Balanced and unbalanced lines

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In summary, balanced transmission lines use a balun to convert between unbalanced and balanced signalling. Unbalanced transmission lines use the shield as the reference. Coaxial cables use the shield as the reference, while a dipole antenna doesn't require a third wire.
  • #1
fisico30
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hello forum,

I am trying to read and learn about balun. Some transmission lines are said balanced while other unbalanced...

In a balanced transmission line the voltage on one wire is always as big as the voltage on the other wire but 180 out of phase.
For example wire 1 is at 50 V while wire 2 is at -50 V...Are these two voltage readings measured in reference to a 3rd reference wire/metal?

I am not sure I fully understand the idea of unbalanced transmission line. Voltage is always measured between two points. Would wire 1 in an unbalanced transmission line show +50 V with respect to the 3rd reference while wire 2 always 0 volts with respect to the 3rd reference metal?

A coaxial cable is an example of a unbalanced line. One of its conductor is always connect to Earth ground. But voltage is a relative concept at the end...
Can someone clarify these ideas please?

thanks,
fisico30
 
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  • #2
In balanced and unbalanced lines, you can take either conductor as the reference and measure the voltage on the other wire relative to this one.

In coaxial cables, it is usual to take the shield as the reference.

In either case, you can also take a third reference point as your reference, as long as you state what you are doing.
One example would be if you were driving the balanced line from a center tapped transformer. In this case, the center tap would be the 3rd reference point.

All that really matters is the difference in voltage between the conductors.
 
  • #3
Hello fisico,

This may help sort the confusion.

Firstly 'a balun' is the term given to a device (usually a transformer) which converts between balanced and unbalanced signalling.

You are correct in realising that balanced signalling requires three wires and unbalanced signalling, only two.

Sometimes one of these two or three wires are connected to earth, sometimes not.

Now the main purpose of the introducing the extra cost and complication of the third wire is to reduce or eliminate noise and other interference.

The idea is the same as using an op amp in differential mode to reject common mode signals.

On a balanced line, each of the two signal wires carries the desired signal (Vs)but in opposite phase.
However the noise (Vv) is picked up equally by both wires in the same phase.

So if we take the difference at the other end will have the following result

Vout = (Vs + Vn) - (-Vs + Vn) = 2Vs

That is the noise is canceled out.
 
  • #4
Studiot said:
Hello fisico,


You are correct in realising that balanced signalling requires three wires and unbalanced signalling, only two.

I can't agree with that.
 
  • #5
A balanced line propagates a differential signal, where both conductors are in close enough proximity to each other that, when exposed to an interference, will have negligible effect to the signal that is transmitted. This is because both conductors of a balanced line are isolated on both sides, using a balun. The signal that propagates from source to destination is only relative to itself when placed across the balun. Since any interference in the transmission line is exposed to both conductors, there is no current flow to represent the interference signal at either end of the transmission line.

In an unbalanced transmission line the shield is used as the reference, and is usually attached to a ground. The shield prevents interference by receiving it and dumping it to a large capacitive sink, or ground. This system is more resistant to outside noise, but more susceptible to noise that is injected to the system through ground.

If your transmission line is less than 1/4 wavelength long, then you only need to match the antenna and transceiver impedances.

If the impedance on the transmission line, transceiver, and antenna are matched, and the transmission line is cut to length based on the electrical wavelength, then a balun is not necessary and would be an efficient transmission of power from your transceiver to antenna. Baluns are used because of poor matching, or sometimes necessary for isolation and noise rejection.
 
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  • #6
I can't agree with that.

Perhaps you can explain why a balanced jack plug or an XLR connector need three terminals for a single balanced signal?

Note the difference in the extract from these instructions for a professional mixer.
 

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  • #7
Studiot said:
Perhaps you can explain why a balanced jack plug or an XLR connector need three terminals for a single balanced signal?

Note the difference in the extract from these instructions for a professional mixer.

It doesn't need the third wire which I believe is connected to chassis ground at the mixer. It is called a drain and is used to reduce noise. It doesn't need to be there to convey the information from the mic. It is only there to reduce noise usually in the form of 60 Hz hum.
-
Feeding a dipole antenna with balanced transmission line requires no third wire. A speaker driven with an output transformer is a balanced system with no third wire.
 
