Coax cable screening efficiency

Coaxial cable isn't perfect. Even with a solid screen conductor, some portion of an interfering external signal will leak into the cable. But how much? Here is a test set (Fig. 1) to measure just that, together with some measurement results for a few common cable types.

Fig. 1: Cable leakage test set

The test set consists of a 1m piece of coax cable mounted concentrically inside a brass tube, so that the tube with the cable screen also forms a coaxial transmission line with a well-defined characteristic impedance. Coaxial connectors B or C permit the injection of an agressor signal. The portion of that signal that leaks into the cable can then be measured at connectors A or D. These latter connections are insulated from the injected signal by a pair of coaxial baluns. The common mode inductance of these baluns are what limits the lowest frequency at which the test set is still usable. All ports are looking into 50Ω source or load impedances.

Some portion of a current flowing over the screen of the coax under test ends up causing a voltage on the inner conductor. The –frequency dependent– transfer impedance Z_T depends on the screen resistance for low frequencies. For medium frequencies, Z_T drops, as the current progressively flows only on the outer surface of the coax under test. For high frequencies, where the leakage inductance of the coax cable becomes significant, Z_T starts to rise again.

Fig. 2: Transfer impedance of 1m of cable

Comments with the measurements

The test jig outer tube has a 24mm i.d. and 1m length. The cable diameters ∅_S are those the stripped screen, i.e., without the outer insulating sheath. This diameter is used to calculate Z_ext and L₀.
R_screen is the DC screen resistance. Z_ext is the characteristic impedance between the outer pipe and the cable screen. For both RG58 and RG214, the screen consists of an Al foil, a tinned copper braid and another Al foil. The UT141 screen is solid copper.

Note that for RG214 and UT141, the resistance of the N-connectors, about a milliohm, is enough to affect the results for low frequency. Also, the reproducibility leaves to be desired. As a result, it is difficult to measure transfer resistances in the lower milliohm range at low frequencies. The transfer resistances of RG214 and UT141 end up at about 7mΩ, rather than the expected R_screen. The superiority of the UT141 with its solid copper screen is evident.

Table 1: Cable parameters

RG214 RG58 UT141 ∅ 10.8mm 5.0mm 3.6mm ∅S 7.3mm 3.5mm 3.6mm Z0 50Ω 50Ω 50Ω Zext 70Ω 115Ω 114Ω L0 238nH 385nH 380nH Rscreen 4.6mΩ 12.5mΩ 6mΩ

References

• International Electrotechnical Commission, IEC Standard Publication 96-1, Radio Frequency Cables, Fourth edition 1986
• D.A. Weston, Electromagnetic compatibility, Marcel Dekker Inc. 2001, ISBN 0-8247-8889-3
• E.F. Vance, Coupling to shielded cables, Wiley 1978, ISBN 0-4710-4107-6
• H. Kaden, Wirbelstrome und Schirmung in der Nachrichtentechnik, Springer Verlag, 1959