balanced and unbalanced lines...

hello forum,

I am reading about baluns. 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 
always connects to earth ground. But voltage is a relative concept at the
end and measure between two points, not three...
Could someone clarify these ideas please?

thanks,
fisico32

fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:
 
> I am reading about baluns. 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?

Usually relative to "ground", which could be the point at infinity.
 
> 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?

For AC measurements it isn't required that it be between two points,
as any point that isn't changing is good enough.  
 
> A coaxial cable is an example of a unbalanced line. One of its conductor 
> always connects to earth ground. But voltage is a relative concept at the
> end and measure between two points, not three...
> Could someone clarify these ideas please?

UTP, unshielded twisted pair, is a common example of balanced line.

Note that you can send gigabit ethernet 100m down an unshielded
cable and have it reliably come out the other end.   It is
transformer coupled, so it is easy to be sure that there is no
common-mode voltage.  Transformer coupling is usual for balanced line.

There is a wikipedia page for it, if you want to know more.

-- glen

On 03/16/2011 04:33 AM, fisico32 wrote:
> hello forum,
>
> I am reading about baluns. 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?

These are modeled assuming a ground reference of some sort, yes.  In 
theory you could but an infinitesimally thick wire exactly in between 
the conductors of your balanced transmission line, and it would remain 
at ground potential at all times.  In reality if you pay attention to 
this at all, you look for unbalanced currents on the line.

> 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
> always connects to earth ground. But voltage is a relative concept at the
> end and measure between two points, not three...
> Could someone clarify these ideas please?

If you have a coaxial cable with one end of the shield grounded, and 
with balanced current flowing, then all of shield current will flow on 
the inside skin of the shield.  When this happens then from the outside 
of the cable it will appear as if no current is flowing at all, and the 
shield will stay at ground potential.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

"fisico32" <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote in 
news:w5idnbZzVJeKPR3QnZ2dnUVZ_r6dnZ2d@giganews.com:

> hello forum,
> 
> I am reading about baluns. 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.
Not necessarily.  The voltage on one wire is Va and the voltage on the 
other wire is Vb, or vice versa.
> 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?
For example, wire #1 is at 3 volts while wire #2 is at 2 volts.  Then, wire 
#1 is at 2 volts and wire #2 is at 3 volts.
Voltage measurements are ALWAYS with respect to some reference point.  This 
reference point is usually marked with a ground symbol on a schematic.  
However, in the example that I gave, you could choose to measure 
"differentially", that is, measure wire #1 with respect to wire #2.  Then 
you will get either +1 volt, or -1 volt.
> 
> 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?
By definition and by the way that it is normally connected, wire #2 is the 
reference point and is connected to "the 3rd reference".  In an ideal 
world, yes, wire #2 is always 0 volts with respect to the 3rd reference.
> 
> A coaxial cable is an example of a unbalanced line. One of its conductor 
> always connects to earth ground. But voltage is a relative concept at the
> end and measure between two points, not three...
> Could someone clarify these ideas please?
Yes, voltage is a relative concept.  Yes, it is measured between two 
points.
As mentioned above, at least in an ideal world, the shield (wire 2) and 
earth ground (3rd reference) are connected together and there are zero 
volts between them.  A measurement from wire 1 to wire 2, or from wire 1 to 
earth ground, will show the same thing.

In the real world, wire resistance and other effects conspire to make the 
voltage between wire 2 and earth ground non-zero.
> 
> thanks,
> fisico32
> 
> 

Just a little add-on to previous replies

On 16.03.2011 14:33, fisico32 wrote:
> 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.

Strictly speaking, a balanced line is the line that has equal impedances 
of conductors along their length and referring to ground. What you're 
describing is a transmission line driven by a symmetric signal. The 
balanced line can be driven by asymmetric signals, as well.
The Wikipedia article about balanced lines
http://en.wikipedia.org/wiki/Balanced_line, which i think Glen was 
referring to, explains this in details.

-- 

Alexander

Alexander Sotnikov <alex.sotnikov@qip.ru> wrote:
> Just a little add-on to previous replies
> 
> On 16.03.2011 14:33, fisico32 wrote:
>> 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.
 
> Strictly speaking, a balanced line is the line that has equal impedances 
> of conductors along their length and referring to ground. 

Such that, given equal and opposite AC voltage, the currents are
also equal and opposite.  And note that as far as the "equal and
opposite" is concerned, it is only the AC part that needs to
be considered.  It is not unusual to have DC currents and voltages
on a balanced line, and the DC source/load resistance may be different 
from the AC (high frequency) impedance.

