Voltage induced into coax by external electric field.

[yungman]

I want to verify the mechanism to which a varying E can induce voltage into a shielded coax.

According to Maxwell

$$\nabla \times \vec E = -\frac{\partial \vec B}{\partial t} \;\hbox { and }\; \nabla \cdot \vec E = \frac{\rho_{free}}{\epsilon}$$

From this, with varying E, you induce varying charge density onto the outer shield which create a varying current onto the shield. But the inner conductor is partially shielded by the shield don't see as much E so the current induced is not as much. Therefore there is a difference in the two currents which create the voltage into the coax.

At the same time, with a 200V/m varying E there MUST be B associated with the E by the Maxwell

$$\nabla \times \vec B = \mu\vec J +\frac{\partial \vec E}{\partial t}$$

But B induce equal current in both the inner conductor and the outer shield. This is common mode and don't matter.


Is this the mechanism?

 

[Naty1]

[But B induce equal current in both the inner conductor and the outer shield./QUOTE]

I don't think so. The field inside the shield is greatly diminished because charge is (almost) equally distributed around the shield. Of course there is some field and some noise...and some real resistance in the shield.

PS: I just saw some newer "RG6" TV coax in Home Depot...called "QUAD 6" which has a different shielding construction...but I did not study the package diagram.Check it out and
see if such offers any insights. Although there were diffent connectors ("RG6" and I think
"RG6Q") they appeared identical in the packages and were priced indentically.

Also: See here
http://en.wikipedia.org/wiki/Electromagnetic_shielding#How_electromagnetic_shielding_works

 

[yungman]

Thanks for the reply. My theory is the copper shield and the inner conductor are both none magnetic and [itex]\mu[/tex]=1. I am using:<br /> <br /> $$\int_S \vec B \cdot d \vec S= \mu I$$<br /> <br /> and this is true for both conductors and this is common mode and don't effect the coax. I don't look at this as shielding. As the article said, the shielding don't even work because it is non magnetic material.<br /> <br /> Thanks<br /> <br /> Alan[/itex]

 

[Mike_In_Plano]

There are three common things I've seen:

1) Shields are typically 90+% shielding and not 100%. Thus, a nearby influence can get in.
2) If the source and destination of the signal have a source of noise current, this travels along the outside of the coax, which isn't of itself an issue. However, at the ends of the coax, a potential can arise when the center portion separated from the shield. This happens a great deal when taking high frequency oscilloscope readings.
3) Many pieces of equipment simply don't like being joined at the hip when inteferring noise is present. Try connecting them on the same AC outlet.