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  • #8
If both balanced and unbalanced signalling was simply the (voltage) difference between two wires or two points, A and B the distinction would be superfluous.

(Electroncis) Dictionary Definition

A balanced line is a transmission line that has conductors of the same length, equal resistances per unit length and equal impedances to Earth and to any other electrical circuits.

Can you draw a diagram showing how this last emboldened condition is met without a third wire or connection?

As I understand the term it stems from the idea that the system is a bit like a see-saw. Two opposing (electrical) forces are set in opposition or balanced, but they have to be both referenced to a common third point. Just as a see-saw has to have a fulcrum.
 
  • #9
Studiot said:
(Electroncis) Dictionary Definition

A balanced line is a transmission line that has conductors of the same length, equal resistances per unit length and equal impedances to Earth and to any other electrical circuits.

Can you draw a diagram showing how this last emboldened condition is met without a third wire or connection?

That definition in no way implies that a third wire is required. I've used transmission lines in my amateur radio hobby for many years and I've never seen or heard of ladder line with three wires. That last condition you are referring to does not have to be perfectly met in order to qualify the transmission line as balanced. If one conductor comes closer to a metal object than the other, then it is an installation problem.
 
  • #10
OK, Thank you for the discussion, gents.

I think I have it now.

Balanced, when talking about a transmission line refers to equality of impedance. It does not refer directly to the signal impressed on the conductors, although it obviously has implications for the signal.

A balanced signal does need a reference to arbitrate the balance however. So for instance a push-pull drive to an output transformer is balanced.
 
  • #11
Studiot in post #3 pointed out that signals in general are composed of both a differential (balanced) signal and a common mode signal. The balanced signal has equal and opposite currents and voltages in the two signal wires. There is a type of coax made specifically for balanced signals. It is a "twinax" cable comprised of two conductors, surrounded by an outside braid shield. See picture and specs in

http://www.awcwire.com/Part.aspx?code=740F23F27J44

It has two signal wires, and a third braided shield. Each signal wire has the same capacitance to the braided shield. The nominal characteristic impedance of the two balanced conductors is about 78 ohms. RG-108 requires special connectors.

Bob S
 
  • #12
Balanced lines use a differential signal, that consists of two wires. The notion of differential is that any noise that occurs on one conductor will appear on the other, and since one signal is referenced to the other a common signal nulls out, leaving only the intended signal.
Unbalanced lines generally use an outer conductor as a shield, that is attached to a reference voltage. This shield helps to eliminate outside noise, but does nothing to help if there is a difference in ground potential at each end of the line.
A third conductor will generally be an external shield, with a balanced line internal to it. This uses the idea of both a shield, and keeps the advantage of having a differential signal. However, the grounded shield has nothing to do with the signal that it surrounds other than acting as a shield.
To answer the OP:
If the voltage in a differential line is to be 50V, assuming RMS, then if you used a voltmeter that is accurate for the frequency that is in the transmission line, you should measure 50VAC. The signals are 180 degrees out of phase but since each signal is used for the reference as the other, a voltage specified is from point A to point B.
I think that the third wire in a transmission line has been discussed.
The statement regarding voltage at the end has to do with the losses in the cable, and differing potentials of ground. This is one reason that some choose to use a balun. It can correct a complex impedance mis-match at the end of the line and provide a differential signal to the antenna. This comes at a cost of inefficiencies.
 
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  • #13
In reality the situation is more complicated than you gentlemen are making out.
Perhaps that is why confusion arises.
In particular a twin wire system may or may not be a balanced line, but it must always carry an unbalanced signal. It is impossible to have a balanced signal on a balanced line without three wires.

I have uploaded some sketches to illustrate this.

Fig 1
Shows the already mentioned loudspeaker fed from a transformer via twin wire.
Since there is no zero reference for the signal, the signal is unbalanced. But the line is balanced.

Fig2
Replaces the loudspeaker with an single power supply amplifier. The transformer supplies an unbalanced signal into the twin wires.

Fig3
The amplifier is now fed from split power supplies. The transformer supplies an unbalanced signal into the twin wires. This is seen by the amplifier as a balanced signal and the amp signal output is also balanced.

Fig4
Returns to the loudspeaker but adds a second one in series. Can anyone claim the line is now balanced as the impedences of each section are now widely different?

Fig5
A variation with the loudspeaker fed from two very different lengths of separate wires.