It used to be common to use 300 ohm balanced line for TV lead in,
where now 75 ohm coax is usually used.  To keep the line balanced
at TV frequencies, it is necessary to keep it away from metal objects,
or at least such that the distance is equal for both wires.

> What you're 
> describing is a transmission line driven by a symmetric signal. The 
> balanced line can be driven by asymmetric signals, as well.

> The Wikipedia article about balanced lines
> http://en.wikipedia.org/wiki/Balanced_line, which i think Glen was 
> referring to, explains this in details.

Also, for coax, which is considered to be unbalanced, one usually
wants equal (AC) currents in the shield and center conductor when
it is desired not to radiate.  (There are come antenna designs
based on radiating from coax.)  Keeping the currents equal and
opposite means that the (AC) current runs on the inside of
the shield, instead of the outside.

-- glen

On 17.03.2011 20:24, glen herrmannsfeldt wrote:
> Alexander Sotnikov<alex.sotnikov@qip.ru>  wrote:
>> Just a little add-on to previous replies
>>
>> On 16.03.2011 14:33, fisico32 wrote:
>>> 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.
>
>> Strictly speaking, a balanced line is the line that has equal impedances
>> of conductors along their length and referring to ground.
>
> Such that, given equal and opposite AC voltage, the currents are
> also equal and opposite.  And note that as far as the "equal and
> opposite" is concerned, it is only the AC part that needs to
> be considered.  It is not unusual to have DC currents and voltages
> on a balanced line, and the DC source/load resistance may be different
> from the AC (high frequency) impedance.
>
> It used to be common to use 300 ohm balanced line for TV lead in,
> where now 75 ohm coax is usually used.  To keep the line balanced
> at TV frequencies, it is necessary to keep it away from metal objects,
> or at least such that the distance is equal for both wires.
>
>> What you're
>> describing is a transmission line driven by a symmetric signal. The
>> balanced line can be driven by asymmetric signals, as well.
>
>> The Wikipedia article about balanced lines
>> http://en.wikipedia.org/wiki/Balanced_line, which i think Glen was
>> referring to, explains this in details.
>
> Also, for coax, which is considered to be unbalanced, one usually
> wants equal (AC) currents in the shield and center conductor when
> it is desired not to radiate.  (There are come antenna designs
> based on radiating from coax.)  Keeping the currents equal and
> opposite means that the (AC) current runs on the inside of
> the shield, instead of the outside.
>
> -- glen

Thank you for correction, Glen.

-- 

Alexander

Thanks eveyone for your inputs.

Just to make sure I get the right idea, let me summarize: 

after leaving the generator, the forward direct current I1 travels through
the inner conductor and reaches the load. It eventually needs to return to
the generator via the coax shield. 
At that point the current has 3 choices: 1)to flow completely on the shield
inner surface, 2) to flow on the shield outer surface or 3) to flow on both
shield surfaces. In the ideal case, what is the physics reason the return
current I2 chooses the inner surface only? 
Also I1=-I2.

In the nonideal, case, some current (how much? Less than I1?) flows on the
outer shield surface (the common mode current). Why? Does it have to do
with the concept of "ground loop".
If we ground the coax shield the return current has an extra path it can
take to go back to the generator, the one through ground (if the generator
is connect to ground. It may be that the generator is not connected to
ground).

thanks
fisico32




>On 17.03.2011 20:24, glen herrmannsfeldt wrote:
>> Alexander Sotnikov<alex.sotnikov@qip.ru>  wrote:
>>> Just a little add-on to previous replies
>>>
>>> On 16.03.2011 14:33, fisico32 wrote:
>>>> 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.
>>
>>> Strictly speaking, a balanced line is the line that has equal
impedances
>>> of conductors along their length and referring to ground.
>>
>> Such that, given equal and opposite AC voltage, the currents are
>> also equal and opposite.  And note that as far as the "equal and
>> opposite" is concerned, it is only the AC part that needs to
>> be considered.  It is not unusual to have DC currents and voltages
>> on a balanced line, and the DC source/load resistance may be different
>> from the AC (high frequency) impedance.
>>
>> It used to be common to use 300 ohm balanced line for TV lead in,
>> where now 75 ohm coax is usually used.  To keep the line balanced
>> at TV frequencies, it is necessary to keep it away from metal objects,
>> or at least such that the distance is equal for both wires.
>>
>>> What you're
>>> describing is a transmission line driven by a symmetric signal. The
>>> balanced line can be driven by asymmetric signals, as well.
>>
>>> The Wikipedia article about balanced lines
>>> http://en.wikipedia.org/wiki/Balanced_line, which i think Glen was
>>> referring to, explains this in details.
>>
>> Also, for coax, which is considered to be unbalanced, one usually
>> wants equal (AC) currents in the shield and center conductor when
>> it is desired not to radiate.  (There are come antenna designs
>> based on radiating from coax.)  Keeping the currents equal and
>> opposite means that the (AC) current runs on the inside of
>> the shield, instead of the outside.
>>
>> -- glen
>
>Thank you for correction, Glen.
>
>-- 
>
>Alexander
>

