Cables and connectors 23/25 Metallization Ferrite Bulkhead Ceramic dielectric Inner conductor Figure 23.32 The feedthrough capacitor: (a) simple capacitor; (b) -section filter In the presence of an electromagnetic field any cable acts as an of this shield is to provide a return path for induced currents which antenna and energy from the field is coupled onto it. The current does not couple onto the signal conductors, or conversely to confine induced on the cable depends on its physical orientation with respect radiating currents that are present on the signal conductors and to the field and any nearby conductive objects, and on its length. In prevent them from coupling with external fields. a reciprocal manner, the field radiated by a cable carrying an r.f. Figure 23.35 shows the ideal connection of a shield to screen in current also depends on these parameters. Cable length is some- which there is no discontinuity between the two. This can be most times under the designer's control, but orientation is not, except nearly achieved when the cable is fixed to the equipment and led occasionally in system design. Therefore, one needs to take steps to through a conductive gland SO that the cable screen makes contact prevent interfering cable currents from affecting circuit operation, all the way round its circumference, through 360°. As soon as a or to prevent circuit operation from generating interfering cable currents. connector is used, some compromise has to be made. Military style connectors are designed as above SO that the cable screen makes 360° contact, but they attract military style prices and 23.15.1 Cable shield termination assembly costs. R.F. coaxial connectors, such as the common BNC type, also make 360° contact, but they carry only one signal line at One approach is to filter the signal lines at the point at which they a time. Many multi-way connectors do not have proper provision enter or leave the cable. Where this is inadequate or impossible, the for terminating the shield, and this is where perform