Fig6
Most would agree with the statement that the centre tapped LHS, of the transformer, is balanced but the RHS is unbalanced. The transformer is a Balanced to Unbalanced device (balun), viewed from the point of view of the signal.

Fig7
The transformer is reversed and now converts Unbalanced to Balanced. To complete this system is is necessary to provide the centre connection to the other end of the line by some means.
 

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  • #14
Sorry Studiot, I can't agree.
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As far as transmission lines go:
Coax can't really EVER be considered balanced transmission line.
Twin-lead, ladder line, zip cord, etc. is considered balanced line but it can be connected in such a way to become unbalanced.
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I've never heard of a a signal being balanced or unbalanced. ALL signals have to be differential.
-
Your example in number 7 means nothing. Where are you going to connect the wire coming from the center tap at the load?
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The summary of unbalanced versus balanced is that balanced is usually picked over unbalanced because most noise introduced onto a balanced transmission line is common mode. This means that when the noise signal voltage on one wire goes up it also goes up on the other wire. This is because the impedance to ground on both wires is the same whether it is infinite (case of no third wire) or some finite value (a center tapped transformer). When both inputs to a differential load such as a loudspeaker, balanced mic input (an actual true differential amplifier), etc. have common mode noise as described above the noise is highly attenuated.
 
  • #15
Coax can't really EVER be considered balanced transmission line.

I don't recall saying it could or couldn't. In fact I don't recall mentioning Coax.

Twin-lead, ladder line, zip cord, etc. is considered balanced line but it can be connected in such a way to become unbalanced.

So you do agree with me.

I've never heard of a a signal being balanced or unbalanced. ALL signals have to be differential.

Just because you have never heard of it doesn't mean it does not exist, I have already described one situation in a previous post. Perhaps you should reread properly rather than making such sweeping statements.

Your example in number 7 means nothing. Where are you going to connect the wire coming from the center tap at the load?

Is it your contention that such transformers do not exist, or did you fail to read Fig3 properly? The wire should be connected to the input zero or Earth of the amplifier. Without this connection the system will not work.
 
  • #16
Studiot said:
I don't recall saying it could or couldn't. In fact I don't recall mentioning Coax.
Just because you didn't mention it doesn't mean I am not permitted to point it out.

So you do agree with me.

I don't agree with you. I am saying that if you connect a zip cord which is considered to be balanced transmission line to an unbalanced output such as a 1/8 inch phone jack for running an external speaker that you are using balanced transmission line but the conductors do not see the same impedance to ground. One is already grounded since typically the sleeve part of said jack is connected to ground.

Just because you have never heard of it doesn't mean it does not exist, I have already described one situation in a previous post. Perhaps you should reread properly rather than making such sweeping statements.

Not sure what to say to this. You can reference any point in a circuit. The closest I can imagine is to say differential vs. single ended. My main point is that a signal has to consist of a voltage between two points. Sorry for any misunderstanding.

Is it your contention that such transformers do not exist, or did you fail to read Fig3 properly? The wire should be connected to the input zero or Earth of the amplifier. Without this connection the system will not work.

On figure three:
The amplifier is now fed from split power supplies. The transformer supplies an unbalanced signal into the twin wires. This is seen by the amplifier as a balanced signal and the amp signal output is also balanced.

I am not saying such transformers do not exist. The transformer is supplying a signal on a balanced transmission line into a balanced load. The fact that the amp has a split supply means nothing. The output is single ended and is unbalanced. Contrary to what you are saying this circuit will work. Of course no negative feedback is shown, I assume it is implied.
 
  • #17
Just did a bit of rereading. Figure 4 doesn't mean much since you don't spec how far away each speaker is from the other. Assume that the speakers are right next to one another (read in the same box) even if the speakers are not at all identical and you can say that it is a balanced system.
-
Figure 5 doesn't mean much since by definition I believe a balanced transmission line requires the conductors to be the same length, at least relative to the wavelengths involved.
-
Figure 6 makes no sense. You have to connect the transformer to something to define it as unbalanced. Alone it certainly is balanced, both the primary and secondary.
-
Figure 7 pretty much falls into the same category as figure 6.
-
I'd say you have some misconceptions in general about all of this.
 
