RE: Coaxial cable
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 gary.mcintu...@worldwidepackets.com 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 ane 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: jrbar...@lexmark.com [mailto:jrbar...@lexmark.com] Sent: Tuesday, November 07, 2000 8:58 AM To: sergioro...@siemens.com.br; emc-p...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org = 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Coaxial cable
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: jrbar...@lexmark.com [mailto:jrbar...@lexmark.com] Sent: Tuesday, November 07, 2000 8:58 AM To: sergioro...@siemens.com.br; emc-p...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Coaxial cable
Richard, The term that I ran across many times while researching my book was optical coverage-- as though you put a light bulb inside the shield and measured what percentage of its light leaked out. The basic assumptions are: 1. Electric and magnetic fields inside the shield are totally blocked by the picks (conductive wires/strips/foil in a group), so the only leakage is through the holes between picks. 2. The fields that leak out are incoherent, and thus add as scalars (sum of magnitudes) for the peak leakage. This is a first-level approximation, and is closely related to a discussion of the shielding effectiveness of arrays of holes/ honeycomb on this mailing list a couple of weeks ago. Over a wide range of frequencies, and far enough away from the shield that the openings seem to blur together, the leakage is approximately proportional to how much of the inside/ other side of the shield is exposed to our view. But at specific frequencies, or if we get very close to a hole in the shield, we get diffraction and constructive-/distructive-interference that cause lobes and notches in the leakage fields. Some companies have tried to take advantage of this for special (high-priced) single-function cables. Optimal braiding selects the gauge and number of wires in each pick (group of wires laid parallel to one another) and carefully controls their crossing angle during construction of the cable. The idea is to create holes with a certain size and shape, and thus polarizability, and with a certain spacing lengthwise and around the cable. The authors of the articles claimed that at the design frequency they would get distructive interference, just like a diffraction grating, making the holes in the shield appear smaller than they really were. The problem that I saw with this scheme was that at other frequencies, or if you bent/pulled/deformed the cable in any way, the geometry changed and you would get an *increase* in emissions. So it always seemed more sensible to me to try to completely seal the electric and magnetic fields inside the cable/shield and not optimize just one tiny operating point. John Barnes Advisory Engineer Lexmark International author of Electronic System Design: Interference and Noise Control Techniques (Prentice-Hall, 1987) richardg%exabyte@interlock.lexmark.com on 11/07/2000 12:11:36 PM To: john_barnes.lexm...@sweeper.lex.lexmark.com cc:(bcc: John Barnes/Lex/Lexmark) Subject: RE: Coaxial cable John, Couldn't quite follow the optical coverage reference. Sounds like it should be optimal coverage for copper wire cable verses optic cable. Thanks. Richard Georgerian Technical Committee 8 Product Safety (TC-8), Vice-chair Colorado Product Safety Technical Committee (CPSTC), Chair Product Compliance Engineer Exabyte 1685 38th Street Boulder, CO 80301 USA tel.: 303-417-7537 fax: 303-417-5710mailto:richa...@exabyte.com -Original Message- From: jrbar...@lexmark.com [mailto:jrbar...@lexmark.com] Sent: Tuesday, November 07, 2000 9:58 AM To: sergioro...@siemens.com.br; emc-p...@ieee.org 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
Re: Coaxial cable
No. The foil has lower inductance at high frequencies. The combination of foil and braid gives the best protection in a flexible cable. -- From: SERGIO LUIZ DA ROCHA LOURES SERGIO sergioro...@siemens.com.br To: emc-p...@ieee.org Subject: Coaxial cable Date: Tue, Nov 7, 2000, 8:14 AM Group We are using a coaxial cable with two shields. One is a metallic mesh and the other is a aluminium foil. This foil is known as static foil. What is the use of this foil? Is this used for electrostatic reasons? Regards Sérgio Rocha Loures Siemens Ltda. - Brazil ICN FL QEL Tel: +55 41 341-5755 Fax: +55 41 341-5058 E-mail: sergioro...@siemens.com.br --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
