This might not directly answer your question, but may put some light on why these questions are difficult to answer in general.
For all cables , the transfer impedance is equal to the Ohmic resistance of the shield at low frequencies. At higher frequencies the braiding effect causes due to the rotating effect of the braiding magnetic field line escape in a direction 90 degrees on the cable. This is additional self inductance that must be added to the transfer ohmic resistance, so at high frequencies the transfer resistance becomes a complex impedance At even higher frequencies the shield cuts itself in an inner shield for the signal current and an outer shield for interference current, due to the skin effect. ( this greatly improves the shielding quality) This of course at frequencies where the "skin depth" is much smaller then the effective depth of the shield. The braiding effect creates here a problem as the wires change with every cm from in- to outside of the shield. The (skin) current WILL stay outside and has to cross from wire to wire. Here oxidation will change cable properties when cable gets old. Spiral (foil) shields rely on these contacts between "windings" to avoid heavy self induction effects due to the winding of the spiral. As the ohmic resistance is higher as copper, at LOWER frequencies these shields are inferior to braided ones. There are however, much less contact points per cm as the braided cable but many contact SURFACE exists so capacitive coupling between spiral winding will reside at high frequencies where oxidation might impair contact resistance. In general these cables do well at high frequencies where self induction part of the transfer impedance is much dominant and where oxidation might happen. Often these shields are combined with a braided shield to get the best of both worlds. Best of all is the massive shield , (semi rigid cable). Regards, Gert Gremmen, (Ing) ce-test, qualified testing =============================================== Web presence http://www.cetest.nl CE-shop http://www.cetest.nl/ce_shop.htm /-/ Compliance testing is our core business /-/ =============================================== >>-----Original Message----- >>From: [email protected] >>[mailto:[email protected]]On Behalf Of >>[email protected] >>Sent: Monday, June 18, 2001 10:55 PM >>To: [email protected] >>Subject: braided vs served shields >> >> >> >>Does anyone have any references or data on the comparative transfer >>impedance between served (spiral) and braided cable shielding at the same >>coverage level? >> >>Thanks! >> >>Brent DeWitt >> >> >>------------------------------------------- >>This message is from the IEEE EMC Society Product Safety >>Technical Committee emc-pstc discussion list. >> >>Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >>To cancel your subscription, send mail to: >> [email protected] >>with the single line: >> unsubscribe emc-pstc >> >>For help, send mail to the list administrators: >> Michael Garretson: [email protected] >> Dave Heald [email protected] >> >>For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >>All emc-pstc postings are archived and searchable on the web at: >> http://www.rcic.com/ click on "Virtual Conference Hall," >>
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