Voltage induced into coax by external electric field.

In summary, when a varying E field is induced into a shielded coax, it creates a varying current onto the outer shield and a lesser current onto the inner conductor. This difference in currents results in a voltage being generated into the coax. However, the magnetic properties of the shield and inner conductor play a role in the effectiveness of shielding. Additionally, noise currents can affect the signal at the ends of the coax, and connecting equipment may also contribute to noise interference.
  • #1
yungman
5,718
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I want to verify the mechanism to which a varying E can induce voltage into a shielded coax.

According to Maxwell

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

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

[tex] \nabla \times \vec B = \mu\vec J +\frac{\partial \vec E}{\partial t} [/tex]

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?
 
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  • #2
[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
 
  • #3
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:

[tex]\int_S \vec B \cdot d \vec S= \mu I[/tex]

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.

Thanks

Alan
 
  • #4
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.
 
  • #5


Yes, the mechanism for voltage induction in a coaxial cable due to an external electric field is explained by Maxwell's equations. When a varying electric field is applied, it induces a varying charge density on the outer shield of the coaxial cable. This results in a varying current on the shield, but the inner conductor is partially shielded and does not experience the same level of electric field. This creates a difference in currents between the outer shield and the inner conductor, leading to a voltage being induced in the coaxial cable.

Additionally, Maxwell's equations also show that a varying electric field must be accompanied by a magnetic field. This magnetic field induces an equal current in both the inner conductor and outer shield, known as common mode. However, this current does not contribute to the voltage induced in the coaxial cable.

Overall, the mechanism for voltage induction in a coaxial cable due to an external electric field is a combination of varying charge density and common mode current induced by the electric and magnetic fields, respectively. This is well-supported by Maxwell's equations and is a fundamental concept in the study of electromagnetism.
 

1. What is Voltage Induced into Coax by External Electric Field?

Voltage induced into coax by external electric field refers to the creation of an electric potential difference within a coaxial cable when it is exposed to an external electric field. This phenomenon is often seen in high voltage power lines, where the electric field from the lines induces a voltage in nearby cables.

2. How does an External Electric Field Induce Voltage in Coaxial Cables?

An external electric field can induce voltage in a coaxial cable through the process of electromagnetic induction. As the electric field interacts with the conductors of the cable, it creates a changing magnetic field which, in turn, induces a voltage along the length of the cable. This voltage is proportional to the strength of the external electric field and the length of the cable.

3. What Factors Affect the Amount of Voltage Induced in Coaxial Cables?

The amount of voltage induced in coaxial cables by an external electric field depends on several factors. These include the strength of the external electric field, the length and orientation of the cable, the composition of the cable's conductors, and the frequency of the electric field.

4. What are Some Applications of Voltage Induced into Coax by External Electric Field?

Voltage induced into coax by external electric field has practical applications in telecommunications, where it is used to transmit signals over long distances. It is also used in power lines to detect faults and monitor the health of the grid. Additionally, this phenomenon is studied in physics and engineering to better understand the principles of electromagnetic induction.

5. How is Voltage Induced into Coax by External Electric Field Calculated?

The voltage induced in a coaxial cable by an external electric field can be calculated using the formula V = E x L, where V is the induced voltage, E is the strength of the external electric field, and L is the length of the cable. This formula is based on the principle of electromagnetic induction and is used to determine the potential difference that is induced in the cable.

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