EMF Cancellation in Coax vs. Twin Speaker Wire

crutschow

Well-Known Member
So you are saying that a coil carrying 100V at 1 amp produces less EMF than a coil carrying 100A at 1V?
Yes.
Electromagnetic radiation is produced by accelerating charges (typically electrons by an AC voltage) in a wire.
The strength of the radiated field is determined by the acceleration rate of the charges (determined by the frequency) and the number of moving charges (current).
The voltage is just there to move the charges in the wire and is otherwise not directly involved in generating the radiation.
can someone please comment on my diagram above showing the series termination resistor as a proposed method to equalize (balance) the resistances of the coax core and shield?
I do not see that as changing the EMF radiation significantly.

pnielsen

Member
I do not see that as changing the EMF radiation significantly.
I believe you previously stated that one of the limitations of coax for EM field cancellation involved the differnce in resistance between the core and shield. My intent with the resistor was to balance this out. You now seem to be saying it does not matter. Please explain.

With regard to acceleration of charges, do you mean to say voltage potential not play a part? I can think of several applications where HV is used to accelerate electrons. I am just trying to get clear on this.

crutschow

Well-Known Member
I believe you previously stated that one of the limitations of coax for EM field cancellation involved the differnce in resistance between the core and shield.
That was not my statement.
With regard to acceleration of charges, do you mean to say voltage potential not play a part? I can think of several applications where HV is used to accelerate electrons. I am just trying to get clear on this.
I'm saying that the voltage is only involved to accelerate the electrons, which creates the EMR.
The electric field from that voltage does not contribute to the radiation.

pnielsen

Member
Sorry that was rjenkinsgb. His statements were:

"With a balanced signal from one end, the coax loses out as the resistance of the two halves of the circuit are different (core vs screen) so the endpoint junction will have voltage on it due to the unbalanced resistive divider."

"Higher voltage just means more emission."

From your most recent post, it appears you do not agree on either point, so I am trying to clarify in my own mind. Anyone's help in doing so would be greatly appreciated.

crutschow

Well-Known Member
Wll, I don't really understand what he is saying so can't help you with that.
Perhaps there is some confusion between EMF (electric and magnetic fields), which are local to the wire, and EMR which is radiated to a far distance from the wire.

rjenkinsgb

Well-Known Member
"With a balanced signal from one end, the coax loses out as the resistance of the two halves of the circuit are different (core vs screen) so the endpoint junction will have voltage on it due to the unbalanced resistive divider."

"Higher voltage just means more emission."
With a balanced signal:
The resistance of the two legs of the conductor form a potential divider.
If that resistance is equal, the far end junction will be at 0V so not overall "antenna effect", for want of a better phrase.

If the resistance is not equal, there will be a residual voltage at the far end junction, so some antenna effect and signal radiation.
As coax is generally not equal resistance comparing core and screen, it will have more radiation than figure of eight style cable.

A higher voltage signal (fed to the same load) means proportionally more current - an unavoidable consequence - so more input power to the load and, if there is any radiation due to load configuration, that will increase as well.

pnielsen

Member
With a balanced signal:
The resistance of the two legs of the conductor form a potential divider.
If that resistance is equal, the far end junction will be at 0V so not overall "antenna effect", for want of a better phrase.

If the resistance is not equal, there will be a residual voltage at the far end junction, so some antenna effect and signal radiation.
As coax is generally not equal resistance comparing core and screen, it will have more radiation than figure of eight style cable.

A higher voltage signal (fed to the same load) means proportionally more current - an unavoidable consequence - so more input power to the load and, if there is any radiation due to load configuration, that will increase as well.
So,, will the solution of a series termination reistor shown in my drawing above solve the problem with the imbalance of resistance between the core and the shield?

My question with regard to current vs voltage specified keeping the wattage the same. Again, in the context of my OP will a coil carrying 100V at 1 amp produce more or less EMF than a coil carrying 100A at 1V?

throbscottle

Well-Known Member
This makes interesting reading. I am reminded that coax is normally used for unbalanced signals and parallel or twisted pairs are normally used for balanced signals. Reading about how the physics of the thing works - insightful.

rjenkinsgb

Well-Known Member
My question with regard to current vs voltage specified keeping the wattage the same. Again, in the context of my OP will a coil carrying 100V at 1 amp produce more or less EMF than a coil carrying 100A at 1V?
100V at 1A = 100 Ohms.
100A at 1V = 0.01 Ohms.

The coils would have to be drastically different to have those current to voltage characteristics.

For magnetic field strength alone, the relative field strengths are given by the ampere-turns ratios.
eg. 100A through a one turn coil will give the same magnetic field strength as 1A through a 100 turn coil.
The voltage is irrelevant as long as the supply voltage and coil resistance allow the appropriate current.

