From: Bruce Griffiths You'd need a rather thick copper jacket to shield effectively against the 50Hz magnetic field. Bruce ============================== Would mu-metal be any use? https://en.wikipedia.org/wiki/Mu-metal http://mumetal.co.uk/ Cheers, David -- SatSignal Software - Quality software written to your requirements Web: http://www.satsignal.eu Email: david-taylor@blueyonder.co.uk Twitter: @gm8arv

On Thursday, 25 August 2016 7:02 PM, David J Taylor <david-taylor@blueyonder.co.uk> wrote:

As long as you don't saturate it, bend it, hit it or drop it. Depending on the ambient field a multilayer shield with outer layers of high saturation magnetic material may be required. Bruce On Thursday, 25 August 2016 7:02 PM, David J Taylor <david-taylor@blueyonder.co.uk> wrote: From: Bruce Griffiths You'd need a rather thick copper jacket to shield effectively against the 50Hz magnetic field. Bruce ============================== Would mu-metal be any use?   https://en.wikipedia.org/wiki/Mu-metal   http://mumetal.co.uk/ Cheers, David -- SatSignal Software - Quality software written to your requirements Web: http://www.satsignal.eu Email: david-taylor@blueyonder.co.uk Twitter: @gm8arv _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.

Bruce wrote: > As long as you don't saturate it, bend it, hit it or drop it. And that is AFTER you form it to shape and then anneal it in a Hydrogen atmosphere. You may (probably would) need several layers, perhaps of different high-permeability alloys, with a thick outermost layer of soft iron. Best regards, Charles

-------- In message <1057836989.2088307.1472104857885.JavaMail.yahoo@mail.yahoo.com>, Br uce Griffiths writes: >You'd need a rather thick copper jacket to shield effectively >against the 50Hz magnetic field. As in: A good-sized fraction of the wavelength if I recall :-) Electric fields are so much easier... One interesting thing here is that across distances like this, there would be significant longitudal currents in such a shield. Not as bad as metal spanning the Mississippi, but getting there. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.

On 25/08/16 18:25, Poul-Henning Kamp wrote: > -------- > In message <1057836989.2088307.1472104857885.JavaMail.yahoo@mail.yahoo.com>, Br > uce Griffiths writes: > >> You'd need a rather thick copper jacket to shield effectively >> against the 50Hz magnetic field. > As in: A good-sized fraction of the wavelength if I recall :-) > > Electric fields are so much easier... > > One interesting thing here is that across distances like this, > there would be significant longitudal currents in such a shield. > > Not as bad as metal spanning the Mississippi, but getting there. > > Skin depth is probably a good place to start with in roughly estimating the thickness needed. In copper at 50 Hz, a quick calculation suggests 9.5 mm, but this just the depth at which the E-M field decreases by 1/e or 8.7 dB. Thus to get 20 dB attenuation this implies a thickness of about 22 mm, etc.

Hi There is also the minor issue of putting the (very thick) layers on in a spiral around the "core". You put one layer on clockwise and the next counter clockwise. Since the materials are quite springy, controlling the whole process through heat treating is a real chore. Bob > On Aug 25, 2016, at 4:25 AM, Poul-Henning Kamp <phk@phk.freebsd.dk> wrote: > > -------- > In message <1057836989.2088307.1472104857885.JavaMail.yahoo@mail.yahoo.com>, Br > uce Griffiths writes: > >> You'd need a rather thick copper jacket to shield effectively >> against the 50Hz magnetic field. > > As in: A good-sized fraction of the wavelength if I recall :-) > > Electric fields are so much easier... > > One interesting thing here is that across distances like this, > there would be significant longitudal currents in such a shield. > > Not as bad as metal spanning the Mississippi, but getting there. > > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > phk@FreeBSD.ORG | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.

Hi, On 08/25/2016 11:04 AM, Bill Metzenthen wrote: > On 25/08/16 18:25, Poul-Henning Kamp wrote: >> -------- >> In message >> <1057836989.2088307.1472104857885.JavaMail.yahoo@mail.yahoo.com>, Br >> uce Griffiths writes: >> >>> You'd need a rather thick copper jacket to shield effectively >>> against the 50Hz magnetic field. >> As in: A good-sized fraction of the wavelength if I recall :-) >> >> Electric fields are so much easier... >> >> One interesting thing here is that across distances like this, >> there would be significant longitudal currents in such a shield. >> >> Not as bad as metal spanning the Mississippi, but getting there. >> >> > Skin depth is probably a good place to start with in roughly estimating > the thickness needed. In copper at 50 Hz, a quick calculation suggests > 9.5 mm, but this just the depth at which the E-M field decreases by 1/e > or 8.7 dB. Thus to get 20 dB attenuation this implies a thickness of > about 22 mm, etc. While interesting, I think you are going overboard. The easy remedy is to move the fiber of the power-cable and put it on some distance, just choose a different path for it in the building. Cheers, Magnus