  • #18
There are some glimmerings of a discussion in your last two posts, but your style is still rather abrasive and dismissive of the other party. For example.

Figure 5 doesn't mean much since by definition I believe a balanced transmission line requires the conductors to be the same length, at least relative to the wavelengths involved.

Of course fig5 means something. I even stated explicitly that both the line and the signal are unbalanced.

You actually seem to agree, so why the snooty response?
 
  • #19
Studiot said:
There are some glimmerings of a discussion in your last two posts, but your style is still rather abrasive and dismissive of the other party. For example.



Of course fig5 means something. I even stated explicitly that both the line and the signal are unbalanced.

You actually seem to agree, so why the snooty response?

The transformer is a balanced output, the speaker is a balanced input. So no, I don't agree. The 'transmission line', if you can call it that, technically isn't by definition a transmission line due to inequalities in length. That is why I said it doesn't mean much.
 
  • #20
It's a real waste of time discussing with someone who simply dismisses what the other person has to say instead of inquiring what is meant or seeking further information or clarification.

You said Fig5 is not a balanced transmission line.

I said Fig5 is not a balanced transmission line.

Yet you deny that we agree.
 
  • #21
You seem to be under the impression that a transformer without a center tap, such as Fig5, is unbalanced. I disagree with that. It is in fact a balanced output driving a balanced input through some wires that convey the signal but by definition is not a perfect transmission line. I am sorry to misunderstand if that is not what you are saying. The way I see it you need to lose the 'balanced signal' talk. It is adding confusion. Transmission lines are balanced or unbalanced, outputs are balanced or unbalanced, inputs are balanced or unbalanced. But the signal itself isn't relevant. I don't know why you feel that a reference through a center tap, or resistive divider (I'm extrapolating, please forgive me) or something like this is a requirement for what you call a balanced signal.
-
For a moment I will give you the benefit of the doubt and we will call figure one a balanced transmission line, balanced output (source), and balanced input (load). We are in agreement up until this point. Where we start to disagree is when you call the signal unbalanced. Why is this? Why does making a center tap on the transformer suddenly make it what you call a balanced signal? Am I correct? Is this your thinking? Is it just the source or the load that you feel this is a requirement and not both? Do you feel that you have to have a common point to reference a scope to before you call the signal balanced? What about when the scope isn't hooked up?
-
Edit: The center of gravity on a tire is nowhere on the tire, yet it exists. We can throw a frisbee with the center cut out (a ring) and it works just perfectly without having a center. It is balanced even though there is no physical connection to its center of gravity. It is the best analogy I can come up with.
 
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  • #22
I posted a series of figures for discussion, not ridicule.

Initially I failed to take seriously the title of the thread - the OP was asking about lines.

I corrected this backalong when I realized I was talking about signals.

In Fig5 for instance I have said nothing whatsoever about the balance or otherwise of either the transformer or the speaker.

I did say that the line is unbalanced. I repeated this as plainly as possible in my previous post.

In fact figs 4 and 5 were introduced to answer another poster entirely who seems to think that it is not necessary for both wires in the line to have sensibly the same transmission impedances and paths.

In fact figs 1 through 5 are variations on a theme about what happens if we vary the conditions of the line or substitute different devices at the ends, but it is still about lines. I would agree that I have also added information about signals, since I introduced that in the first place.

As an aside, not all signals are differential - again if that were the case there would be no need of the term - for instance in certain (usually logic) circuits an indeterminate state can arise.
 
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  • #23
Studiot said:
As an aside, not all signals are differential - again if that were the case there would be no need of the term - for instance in certain (usually logic) circuits an indeterminate state can arise.

Please explain this farther.
 
  • #24
There is no fundamental reason why a balanced line needs a third conductor. What would it achieve as long as the balanced pair take the same route (very often they're twisted to achieve this)? Any signal induced in one of the pair will also be induced in the other - so the voltage difference will be the same. Of course, there is a limit to how 'balanced' the line can be constructed (to both magnetic and electric field interference) as the total length of each line needs to be very closely matched.* Eventually, common mode signals can end up as difference signals if you're not careful. It is often convenient to introduce a third conductor, which is sometimes run along with the signal pair or can be a screen - or even, as with some HF feeders, a rectangular trunking. This is common at the output run from a transmitter hall in which there are a number of transmitters involving high levels of local interference, which would not be wanted (even at levels of -50dB or better) to be radiated on the wrong array in the wrong direction.
* In long cables carrying multiple twisted pairs it is common to swap the relative positions and polarities of pairs in order to reduce the effect of crosstalk. This, I guess you could say, is because the simple balanced lines are not truly balanced - which they are not, because the impedance of each side to ground or to anywhere else cannot be the same all the way along. Any imbalance in impedance can be the source of interference or crosstalk.