It gets more complex with AC as the impedance of the coil increases with the square of the turns ratio; a 100 turn coil will have around 10,000 times the impedance of a similar size one turn coil...

pnielsen

Member
I think you answered by second question in my most recent post.
Regarding the first, what about using a resistor as shown above to compensate for the imbalance in the coax between the core and the shield? The value would need to be selected based upon resistance measurements in each case. Note the coax is being energized in current cancellation mode as per my experiment.

MikeMl

Well-Known Member
In the RF world, we sometimes use two-closely spaced wires as a transmission line (called "balanced-line, twin-lead, or ladder-line" to get RF from a transmitter to an antenna. In antenna work, you normally do not want the transmission line to "radiate" (i.e. to become an element of the antenna itself). This is prevented if the two currents are of equal magnitude, but opposite phase.

If there is a net difference between the two currents, then that difference is called "common-mode current", and it causes the feedline to radiate. It is one thing to terminate a balanced line with a "floating" load impedance; it is something else if the load has a path to "ground" from one end of the load that is different than the path from the other end to ground; that is what causes the currents in the balanced feedline to not cancel, leaving a common-mode current which radiates like any other current-carrying conductor which is part of an "antenna".

If you use coax instead of balanced feeder, life gets more complicated. The coax transports energy in two different modes. One mode is a standing wave (differential mode) that forms between the center conductor and the inside of the shield conductor. The other mode is a standing wave (common-mode current) that forms on the outside of the shield, which is never canceled by whatever is happening inside the coax. The only thing that stops common-mode current from flowing along the outside of coax is putting a common-mode choke (usually ferrite core with multiple turns of the coax going through it, or ferrite beads slipped over the outside of the coax). Ferrite common-mode chokes can be applied to balanced-lines, as well as to coax.

In audio, far more problems are caused by connecting disjoint grounds together than by radiated fields. The way to solve these problems is by avoiding "ground loops" (i.e using "single-point-grounding").

unclejed613

Well-Known Member
So you are saying that a coil carrying 100V at 1 amp produces less EMF than a coil carrying 100A at 1V?
in both cases the power is 100 watts. the DC resistance of the first is 100 ohms, and the resistance of the second is 0.01 ohms.

pnielsen

Member
What I was interested in is the EMF cancellation dynamics of the cable from which the coil is made, i.e. side-by-side, twisted or coax. IOW not the specs of the coil itself. Think of coils of the same internal resistance, but passing the same wattage due to external factors.

I am seeking to clarify this because of earlier responses suggesting that for any identical wattage high current, as opposed to HV, radiates the strongest field. This I did not understand.

I am still trying to figure out if the inherent imbalance of coax, at audio frequency, can be solved external to the cable with a series resistor on the return side.

MikeMl

Well-Known Member
[QUOTE="pnielsen, post: 1350993, member: 218013"...
I am still trying to figure out if the inherent imbalance of coax, at audio frequency, can be solved external to the cable with a series resistor on the return side.[/QUOTE]
That problem is best solved by grounding the coax shield at only one end... At RF frequencies, you can get sufficient choking action due inductance; not so at audio frequencies.

pnielsen

Member
So, as per my previously attached drawing but without the resistor. You seem to be saying that the core and shield will still present differing resistances (voltage divider) but the antenna effect does not come inot play for audio. frequencies.

unclejed613

Well-Known Member
I am seeking to clarify this because of earlier responses suggesting that for any identical wattage high current, as opposed to HV, radiates the strongest field. This I did not understand.
the thing that determines the strength of the magnetic field is Ampere Turns. since in the example of a 100 ohm coil vs a 0.01 ohm coil, the number of turns is unknown in each, it's only possible in a narrow set of parameters to have the magnetic field on each of the coils be the same strength.

edit: if you have a single turn with 100A, or 100 turns at 1A, the magnetic field is the same strength.

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MikeMl

Well-Known Member
So, as per my previously attached drawing but without the resistor. You seem to be saying that the core and shield will still present differing resistances (voltage divider) but the antenna effect does not come inot play for audio. frequencies.
Perhaps if you told us what problem you are trying to fix, we could make better suggestions...

BobW

Active Member
If the frequency is low enough that the current is uniform along the length of the conductors (i.e., the conductor length is much much shorter than the wavelength of the signal), then there will be zero magnetic field outside of the coax.
For the case of the parallel twin conductor cable, then the magnetic field will only be significant at a distance comparable to the spacing between the conductors. Further away, it will quickly drop to zero.

crutschow

Well-Known Member
If you tell us exactly what problem you are concerned about, we won't have to play 20 questions.

pnielsen

Member
There is no problem. My questions were theoretical. But since they do not comply with accepted practice, the responses have not addressed them completely. I think some people were referring to RF effects and that confused me. Anyway, I am now satisfied that enough information has been provided. Thank you for your patience.