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

[Magnus Danielson]

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 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.

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