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EuroWire – July 2007
96
english
Performance of bi-metallic
braid wire in electronic
cable shielding
By Randall D Stevens, CommScope Inc, North Carolina, USA
Introduction
Traditionally copper is the preferred
material used in braided coaxial cable
shielding applications where solderability
and lower contact resistance at the
connector interface are important.
Until recently copper clad aluminium
has not been readily available in fine
wire sizes suitable for braiding. Recent
improvements in drawing fine gauge
copper clad aluminium has made these
materials available for new braided
shield designs.
Bi-metallic materials may be used in high
frequency applications without loss of
transmission performance because the
majority of the RF signal current is carried
on or near the surface of the conductor.
The depth of the current gradient near
the surface of the given conducting
material is known as the skin depth.
Tests were performed on RG 59 coaxial
cables to evaluate electrical performance
of the copper clad aluminium wire as
shielding material.
Prototypes of RG 59 type low loss 75 ohm
video coaxial cables were prepared for
these comparisons. The coaxial cables
consisted of solid copper 20 AWG centre
conductors with a foam dielectric and
95% optical braid coverage.
One cable shield used 34 AWG solid copper
shielding. The other used 34 AWG copper
clad aluminium shielding wire. Screening
attenuation comparisons were made along
with basic NTSC video measurements.
Measurements of basic NTSC video
parameters show the copper clad
aluminiummaterial to be virtually identical
in performance to the solid copper
materials. Higher DC resistance values
may limit the use of copper clad
aluminium for low frequency applications
such as in-line power inserters. Shielding
effectiveness is comparable for the two
materials over a wide frequency range.
Overall, the copper clad aluminium offers
the advantage of being significantly lower
weight than copper while delivering
equivalent electrical performance for
many applications.
The copper clad aluminium used for the
coaxial shield was 34 AWG Class 10H per
ASTM B 566.
Figure 2
:
Low Frequency Screen Attenuation
▲
Figure 1
:
High Frequency Screening Attenuation
▲
Shielding Method: Screening Attenuation Comet Tube IEC 62153-4-4
Type Cable: 59 95% CCA vs RG 59 95% Cu 5 MHz to 1000 MHz
Shielding Method: Screening Attenuation Comet Tube IEC 62153-4-4
Expanded Low Frequency
Type Cable: RG 59 95% CCA vs RG 59 95% Cu .3 MHz to 5 MHz