A Method of Determining the Comparative Shielding Effectiveness of Cables, Duct and Conduit

摘要

To make comparative measurements on coaxial, a length of the cable is confined in a shielded room, shorted at the far end and connected to a matching signal generator placed outside the room. The resulting leakage radiation from the cable is then measured with a frequency selective voltmeter. Another cable with different shielding qualities is then subjected to the same measurement and the difference between the two measurements is an indication of the comparative shielding qualities of the cables. Duct and conduit can also be measured for shielding effectiveness in the same manner by making them into a type of coaxial cable. This is accomplished by running a conductor down the interior of the duct or conduit. This conductor is of such a size, and place in such a position to make the enclosure characteristic resistance match the signal generator. A time domain reflectometer is used in assembling these enclosures. With this instrument the positioning of the conductor becomes an easy task because it displays the variation in impedance along the full length of the enclosure. Adjustment for a good match is accomplished easily. The technique is very useful when one type of cable or duct is to be compared with another, or to show up improvements in duct or conduit designs. (Author)