Background and introduction to EM screening (shielding) behaviours and measurements of coaxial and symmetrical cables, cable assemblies and connectors

The theory and related parameters of screening (shielding) are identified and behaviour explained. The concepts of the square root of power waves, general coupling functions and operational attenuation are introduced. The coupling mechanisms between parallel lines are explained and the influence of cable (coupling) lengths and frequencies is outlined and a summing function envelope called screening attenuation introduced. Theoretical curves with different cable screening parameters are shown. The mechanism of the leakage of a single braid is explained and shows that a perforated tube cannot be used as a model for a braided screen. The advantages and shortcomings of 16 different test methods of screen have been stated. Some preliminary statements of the measurement of the coupling attenuation of balanced cables are made. It seems to be an agreement between screening specialists that ITU-T Recommendation G 117 [32 ITU-T] could be used as definitions and principal measurement set-ups for the symmetry to earth of balanced cable pairs also in the 100s of MHz transmission region. Since both screening attenuation and coupling attenuation are dependent on measurement set-ups the results have to be measured in standardised conditions or test results have to be reduced to standardised conditions. However, it is recommended that both series of test results are given to avoid misinterpretation. Also in near and far end unbalance attenuations the capacitive and inductive coupling add towards the near end and subtract towards the far end