[TowerTalk] Fields and coax

[Jim Brown]

On Sun, 24 Jul 2005 06:59:47 -0400, Tom Rauch wrote:

>The time-varying magnetic field is fully shielded by the
>shield just as the time-varying electric field is. Both are
>a "partner" of each other, and in the case of TEM mode
>transmission in a transmission line if you take the electric
>field to zero you take the magnetic field to zero you take
>the electric field to zero, and vice versa.

Yes. BUT:  

Below radio frequencies, a cable "shield" provides magnetic field 
rejection NOT by shielding (because it is far too thin in terms of 
skin depth to provide shielding), but by virtue of the perfect 
coupling coefficient between the shield and the center conductor 
(i.e., k=1). Thus any current flowing on the shield induces an 
equal current in the center conductor, the fields buck, and the 
currents cancel. 

At low audio frequencies, however, the resistance of the shield is 
of greater magnitude than its inductive reactance, so much of the 
voltage along the shield appears across resistance rather than 
inductance. In that condition, there is poor cancellation. The 
"turnover" frequency where R and X are equal is called the shield 
cutoff frequency, and for typical copper braid shielded coax is 
between 500 Hz and 1 kHz. Ott has measured data in his book on 
EMC. Bill Whitlock talks about coaxial cable as a common mode 
choke that becomes effective at frequencies above about 5X the 
cutoff frequency. Note also that foil shields that do not include 
a robust copper braid have very poor performance at low 
frequencies, which is why VIDEO coax has a robust copper braid 
(sometimes two of them). 

I didn't read the original post in this thread, so I don't know 
the application, but it is critical to understand that 

1) cable shields provide no magnetic shielding until the frequency 
rises high enough for skin depth to come into play (nowhere near 
audio)

2) virtually all power-related fields are magnetic fields, and all 
home entertainment systems are in the near field (the near field 
of 100 Hz is within roughly 200 miles of a source). 

I would say that given the above facts, coiling up a piece of 
coaxial cable that carries AUDIO (or baseband video) in the 
presence of a strong AUDIO magnetic field is a really dumb idea, 
and likely to induce lots of hum and buzz into the system in 
question. 


Jim Brown  K9YC