A good mechanical analogy to a balanced line would be a bowden l conbtrocable in which the force / motion is between the inner and outer and is independent of the movement of the engine / suspension etc. This is much better than just having a single wire or rod for which the position of the end also depends on the movement of all the rest of the mechanism.
 
  • #25
I view the inner conductor of a coax cable as the going path while the outer conductor the return path for the current in the circuit.

If I take a voltage source and connect it to a load (say a light-bulb) by connecting one of the battery terminals to the inner conductor of the coax and the other terminal to the outer coax conductor, will the coax cable become a balanced line in that case?

if we put an ammeter on the outer conductor will we be able to detect a current?What if we simply connect the outer conductor to earth? Will not current flow in the outer conductor then? Why?

thanks
fisico30
 
  • #26
If the signal is RF (at high frequencies), the current will flow on the INSIDE of the outer conductor (ie the shield). This is the normal way coaxial cable works.

You can also have a different current flowing on the OUTSIDE of the outer conductor, due to skin effect, if the cable is passing too close to an antenna and parallel to it rather than approaching the antenna at right angles.

Current on the outside of the coaxial cable shield is the main reason for using baluns with antennas.
Even feeding a dipole antenna with coaxial cable will result in RF current flowing on the outside of the coaxial cable and back towards the transmitter.
This can cause transmitter cases to become "hot" with RF voltages on them and they can give an RF burn if you touch them.

RF current on the outside of coaxial cable shield can also affect the radiation pattern of the antenna.

will the coax cable become a balanced line in that case?
Coaxial cable with a single inner conductor is never a balanced line. This is mainly a question of symmetry. You can see that the inner and outer conductors are nothing like each other so you can expect them to behave differently.
 
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  • #27
fisico30 said:
I view the inner conductor of a coax cable as the going path while the outer conductor the return path for the current in the circuit.
The signal power flows between the inner and outer conductors of a coax cable as radial electric (E) fields and azimuthal magnetic (H) fields. The vector cross product of the E and H fields, called the Poynting vector (E x H), represents the energy flow.

It is convenient to think of the currents as flowing in the conductors, but in actually, all the signal power is in the dielectric. Suppose the copper were superconducting; no ac currents can flow in a superconductor.

Bob S
 
  • #28
Hello vk6kro,

thanks for the good reply. Please have patience and try to fix the misunderstanding I have.

In the DC regime, given a voltage source connected to a single load (light-bulb) by twp simple wires, is it correct to think of the same current I as flowing through the circuit and as one of the wires, from one of the battery terminals to one of the load terminals as the return wire while the other wire as the going wire?

I know that when we move to AC regime (low or high frequency, depending on how long the transmission line is) things behave differently. For instance an open circuit can become something other than an open circuit and a short circuit may not be a short circuit anymore either...

Why is it that if we are dealing with high freq. RF there is not going to be current in the inner cylindrical conductor of the coax?
You mention that current will flow "INSIDE of the outer conductor (ie the shield) "... I am not sure about what that means: do your intend that the current flows on the surface of the outer cylindrical conductor of the coax?

Also, you mention that "...Even feeding a dipole antenna with coaxial cable will result in RF current flowing on the outside of the coaxial cable and back towards the transmitter..."
Is it not ok for the current to loop back to the transmitter the same way it would happen in a simple DC circuit? Why is it not ok and why does it not happen?
I continue to think that for current to flow, be it AC or DC, a circuit must be closed and the current always has to go back to its source...
I guess I am wrong...why?
thanks for any tutorial help
 
  • #29
Bob S said:
The signal power flows between the inner and outer conductors of a coax cable as radial electric (E) fields and azimuthal magnetic (H) fields. The vector cross product of the E and H fields, called the Poynting vector (E x H), represents the energy flow.

It is convenient to think of the currents as flowing in the conductors, but in actually, all the signal power is in the dielectric. Suppose the copper were superconducting; no ac currents can flow in a superconductor.