On 5/7/2011 9:51 AM, fisico32 wrote:
> Thanks eveyone for your inputs.
>
> Just to make sure I get the right idea, let me summarize:
>
> after leaving the generator, the forward direct current I1 travels through
> the inner conductor and reaches the load. It eventually needs to return to
> the generator via the coax shield.
> At that point the current has 3 choices: 1)to flow completely on the shield
> inner surface, 2) to flow on the shield outer surface or 3) to flow on both
> shield surfaces. In the ideal case, what is the physics reason the return
> current I2 chooses the inner surface only?
> Also I1=-I2.
>
> In the nonideal, case, some current (how much? Less than I1?) flows on the
> outer shield surface (the common mode current). Why? Does it have to do
> with the concept of "ground loop".
> If we ground the coax shield the return current has an extra path it can
> take to go back to the generator, the one through ground (if the generator
> is connect to ground. It may be that the generator is not connected to
> ground).
>
> thanks
> fisico32


I think we all need to be very careful here.  There are two or three 
"conventions" of language:
- there are voltage and current referred to at a single frequency .. 
this is very common in communication systems.  "impedance" as a single 
quantity is an example of this (limited) convention.
- there is "total" voltage and current meaning "at all frequencies" 
which gets a lot more complicated and usually requires simplifying 
models that can be dealt with analytically or computer simulations that 
might allow more complicated models.
- there is "DC".

In this question you mention "forward direct current".  If that's what 
you mean then I don't think there will be any "inner surface / outer 
surface" stuff at all - will there?  I believe the current will be 
equally distributed through the cross-section of the conductor (to use a 
simple model of the conductor.  Maybe I need to learn something new here....

Fred

Hello Fred,

I think you are right, but only if you are referring to the DC current
case:

then current I1 flows in the inner conductor (taking advantage of the
conductor full cross section) and an equal but oppositely traveling current
is on the shield conductor, across its full cross section too..

But in the case the source is a RF generator things get different: you get
surface currents (or currents traveling on a thin surface layer of the
conductor) due to skin effect.....

fisico32










>On 5/7/2011 9:51 AM, fisico32 wrote:
>> Thanks eveyone for your inputs.
>>
>> Just to make sure I get the right idea, let me summarize:
>>
>> after leaving the generator, the forward direct current I1 travels
through
>> the inner conductor and reaches the load. It eventually needs to return
to
>> the generator via the coax shield.
>> At that point the current has 3 choices: 1)to flow completely on the
shield
>> inner surface, 2) to flow on the shield outer surface or 3) to flow on
both
>> shield surfaces. In the ideal case, what is the physics reason the
return
>> current I2 chooses the inner surface only?
>> Also I1=-I2.
>>
>> In the nonideal, case, some current (how much? Less than I1?) flows on
the
>> outer shield surface (the common mode current). Why? Does it have to do
>> with the concept of "ground loop".
>> If we ground the coax shield the return current has an extra path it
can
>> take to go back to the generator, the one through ground (if the
generator
>> is connect to ground. It may be that the generator is not connected to
>> ground).
>>
>> thanks
>> fisico32
>
>
>I think we all need to be very careful here.  There are two or three 
>"conventions" of language:
>- there are voltage and current referred to at a single frequency .. 
>this is very common in communication systems.  "impedance" as a single 
>quantity is an example of this (limited) convention.
>- there is "total" voltage and current meaning "at all frequencies" 
>which gets a lot more complicated and usually requires simplifying 
>models that can be dealt with analytically or computer simulations that 
>might allow more complicated models.
>- there is "DC".
>
>In this question you mention "forward direct current".  If that's what 
>you mean then I don't think there will be any "inner surface / outer 
>surface" stuff at all - will there?  I believe the current will be 
>equally distributed through the cross-section of the conductor (to use a 
>simple model of the conductor.  Maybe I need to learn something new
here....
>
>Fred
>