EoW July 2007

english

Here the data curves begin to merge at approximately 4.5 MHz and the solid copper material begins to drop at a slightly greater rate. The differences in the curves at both the high and low frequencies are relatively small and may fall within the variability of the tests. According to Formula 1 , the skin depth at 4.2 MHz is 21 microns. 4.2 MHz is the highest frequency in most security video applications. There appears to be no problem with the RF current carrying capacity of the copper clad shielding conductors at the higher frequencies used in standard NTSC video waveforms. However, the low frequency component of the waveform and the relationship to the high frequency components need to be examined.

Composite video testing focused on parameters that may show any tendency toward dispersion using the copper clad aluminium conductors. Test methods were taken from ANSI Standard T1.502- 2004, System M-NTSC Television Signals – Network Interface Specifications and Performance Parameters. Tests were performed using a Tektronix TSG95 Video Test Signal Generator driving the cable under test, which was connected to a Tektronix VM700 Video Measurement Test Set. All measurements, with the exception of the multi-burst were taken using an NTC-7 composite test signal. Test results are shown for 700 ft (213m) and 500 ft (152m) lengths of cable. Note that video signal levels are expressed in IRE units. Zero IRE in an NTSC video pulse is defined as the blanking level with 100 IRE equalling full white.

Formula 1

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Class 10 H is hard drawn material with 10% copper cladding by volume. The aluminium core is EC grade aluminium. Copper thickness for this material is 3.5% of the radius of the wire. The com- bined materials have a specific gravity of 3.32 compared to 8.89 for solid copper. Cladding thickness of the finished 34 AWG conductor is only 32 microns. However, when considering the resistance of the shield, the resistance of the outer layer must be taken into account. This is due to the skin effect. In a cylindrical conductor, the skin effect causes the current gradient to concentrate towards the surface of the conductors. This effect can be quantified in formula 1 as shown above. While the overall conductivity is 62% that of copper, the skin effect results in equivalent resistance at higher frequencies. Screening attenuation measurements were conducted according to IEC 62153-4-4. A commercially available triaxial fixture known as a CoMet tube was used for the measurements. Figure 1 shows a screening attenuation comparison of the copper clad aluminium, CCA, braided coaxial cable to the standard copper, Cu, braided shield. This chart covers the frequency range from 5MHz to 1GHz. The traces are an average of five measurements on five separate two-metre samples of each shield design. Note that the overall curves are very similar with the CCA braided samples showing slightly better results than the standard. Both shields show a sharp improvement in screening attenuation at the lower frequencies, dropping into the -75 dB range. Figure 2 expands the data from the higher frequency screening attenuation measurements. The data from 0.3 to 5 MHz was collected during the same measurements as the data from Figure 1 . Overview of testing and results

RG 59 Type 95% Cu

RG 59 95% CCA

72.8

700 feet (213 metres)

73.8

79.6

500 feet (152 metres)

80.4

T1.502 Specifications (Long Hall) +/- 7 IRE (93% – 107%)

Table 1 : Chrominance/Luminance Gain Inequality

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RG 59 Type 95% Cu

RG 59 95% CCA

700 feet (213 metres)

-51

-51

500 feet (152 metres)

-37

-36

T1.502 Specifications (Long Hall) +/- 54 ns

Table 2 : Chrominance/Luminance Delay Inequality ▲

RG 59 Type 95% Cu

RG 59 95% CCA

700 feet (213 metres)

2.0

2.0

500 feet (152 metres)

1.5

1.4

T1.502 Specifications (Long Hall) 1.5 IRE P-P (+/-1.5% P-P)

Table 3 : Line Time Waveform Distortion

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RG 59 Type 95% Cu

RG 59 95% CCA

700 feet (213 metres)

94.2

93.6

500 feet (152 metres)

96.0

95.3

T1.502 Specifications (Long Hall) +5.9 IRE to -5.5 IRE (+105.9% - 94.5%)

Table 4 : Insertion Gain

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97

EuroWire – July 2007