Re: [Phys-l] grounding and shielding (was: capacitance problem)

[Bernard Cleyet <bernardcleyet@redshift.com>]

Yes,  Except for microwave frequencies on must use an unshielded loop.


Probe antennas:

http://scitation.aip.org/getpdf/servlet/GetPDFServlet? 
filetype=pdf&id=AJPIAS000039000001000121000001&idtype=cvips


bc an audio ground loop expert, and notes the completed circuit  
(above) w/ a likely less than one pF mica cap.

p.s. question for JD:  by using a broadly  resonant loop (low Z) is  
the H (B) probe immune to E field?



On 2008, Mar 29, , at 07:23, John Denker wrote:

> By the way, it is also possible to measure the stray
> *inductive* contributions using an easily-made device
>    http://www.av8n.com/physics/img48/stray-field-pickup.png
> plus an oscilloscope and a couple pieces of coax.  Can
> you see how it works?  For starters, note that the shield
> of the loop of coax protects the circuit against stray
> capacitive contributions, so it is only sensitive to stray
> inductive contributions.  Remember that
>        V = Phi dot
> is one of the Maxwell equations.  Here Phi is magnetic flux.
>
> In many classroom and/or laboratory settings, you will
> find that these inductive contributions are much larger
> and much harder to shield than the previously-considered
> capacitive contributions.  That's because the inductive
> guys have a near-zero Thévenin impedance, whereas the
> capacitive guys have a near-infinite Thévenin impedance.
> This is the physics behind the dreaded "ground loops".
> ________