I measured impedances of 15 - 25 Ohms. The separation between twisted shielded pair center conductor(s) and shield is the thickness of the center conductors' insulation - very thin. This makes the capacitance higher than for 50 Ohm coax. At the same time the close proximity of center conductors and shield reduces the inductance, relative to 50 Ohm coax, and since the impedance is the square root of the ratio of inductance by capacitance, it seems clear to me that the impedance has to be lower than 50 Ohms. But the impedance is not the major issue here. How am I getting so much loss in the very thin conductor insulation? Or is there another loss mechanism of which I am unaware?
---------- >From: Cortland Richmond <72146....@compuserve.com> >To: Ken Javor <ken.ja...@emccompliance.com>, ieee pstc list <emc-p...@ieee.org> >Subject: Re: Stumped >Date: Tue, Oct 15, 2002, 2:52 AM > > Ken, a few thoughts. > > Did you account for radiation resistance? You have described not merely a > single-wire transmission line but ALSO, a fairly good antenna. > > The impedance is probably higher than you calculate. A coax cable with the > same ratio of shield radius (height above ground) to inner conductor size > will be higher impedance tan your 15-25 ohms. You have greater spacing -- > the ground isn't concentric -- and the impedance HAS to be higher. > > For all but the lowest frequencies in the range you mention, the chamber > prevents current on the cable from flowing as on a transmission line; its > resonances couple differently to the line than operating over an unenclosed > ground plane. But assuming your matching is correct, you SHOULD see only > a travelling wave on the cable, and current or voltage that does not vary > along the line (except due to loss). If the match is incorrect, you will > have standing waves, and this you can confirm pretty easily with a current > probe. The chamber resonances may obscure this. > > It is possible to find loss in common mode simply by measuring common-mode > current at a peak near one end of a suspended conductor and again at a peak > near the other end. The location of current nodes will depend (outside a > chamber!) on wavelength, but the difference between them over length will > depend on radiation and other losses. > > Cortland > ------------------------------------------- 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"
