EoW July 2007

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

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

Figure 1 : High Frequency Screening Attenuation

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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

Figure 2 : Low Frequency Screen Attenuation

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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.

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EuroWire – July 2007