Cylindrical Shielding and Its Measurement at Radio Frequencies

The effectiveness of shields from the point of view of the wave theory of shielding is discussed. Specific consideration is given to cylindrical shielding against low-impedance fields and its measurement at radio frequencies. Various methods and concepts of measurement are discussed briefly; inadequacy of probe-type tests and the advantages of an integrating-type test are pointed out. Equipment of the integrating type suitable for production testing of specimens of cylindrical shielding from 3/16 to 2 inches diameter at 3 megacycles is described and illustrated. With this equipment, shielding effectiveness of the unknown is determined in terms of the effectiveness of a specified rigid metal-tube standard. Sensitivity is sufficient, to measure the leakage through 0.024 inch of copper at the test frequency. A shielded room is not required. Experimental results obtained with this and similar equipmen from 200 kilocycles to 10 megacycles are given. Tests at various frequencies on thin-wall copper tubes of different thicknesses are shown to be in agreement with the results predicted by theory. Included are data on metal tubes, wire braids, coaxial cable, and flexible-shielding conduits. Test results are shown to be independent of current through the specimen, receiver gain or adjustment, and various other factors. Results are shown also, in general, to be independent of the length of specimen tested and its impedance. Various factors affecting test results are considered and formulas are given for correcting results obtained on exceptional specimens having abnormally high resistance.