out of the
enclosure are properly routed and connected, the metal
enclosure can keep RF from external fields out and RF from
internal signals in. Furthermore, a metal enclosure provides
mechanical strength to your project.
Shielding is important for sensitive or important
circuits, and to be a “good neighbor” to other equipment
that might be disrupted by radiated or conducted EMI
(electromagnetic interference). As an example, if you have
a general coverage receiver and live in a typical suburban
or urban neighborhood, connect a dipole or other antenna
and tune to 14.030 MHz — you will hear many weak
“birdies” (single tones, often slightly wobbly or jumpy)
from Ethernet network equipment being radiated by cables
connected to unshielded or poorly shielded equipment.
(Computer equipment is notorious for creating and
responding to EMI into the UHF range.)
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
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How Effective is Your Shield?
A magnetic field is attenuated by absorption as it passes through
the thickness of a shield by 8. 7 dB for each skin depth of thickness.
(See the sidebar on skin depth.) Figure 2 is a graph of skin depth vs.
frequency for common shielding materials. The smaller the skin depth
for a shielding material, the greater the absorption it provides for a
Only magnetic materials can provide magnetic shielding at AC
power frequencies. Copper and aluminum thick enough to form a rigid
chassis or enclosure begin to provide magnetic shielding around 10
kHz and are quite effective above 1 MHz.
Electric field shielding occurs by reflection, and the loss can be
R = 20 log [(ZW / 4ZS) cos F] dB
where R is the reflection loss, ZW is the wave impedance (377W in free
space), ZS is the impedance of the shield, and F is the angle between
the field and the shield.
For common shield materials like aluminum and copper, ZS is <1W
so even a very thin shield can provide more than 70 dB of shielding.
Electric field shielding is quite easy — especially at low frequencies.
An electric field shield must be continuous and must completely cover
the circuit to be shielded.
What is Skin Effect?
Because of how AC fields interact with excellent conductors,
metals conduct AC only to a certain skin depth, d, that is inversely
proportional to the square of the frequency. This is called the skin
effect and it decreases the cross-section area of the conductor that
conducts AC, increasing its resistance.
d = 1 √ vpfµs
µ is the conductor’s permeability and s is the conductor’s
The skin effect begins to have a significant effect above 1 MHz.
By 10 MHz, a copper wire only conducts current in its outer 0.02 mm.
At UHF and above, a thin layer of metal plating is sufficient.