Bob S

Hi Bob,
thanks for your reply too.

I have actually known about the TEM mode that a coax supports with both E and H fields orthogonal to each other and to the conductor axes.

If we put an ammeter on the inner or outer conductor of the coax would we measure any actual current?

Why do you say that if the coax was made of superconducting material there would be exactly no current? I don't get that...
Thanks and sorry for my lack of faster understanding
 
  • #30
In the DC regime, given a voltage source connected to a single load (light-bulb) by twp simple wires, is it correct to think of the same current I as flowing through the circuit and as one of the wires, from one of the battery terminals to one of the load terminals as the return wire while the other wire as the going wire?

Yes, of course. These are just conductors and they will conduct DC current like any copper conductor will do.

The situation gets more complex at higher AC frequencies. Currents flow in the inner conductor (but more and more on the outside of the inner conductor as the frequency increases) and on the inner surface of the outer conductor. These two surfaces carry the forward and return paths of the signal.

Current induced on the outer side of the coaxial shield will return to the outside of the the transmitter case causing RF burns etc. This does not happen with the normal outgoing signal, both sides of which originate inside the transmitter case.

Even if the transmitter case is grounded, this will normally involve a length of wire and this wire will have some impedance at the frequency of the signal.

So, even a transmitter case that is grounded for mains frequencies will not normally be grounded for radio frequencies.

I know it probably sounds weird that the same conductor can have different voltages on different parts of it without a current flowing between them, but this is a consequence of the frequencies involved.
 
  • #31

The situation gets more complex at higher AC frequencies. Currents flow in the inner conductor (but more and more on the outside of the inner conductor as the frequency increases) and on the inner surface of the outer conductor. These two surfaces carry the forward and return paths of the signal.


I guess that is the so called skin effect...

Current induced on the outer side of the coaxial shield will return to the of the the transmitter case causing RF burns etc. This does not happen with the normal outgoing signal, both sides of which originate inside the transmitter case.

What do you mean by both side of the outgoing signal?

I know it probably sounds weird that the same conductor can have different voltages on different parts of it without a current flowing between them, but this is a consequence of the frequencies involved.

It is surely weired. I have read about the strange effects of transmission lines. Along a line there could be standing waves so the voltage at certain points can be different from other points. But voltage remains a relative concept so it must always be measured between two points...
When you mention that "the same conductor can have different voltages on different parts of it without a current flowing between them" are you referring to which conductor, the inner cylinder or the outer one?
 
  • #32
I guess that is the so called skin effect...

Yes.

What do you mean by both side of the outgoing signal?

A signal is generated inside the transmitter. This may be coupled to the outside like this:

[PLAIN]http://dl.dropbox.com/u/4222062/RF%20output.PNG [Broken]

A small coil is placed near the end of a larger coil and this picks up a signal for transmission.

Both ends of the coil must be taken to the transmission line, even if one side of the line is grounded as it passes through the case, as is usually the case with a coaxial connector.

When you mention that "the same conductor can have different voltages on different parts of it without a current flowing between them" are you referring to which conductor, the inner cylinder or the outer one?

Mainly the outer one. The shield may be only a few tenths of a millimeter thick, yet it can have totally different signals on the inside and outside of it.
 
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What is the difference between balanced and unbalanced lines?

Balanced lines have two conductors that carry equal and opposite signals, while unbalanced lines have only one conductor and a ground. This allows for better noise cancellation in balanced lines.

What are some examples of balanced and unbalanced lines?

Examples of balanced lines include twisted pair cables, which are commonly used in Ethernet connections, and XLR cables, which are used in professional audio equipment. Unbalanced lines include coaxial cables and RCA cables.

What are the advantages of using balanced lines?

The main advantage of using balanced lines is their ability to reject noise and interference. This makes them ideal for long distance transmissions and in environments with high levels of electromagnetic interference.

How do you convert an unbalanced line to a balanced line?

This can be done by using a device called a balun, which stands for balanced-unbalanced. A balun converts the unbalanced signal into a balanced one by using a transformer or other electronic components.

Are there any disadvantages to using balanced lines?

One potential disadvantage is that balanced lines require more complex circuitry and equipment, which can make them more expensive. They also require more cables and connectors, which can make them less convenient for certain applications.

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