Actually it is called a "Z" fold as it imitates a squashed letter z when folded. The foil is folded that way so it makes continuous circumpherential contact. Not all manufacturers employ a "Z" fold or equivalent method and their coax cable's performance is degraded. The "Z" fold is much better than an overlapping non-contacting or a spiraling foil.
Hans --- Gary McInturff <[email protected]> wrote: > > Nicely put, but I want to emphasize one point. The foil in most cables > doesn't have a metal to metal contact. Where the foil overlaps itself the > polyester on the "underside" is what is in contact with the foil "outside". > The type cable you are mentioning is called an "e" fold if I remember > correctly, but I don't kno why. A lot of folks don't realize that they are > not getting foil to foil contact, just and overlap of material. > Gary > > -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: Tuesday, November 07, 2000 8:58 AM > To: [email protected]; [email protected] > Subject: Re: Coaxial cable > > > > Sergio, > A "foil & braid" shield is quite common on high-speed cables. If a cable is > properly terminated and you don't have common-mode problems, most of its > radiated emissions will be from holes in the shield. Thus "optical > coverage", > the percentage of the shield's nominal area that is actually covered by > wires/conductive foil, is a reasonable approximation to the shielding > effectiveness. > > It is very difficult to braid wires in a way that achieves over 95% optical > coverage. A foil shield, with the overlap folded over so the conductive > surfaces touch, can easily achieve 100% optical coverage, but is fragile. > If a > foil-shielded cable vibrates, or is repeatedly bent, the foil will > eventually > tear. Even if end-to-end continuity is retained, this hole in the shield > can > cause a great increase in radiated emissions. By braiding wires over the > foil, > you start out with 100% optical coverage, and if/when the foil tears degrade > in > just that area to the 90-95% optical coverage of the braid. > > We used to use a type of parallel cable for Electromagnetic Compatibility > (EMC) > testing that had a foil shield. We would get about three weeks use out of > these > before they went bad and had to be thrown away because of excessive radiated > emissions. I helped develop and release an IEEE-1284 parallel cable in > 1994 > (Lexmark partnumber 1329605) that used a foil & braid shield, and we put > these > in our EMC lab. It took nine months of heavy use before the first of these > cables exhibited a noticeable increase in emissions over brand-new cables. > > John Barnes Advisory Engineer > Lexmark International > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > ===== Best Regards Hans Mellberg __________________________________________________ Do You Yahoo!? Thousands of Stores. Millions of Products. All in one Place. http://shopping.yahoo.com/ ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
