Why is Coax Shielded? Exploring the Different Modes of Current in Coaxial Cables

[Fixar Frazze]

I know that the signal in a coax is shielded. As I understand the EM waves only induce currents on the outer shield, but why does that not affect the signal in the inner conductor?

 

[Averagesupernova]

As far as currents on the shield affecting the inner conductor: Think about the flux lines inside the shield. To simplify, imagine 2 wires on opposite sides of a single conductor with current flowing the same direction in each. Flux lines from each conductor are doing what to the center conductor?

 

[es1]

That's part of it. coax cable is also an efficient geometry for moving high speed signals, and as a bonus is it easy to manufacture and make connectors for.

This link has more:
http://blog.lamsimenterprises.com/tag/coaxial-cable/

 

[yungman]

Coax cable is a guided structure for EM wave to propagates in. The voltage and current is only the consequence of the boundary condition of the E and B field. You need two conductor to form a guided structure.

It is not true that only current induced in the outer shield. To every forward current, there is always a return current in the opposite direction. The forward traveling current in the inner conductor has to be exactly the same as the return current in the outer shield.

 

[Averagesupernova]

Coax cable is a guided structure for EM wave to propagates in. The voltage and current is only the consequence of the boundary condition of the E and B field. You need two conductor to form a guided structure.

It is not true that only current induced in the outer shield. To every forward current, there is always a return current in the opposite direction. The forward traveling current in the inner conductor has to be exactly the same as the return current in the outer shield.

I think you have missed the point, or at least the point I made. I will elaborate. It is quite possible to have currents flowing on the shield that are not flowing in the center conductor due to ground loops, etc. I didn't say that it is impossible to have currents induced into the center conductor by an external source. Alot of equipment will have the outside shield connected to conduit ground so it is possible to have certain ground currents flowing in the shield from one piece of equipment to the next, but not in the center conductor. Now go back and read my first post and apply what I said in this post. I did want the OP to realize that the symetry of the shield has advantages over a single wire such as twisted pair which is why posted in the manner I did.
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Yungman, I realize you were probably referring to the OP who does seem to have the misconception that no unwanted current can ever flow in the center. But I didn't want any confusion.

 

[yungman]

I think you have missed the point, or at least the point I made. I will elaborate. It is quite possible to have currents flowing on the shield that are not flowing in the center conductor due to ground loops, etc. I didn't say that it is impossible to have currents induced into the center conductor by an external source. Alot of equipment will have the outside shield connected to conduit ground so it is possible to have certain ground currents flowing in the shield from one piece of equipment to the next, but not in the center conductor. Now go back and read my first post and apply what I said in this post. I did want the OP to realize that the symetry of the shield has advantages over a single wire such as twisted pair which is why posted in the manner I did.
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Yungman, I realize you were probably referring to the OP who does seem to have the misconception that no unwanted current can ever flow in the center. But I didn't want any confusion.

I was not referring to any of your post, I answer the OP's post.

 

[Bob S]

... The forward traveling current in the inner conductor has to be exactly the same as the return current in the outer shield.

This is not always true. If the outer shield of a coax is one of two or more conductors coupling two shielded enclosures, and a stray EM field induces a common mode current I_stray in that loop, the I_strayR voltage drop between the chassis gets superimposed on the signal inside the coax. We often will wrap the coax (e.g., RG-58) through a toroid or use a clamp-on toroid to increase the ac resistance of the loop and reduce the common-mode current in the coax. See http://www.kf7p.com/KF7P/Ferrite.html

 

[Leland]

I know that the signal in a coax is shielded. As I understand the EM waves only induce currents on the outer shield, but why does that not affect the signal in the inner conductor?

The outer shield block the EM waves from the environment from influencing the inner conductor.

You can look up the "Electromagnetic shielding" entry in Wikipedia for more details.

 

[yungman]

I know that the signal in a coax is shielded. As I understand the EM waves only induce currents on the outer shield, but why does that not affect the signal in the inner conductor?

Do you mean external EM wave, not the EM wave traveling inside the coax? If so, I miss understood your question in my first post. You know that it is also EM wave that travel inside the coax, not voltage and current like conventional thinking.

If you refer to the external EM wave penetrate into the coax. It really depends on the frequency of the EM wave. There is something called the skin effect that the higher the frequency, the less it can penetrate the good conductor ( metal like the shield of the coax). So the higher frequency of the EM wave will get blocked by the outer shield and never reach the inner shield.

But as frequency goes lower, the dept of penetration getting larger. At low frequency, EM wave can penetrate through the shield and reach the inner conductor also. That will induce current in the inner conductor by the boundary condition.

That's the reason the lower frequency in certain way, more troublesome. EM wave at audio frequency can penetrate thick metal. that's the reason some people use thick aluminum pickguard for guitar to shield the EM wave. that's the thing I am dealing with at the moment.

 

[the_emi_guy]

Sometimes it is easiest to describe this to a inquisitive novice if we forget RF and bring it down to DC:
Take a length of coax, connect 50 ohms between the center conductor and the shield at the far end, and a battery between the center conductor and the shield at the near end. Current will flow down the center conductor then return back to the battery on the shield. Equal currents since they are part of the same circuit. We call this "differential mode" of "metallic" current.
Or, we can connect 50 ohms between the shield and ground at the far end, and a voltage source between the shield and ground at the near end. Current will flow down the shield and return through ground. No current will flow in the center conductor, its not connected to anything. We call this "common mode" or "longitudinal" current.

Armed with this basic understanding we can go deeper:
1 - differential mode is our intended signal, common mode is generally unwanted and often bad, both can exist simultaneously.
2 - as we increase the differential mode frequency, lots of interesting things occur, characteristic impedance, skin effect, loss, but the basic idea is consistent with the DC case.
3 - If common mode is high frequency, it is confined to outer skin, isolating it from differential signal.
4 - Fields in interior are governed by Maxwell equations.
etc.