The cables in question all used braided shield construction and were aerospace quality. The individual center conductors are insulated, of course, but there is nothing else between the insulated wires and the shield braid. The shield braid itself appears to be Kapton coated. It is indeed true that the twisted pair as a center conductor has an irregular shape relative to a single coaxial conductor. However there is an expression for the characteristic impedance of just such a cable (except for the twist) in the Reference Handbook for Radio Engineers. In my 1953 copy this type of transmission line is termed "wires in parallel - sheath return." It is a complex formula but the point is that even with an irregular center conductor there is a defined transmission line characteristic impedance, which means that whatever the impedance actually is, if properly matched, mismatch losses may be bounded. If it were "mismatch loss" wouldn't the loss be strongly frequency dependent in a periodic fashion? The losses I measured increased with increasing frequency, and the nature of the increase was identical to that specified for the RG-XX coaxial types (with larger magnitude as previously noted). I can send a test data attachment to anyone interested. ---------- >From: "Wan Juang Foo" <f...@np.edu.sg> >To: Ken Javor <ken.ja...@emccompliance.com>, <emc-p...@majordomo.ieee.org> >Subject: Re: Stumped (TSP or FTP (Foil screened Twisted Pair) ) >Date: Mon, Oct 14, 2002, 9:11 PM >
> > Ken, > I am only making a guess in the absence of the cable's specifications. Is > your TSP a FTP (Foil screened Twisted Pair) type of cable constructed from > copper wires coated with polyethylene and wrapped by Mylar tape (a > transparent and mechanically tough film) between the TP and the screen? I > think it is just mismatch losses due to the eccentricity of the cable. > Imagine what kind of performance one would expect from a coax with an inner > conductor that exhibit irregular cross sectional radius. > > IMHO, it's the inherent 'mismatch' losses of the 'cable' to CM signal. > Given the inherent twisting (eccentricity) of the conductors within the > 'screen', it is more like a coax with a screw shaped (how would I describe > it???) distribution of Zo along the length of the cable. The TP > configuration in the CM situation should be low loss only to circularly > polarised electromagnetic waves (if there is such a phenomenal in > electromagnetic propagation within a Tx line). For propagation modes of > anything else approaching something that may represent a substantial > fraction of Lambda would be, (or shall I say, should be) presented (or > seen) as a lossy line due to the changing cross sectional characteristic of > a FTP. > >> .... I expected losses that would be on the same order or lower than >> that associated with off-the-shelf coax types like RG-58. Instead my > losses >> were dramatically higher. > > In terms of CM performance (e.g. input impedance, losses, etc... ), a FTP > due to it's eccentricity, I suppose cannot be compared to a RG-58. I have > come across some FTP with 0.52 mm i.d. copper with a final diameter of 6.1 > mm for the cable. Let me know if this description fits your bill. I have > some information on their fabrication. > > Just another of my 2 ยข ... > > regards > > Tim Foo > > > > > Ken Javor > > <ken.javor@emccomplian To: > emc-p...@majordomo.ieee.org > ce.com> cc: (bcc: Wan > Juang Foo/ece/staff/npnet) > Sent by: Subject: Stumped > > owner-emc-pstc@majordo > > mo.ieee.org > > > > > > 10/15/02 05:31 AM > > Please respond to Ken > > Javor > > > > > > > > > > > Forum, > > I have made some measurements and gotten results which are at odds with my > intuition. I am wondering if someone out there can shed some light on this > subject. > > I was interested in the losses associated with rf traveling on a twisted > shielded pair cable. The scenario is that a length of this TSP cable is > exposed to an rf environment (as in a test chamber during IEC 1000-4-3 > testing) and then the cable penetrates a bulkhead using a grounded > connector > that provides excellent shield termination, and the cable continues on the > other side in the pristine rf environment of a shielded control chamber, > say > for several meters. The question is, how much rf signal is at the final > destination point vs. at the bulkhead. The concern is common mode, not > differential mode. Meaning that the twisted pair can be looked at like > coax, with an identical signal on both inner conductors relative to the > shield. I expected losses that would be on the same order or lower than > that associated with off-the-shelf coax types like RG-58. Instead my > losses > were dramatically higher. > > Following is my measurement technique. > > <snip> > > > > > > > ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
