# Why does common mode current flows on outer surface of the coaxial cable?

#### Dong-gyu Jang

Joined Jun 26, 2015
115
Hello.

I've read that common mode current (maybe noise) flows on outer layer of the outer shield of the coaxial cable and normal current as differential mode current flows on central conductor and inner layer of the shield in coax. I don't understand why the common mode selects outer layer of its path and why normal current is 'closely' confined?

#### nsaspook

Joined Aug 27, 2009
13,448
It's easier to visualize 'why' by thinking about how the electromagnetic field internal and external to the coax moves as a waveguide.

TEM00 (Transverse Electric Magnetic) mode https://en.wikipedia.org/wiki/Transverse_mode

Then you have to understand Skin Effect on how the shield has an inner and outer surface conductor: http://www.sigcon.com/Pubs/edn/modelingskineffect.htm

Now we can examine the complete field structure.

http://blog.lamsimenterprises.com/2011/02/22/coaxial-transmission-line-geometry/
Eddy current-induced magnetic-field line patterns look exactly like magnetic-field lines (grey) from imaginary currents surrounding the outer conductor. These imaginary currents are referred to as image currents, and have the same magnitude as the real current; except they are in the opposite direction [1]. For simplicity, there are only eight image currents shown. But in reality, there are many more; forming a continuous loop of imaginary currents on a radius equal to twice the radius of the outer conductor to the center of the circle. The image currents create associated image magnetic-field lines in the opposite direction of the real field lines. As a result, the real magnetic-field lines are compressed and are entirely contained within the outer conductor.

The outer conductor thus forms a shield preventing external magnetic-fields from coupling noise onto the main signal and likewise, prevents its own magnetic field from escaping and coupling to other cables or equipment. This is why it is a popular choice for RF applications.
When this field geometry is disturbed by common mode signals the fields no longer cancel on the outer shield surface, this gives rise to a electric field and current flow on the outer shield surface conductor circuit as a noise current.

This external field shield current can be useful if used in the correct way.
http://www.antenna-theory.com/definitions/infinite.php

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#### Dong-gyu Jang

Joined Jun 26, 2015
115
It's easier to visualize 'why' by thinking about how the electromagnetic field internal and external to the coax moves as a waveguide.

TEM00 (Transverse Electric Magnetic) mode https://en.wikipedia.org/wiki/Transverse_mode

Then you have to understand Skin Effect on how the shield has an inner and outer surface conductor: http://www.sigcon.com/Pubs/edn/modelingskineffect.htm

Now we can examine the complete field structure.

http://blog.lamsimenterprises.com/2011/02/22/coaxial-transmission-line-geometry/

When this field geometry is disturbed by common mode signals the fields no longer cancel on the outer shield surface, this gives rise to a electric field and current flow on the outer shield surface conductor circuit as a noise current.

This external field shield current can be useful if used in the correct way.
http://www.antenna-theory.com/definitions/infinite.php
Hello.

Thanks to give me very kind explanation.

I'd known skin effect and your referred materials gives some fresh aspect of this.

So in normal operation, return current should be on the inner layer of the shield in coax. If they're on the outer layer of the shield, because of the skin effect which limits field-penetrating depth, the return current is no more associated with the center conductor in terms of the field. It is as if there is no return current in view of signal current on center conductor and is not physically right situation. I guess this is what you want to say.

However, still, I don't understand why noise current flows only on outer layer of the shield. I imagines that this current flows on center conductor and the magnetic field associated with it will be confined within inner layer of the shield according to your reference. No problem at all.

Did I miss something else?

Beside, I'm now confused with my original terminology, "common mode current", The typical definition of this may be the currents flowing both hot and neutral lines. Maybe my original meaning of "common mode current flows only on outer layer of the shield" is wrong and this noise flows on both conductors in current?

#### nsaspook

Joined Aug 27, 2009
13,448
Beside, I'm now confused with my original terminology, "common mode current", The typical definition of this may be the currents flowing both hot and neutral lines. Maybe my original meaning of "common mode current flows only on outer layer of the shield" is wrong and this noise flows on both conductors in current?
How and where the 'noise' current flows is a complex subject (EMC) that depends on the source/load connections and a host of other factors.

The primary methods to eliminate unwanted signals are redirection, dissipation, reflection and cancellation (generation of an equal but opposite masking field) of energy. Each of these might have different effects on a "common mode current" path.

http://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf
http://www.analog.com/media/en/training-seminars/tutorials/MT-095.pdf

#### Dong-gyu Jang

Joined Jun 26, 2015
115
How and where the 'noise' current flows is a complex subject (EMC) that depends on the source/load connections and a host of other factors.

The primary methods to eliminate unwanted signals are redirection, dissipation, reflection and cancellation (generation of an equal but opposite masking field) of energy. Each of these might have different effects on a "common mode current" path.

http://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf
http://www.analog.com/media/en/training-seminars/tutorials/MT-095.pdf

So...basically noise can go anywhere rather than going along certain side of the circuit like outer layer of the shield, right? I'm happy to at least break my wrong knowledge.

#### nsaspook

Joined Aug 27, 2009
13,448

So...basically noise can go anywhere rather than going along certain side of the circuit like outer layer of the shield, right? I'm happy to at least break my wrong knowledge.
A perfectly terminated coax with perfect shielding will exclude the noise signal from the inner conductors so you're not wrong but things are never perfect.