[time-nuts] Optical link connects atomic clocks over 1400 km of fibre

[Bill Metzenthen]

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.