to shield against DC and low frequency magnetic field usually high permeability magnetizable material -- Permalloy, Mu-metal or similar is used, the field concentrates in the high permeability material and "behind it" is no left over magnetic field 73 Alex On 8/25/2016 11:10 AM, Magnus Danielson wrote: > Hi, > > On 08/25/2016 11:04 AM, Bill Metzenthen wrote: >> On 25/08/16 18:25, Poul-Henning Kamp wrote: >>> -------- >>> In message >>> <1057836989.2088307.1472104857885.JavaMail.yahoo@mail.yahoo.com>, Br >>> uce Griffiths writes: >>> >>>> You'd need a rather thick copper jacket to shield effectively >>>> against the 50Hz magnetic field. >>> As in: A good-sized fraction of the wavelength if I recall :-) >>> >>> Electric fields are so much easier... >>> >>> One interesting thing here is that across distances like this, >>> there would be significant longitudal currents in such a shield. >>> >>> Not as bad as metal spanning the Mississippi, but getting there. >>> >>> >> Skin depth is probably a good place to start with in roughly estimating >> the thickness needed. In copper at 50 Hz, a quick calculation suggests >> 9.5 mm, but this just the depth at which the E-M field decreases by 1/e >> or 8.7 dB. Thus to get 20 dB attenuation this implies a thickness of >> about 22 mm, etc. > > While interesting, I think you are going overboard. The easy remedy is > to move the fiber of the power-cable and put it on some distance, just > choose a different path for it in the building. > > Cheers, > Magnus > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2016.0.7752 / Virus Database: 4647/12877 - Release Date: > 08/25/16

Let's know if they get to the bottom of this, Magnus- its interesting to speculate on the cause but hopefully they figure out the real issue. DaveB, NZ ----- Original Message ----- From: "Magnus Danielson" <magnus@rubidium.dyndns.org> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Cc: <magnus@rubidium.se> Sent: Wednesday, August 24, 2016 11:06 AM Subject: Re: [time-nuts] Optical link connects atomic clocks over 1400 km of fibre > Don't over-interpret the 50 Hz aspect, I don't remember those details from > 4.5 months back or so, as I already indicated. I can ask on the details > tomorrow. I think they discussed the Kerr effect: > https://en.wikipedia.org/wiki/Kerr_effect > The PTB folks asked me the same question essentially. > > Would be nice to verify it. > > Cheers, > Magnus > > On 08/24/2016 12:11 AM, David wrote: >> I could not find it in the links but Magnus mentions 50 Hz instead of >> 100 Hz. >> >> I would expect a 100 Hz noise signal if it was vibration coupled from >> magnetostriction in a transformer; magnetostrictive strain depends on >> the magnitude of the magnetic field strength and not the sign which is >> why 50/60 Hz transformers hum at 100/120 Hz. 50 Hz however fits with >> piezomagnetism if the optical fiber was in an oscillating magnetic >> field and antiferromagnetic; for piezomagnetism, the strain does >> follow the sign. >> >> https://en.wikipedia.org/wiki/Magnetostriction >> https://en.wikipedia.org/wiki/Piezomagnetism >> >> I do not know if optical fibers are even slightly antiferromagnetic >> but maybe doping can make them susceptible? >> >> On Wed, 24 Aug 2016 09:31:57 +1200, you wrote: >> >>> What is the coupling mechanism giving rise to the 50Hz disturbance? >>> DaveB, NZ >>> >>> ----- Original Message ----- >>> From: "Magnus Danielson" <magnus@rubidium.dyndns.org> >>> To: <time-nuts@febo.com> >>> Cc: <magnus@rubidium.se> >>> Sent: Wednesday, August 24, 2016 8:54 AM >>> Subject: Re: [time-nuts] Optical link connects atomic clocks over 1400 >>> km of >>> fibre >>> >>>> ... >>>> >>>> These links is in principle not very complex, but they are regardless >>>> somewhat sensitive. One link experienced excessive 50 Hz disturbance, >>>> which they could trace to the fact that for a short distance the fibre >>>> was >>>> laying alongside the house 400V three-phase feed-cable with quite a bit >>>> of >>>> current in it. >>>> >>>> ... >>>> >>>> Cheers, >>>> Magnus >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.