Test fixture design and shielded screening attenuation performance of CATV coaxial cable

This paper describes a practical test fixture that has been developed for the measurement of shielded screening attenuation (shield effectiveness) of community antenna television (CATV) coaxial cable. The theoretical base of the fixture is contained in the draft Amendment 1 to International Electrotechnical Commission (IEC) Standard 61196-1 and the original German development publications. The test fixture housing is a cylinder that forms a triaxial test configuration with the cable sample shield and centre conductor. The test fixture is thus composed of two regions; the test chamber and the cable sample. The test chamber diameter ratio determines the upper frequency test limit and the lower frequency limit is determined by the length of the cable sample. The sensitivity of the test method is limited by contact resistance where the cable sample shield forms a short circuit with the cylindrical test fixture end plate and by leakage from the cable sample termination which is located within the fixture test chamber. These factors have been minimised to achieve a measurement capability greater than 140 dB over the 1-1000 MHz frequency range. Test results are presented for a selection of braid and combination foil-braid shield designs for RG 6 type coaxial cable measured in lengths up to 6.7 meters over the frequency range 1-1000 MHz. The paper concludes that the test fixture is a satisfactory implementation of the shielded screening attenuation test method. However, recommendations are given for further study of combination foil-braid shield performance