Magnetic Field Pickup by Flexible Braid Coaxial Cables

Cabling systems have long been known to be a source of unwanted pickup of radio-frequency interference. A typical source of pickup occurs when the outer sheath of a coaxial cable system forms part of a ground return for potentially interfering electromagnetic energy. Another source of penetration arises from a magnetic field which is perpendicular to the axis of the coaxial cable. A previous study demonstrated that the field emanating from a coaxial cable could be characterized as an equivalent open-wire pair. This particular effect is caused by the eccentricity between the centers of the outer and the inner conductors or because of other asymmetrical characteristics. The susceptibility of a coaxial cabling system to perpendicular magnetic fields can also be characterized in terms of an equivalent open-wire pair for short runs. The purpose of this investigation was to measure the shielding effectiveness of a typical flexible braid coaxial cable to magnetic fields perpendicular to its axis and to demonstrate that a coaxial cable may be considered as an equivalent open-wire pair in a cylindrical shield. A test setup employed a parallel plane transmission line to generate a uniform magnetic field perpendicular to the axis of the coaxial cable. The shielding effectiveness of the braid was measured for several classes of braid conditions. Then, using the same braid, a coaxial cable with an experimentally induced known eccentricity was employed and the overall pickup measured. The results confirmed the objectives of the investigation.