Coaxial conductor of high screening capability

Abstract

The simplest coaxial conductor consists of a homogeneous tube, often termed ‘the screen’, with a central conductor running axially through it. The geometry of this arrangement ensures that the coaxial conductor is virtually immune from direct interference arising from external sources of electric or magnetic fields. However, currents may be induced in those external paths of which the screen forms a part. In that case the voltage drops along the screen associated with these currents will cause interference currents to flow around the coaxial conductor circuit. This interference mechanism is analysed, and it is shown that with a simple modification, namely the reduction of the wall thickness of the outer conductor over an appropriate length, the interference current over a specified frequency range can be substantially reduced. Conversely, the signal current in the coaxial circuit does not directly cause interference in outside circuits, but the associated voltage drop along the screen can cause interference currents to flow around external paths. Reciprocity applies, and the carrying out of the simple modification to which allusion has been made also brings about the same substantial fractional reduction in this external interference, over the same specified frequency range.