[time-nuts] Optical link connects atomic clocks over 1400 km of fibre
billm713 at gmail.com
Thu Aug 25 05:04:38 EDT 2016
On 25/08/16 18:25, Poul-Henning Kamp wrote:
> In message <1057836989.2088307.1472104857885.JavaMail.yahoo at 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.
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