# [PSES] Doug Smith's second teaser : THE ANSWER

```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
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```