Hi Paolo and the group, It turns out I did reply to the SI-List but either not to this list or in html mode and it got rejected. It takes me more effort to reply to this group as I have to reconfigure my email program to use this list in 1970s style text mode and then back again for the rest of the world and some times I forget and some of my posts here bounce.
Can't post a picture so a few words will have to suffice. First I will talk about a coax cable, but a practical case might be where you have a pair of wires in a shield to measure the resistance of a temperature sensor and we are worried about 80 MHz common mode noise on the pair so we add a shield. A coax cable is composed of three conductors: the center conductor, the inside surface of the shield, and the outside surface of the shield. The two surfaces of the shield are independent conductors of current at practical frequencies because of skin effect. If I extend the center conductor outside of the shield, the wire will conduct current and form an antenna. I can replace everything inside of the coax (center conductor, inside surface of shield, and 50 Ohm source driving the cable) with a single 50 Ohm source (that is the definition of a 50 Ohm transmission line). If I do that, I have a 50 Ohm source driving an asymmetrical dipole: the outside of the shield on one side and the extended center conductor on the other. If the extended center conductor is very short (say 1/100th wavelength) current will not much flow out of the coax onto the center conductor and neither will there be much image current on the outside of the shield. In fact, this structure is an E-field probe! The source does not have to be 50 Ohms, if it is not that just means the dipole will have some other impedance source driving it. The shield in this case, open and not connected to anything at the end, works to suppress radiation from the current flowing on the center conductor (which would be a great antenna if the shield were not present). But as the extended center conductor becomes longer it will accept current and its image will be on the outside of the shield as well, the other half of the dipole. In fact, at this point, you have equal currents flowing in opposite directions on the inside and outside surfaces of the shield! I have some great pictures of this and some great scope plots from my monthly classes in Boulder City and at Oxford University, but cannot post them to this 1970s email system. :-( Of course we are assuming the frequency is low enough that the shielded cable does not have strange modes where it can radiate right out of the end of the coax like a waveguide. A reasonable assumption. By application of Kirchoff's current law over very small distances (<<wavelength) and skin effect, the current patterns become quickly obvious if drawn out on paper. Skin effect is a very powerful effect! The practical version of this is a small temperature or pressure sensor poking out the end of the cable but is very small and not capacitively coupled to nearby metal in a substantial way. In this case, we can just end the shield and it works. I have pretty much tested out in the lab most every technical concept I have learned starting with some of Marconi's wireless work when I was age 11 (it worked). What I find is that everything I have been taught is mostly correct, but, there are some interesting and sometimes important exceptions. Doug University of Oxford, Course Tutor Department for Continuing Education Oxford, Oxfordshire, United Kingdom -------------------------------------------------- Doug Smith P.O. Box 60941 Boulder City, NV 89006-0941 TEL/FAX: 702-570-6108/570-6013 Mobile: 408-858-4528 Email: d...@dsmith.org Web: http://www.dsmith.org -------------------------------------------------- On Mon, 19 Feb 2018 12:00:02 +0100, Paolo Roncone wrote: --- ------- Forwarded message ---------- From: Mark Hone <mark.h...@tpgroup.uk.com> Date: Mon, Feb 19, 2018 at 9:48 AM Subject: Re: [PSES] Doug Smith's second teaser To: EMC-PSTC@listserv.ieee.org I’d like to second John’s query, as I haven’t seen the explanation either. If it has been posted, are EMC-PSTC postings being lost somewhere? I have checked our company spam filters! Regards, Mark From: John Woodgate [mailto:j...@woodjohn.uk ] Sent: 08 February 2018 14:15 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Doug Smith's second teaser I didn't see an explanation from Doug about the open-ended shield of a balanced twisted pair being OK. Did I miss it or is the solution still awaited? -- John Woodgate OOO-Own Opinions Only J M Woodgate and Associates www.woodjohn.uk Rayleigh, Essex UK Disclaimer TP Group plc, A2/1064 Cody Technology Park, Farnborough, Hampshire, GU14 0LX, United Kingdom. Registered No. 3152034 This email and any attached files are private and confidential and for the use of the addressee(s) only. If you are not the intended recipient, please notify the sender immediately and delete the material from any computer. 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