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Common and Differential Modalities

wrote in message
...
On Sep 3, 3:59 pm, Richard Clark wrote:
On Mon, 1 Sep 2008 09:26:26 -0700 (PDT), wrote:
In a perfect situation, with a balanced feedline, the only kind of
current and voltage you have IS common mode!

This statement above contains a serious error of perception while
trying to inhabit the debate over BalUns - and it probably corrupts
that topic too.

First - a circuit has at a minimum two conductors extending from a
source. A circuit by its nature is circular: for every charge carrier
that enters it, one must exit it. Continuity is a necessary condition
for a circuit. No continuity, no conduction, hence an "Open Circuit."

Second - those two conductors, if viewed at a remote point where they
are joined, have equal and opposite paths of current conduction - to
and from that point. This is from Kirchoff's law of currents.

Third - this is called Differential Mode current in anticipation of a
common modality.

Fourth - if that remote point of connection is replaced with a load,
there is a voltage across that load characterized by both the
unaltered directions of current, and its now altered magnitude of
current.

Fifth - this is called Differential Mode voltage in anticipation of a
common modality.

This completes the discussion of the Differential Mode.

If we expand upon this simple model of a source, two wires, and a load
and put it into the context of life as we know it; then the circuit
operates in the proximity of ground. By convention, ground is called
Common.

Ground, by convention is an infinite sink of charge of infinite
extent. Hence as a conductor, it is available everywhere - Common.
This ground may have either deliberate or accidental conductive
relationships to the Differential Circuit.

First - the linkage of ground to the differential circuit can be
through an Ohmic path, or by an inductive path, or by a capacitive
path. To support conduction, the circuit must contain two conductive
paths to ground through any combination of these linkages, and that
path must be complete. The apparent source driving conduction through
that path will be a combination of the differential source and the
differential load as each will have some relationship to ground.

Second - those two conductive paths, if viewed at a remote point
where they are joined, have equal and opposite paths of current
conduction - to and from that point. This is from Kirchoff's law of
currents.

Third - this is called Common Mode current.

Fourth - as the differential circuit is original and establishes both
the source and the load; then through the introduction of ground, this
Common Mode current is mixed with the original Differential Current
and analysis must be performed by substitutions to separate them.

Fifth - the apparent source presents the Common Mode voltage.

This completes the discussion of Common Mode.

The applications of a choke to either circuit is commonplace to
control each mode's current. It would appear through the context of
discussion in other threads that there is some confusion in what is
being choked, and how a choke is properly applied is confounded by
that confusion.

It follows that if the transmission line from the source to load
suffers from Common Mode currents, that this must be due to a Common
Mode voltage gradient extending from the source to the load. If
either lead of that transmission line pair were choked, this would
disrupt the Differential Mode. If both leads of the transmission line
pair were independently choked, this would only double the disruption.
However, if both leads were choked in parallel (both lines either
coiled as a pair rather than individually, or both lines penetrate a
lossy core) then their fields would be contained between them in the
Differential Mode, but their Common Mode path (they both share equal
conduction in the same direction due to the Common Mode voltage
gradient) will be snubbed.

Some BalUns employ these techniques - some don't. BalUns fail by the
degree that they don't when Common Mode, as a problem, is injected
into the circuit through imbalances. As balance in the proximity of
earth and many confounding nearby structures is a forgone failure,
choking is a practical necessity for correct BalUn performance. Any
issues of BalUn heatings are proof of this choking necessity, and
further proof of the demand for additional choking at that point (and
frequently elsewhere at wavelength relationships along the affected
line).

73's
Richard Clark, KB7QHC

Let me put it this way (again very simplifed): How do you explain a
residential 208V power source where you have 120 V from line 1 and
line 2 to ground but 240V with respect to each other. You 240V
household appliances operate this way. Ecept for the fact that the
lines are 120 degrees out of phase phase (insread of 180 because we
use a delta system instead of Y, but this is not that important for
this discussion) this is nearly a BALANCED feed, where lines 1 and 2
degrees out of phase at 60 Hz and the voltage of interest is the
summation of the two lines. Nearly every home in the USA operates this
way. In Europe, 240 V is usually obtained by the voltage difference
between line 1 (240V) and earth (0V). That is an unbalanced feed. You
can insert a 1:1 isolation transformer using the European system at
the input and create the balanced USA system at the output by drawing
+120 and -120 at from the output windings assigning imaginary isolated
earth at centertap. The isolated ground CANNOT conduct tio real ground
if the winding to winding impedance is infinity. Basically, hams' 1:1
baluns do much the same thing: They isolate the real ground as such
and prevent currents from flowing down the input ground shield. On
this ng in a short space I cannot think of a simpler way to express
this although I expect to see many statements (you can't equate 60Hz
to RF!). Yes you can; a transformer operates as a transformer in the
same wayat any frequency providing you design it properly for the
frequency of interest. This subject is not nearly so complicated as
some in this group makes it out to be and the topic certainly does not
rate articles in amateur publications any more than basic application
of ohm's law does.

Hi Dfinn

I realize that you have asked this question to Richard, who is far better
prepared to answer than I am. But, it seems that you are confused about
how two sine waves add. Maybe I am wrong, and you do know how two sine wave
voltages generated at different times and are connected in series combine to
being other than 180 degrees from terminal to terminal.
All the 208 power lines I am familiar with *are* 208 from terminal to
terminal when each leg is 120 "terminal to center". Where did you get the
"240"?

Jerry KD6JDJ

Common and Differential Modalities

"Jerry" wrote in
news:s5Ivk.448$sq3.441@trnddc07:
....
I realize that you have asked this question to Richard, who is far
better
prepared to answer than I am. But, it seems that you are confused
about how two sine waves add. Maybe I am wrong, and you do know how
two sine wave voltages generated at different times and are connected
in series combine to being other than 180 degrees from terminal to
terminal.
All the 208 power lines I am familiar with *are* 208 from terminal
to
terminal when each leg is 120 "terminal to center". Where did you
get the "240"?

Jerry KD6JDJ

Jerry, in the three wire system with 180° phase difference between Line1
and Line2 wrt Neutral (the centre wire) the voltage Line1 wrt Line2 is
exactly twice the Line1 to Neutral voltage.

You probably don't use the word Neutral over there, it is the word used
for four wire three phase systems and it is equally applicable for a
three wire two phase system where the Neutral wire carries the
"imbalance" current.

None of this is a good analogy to a two wire open transmission line at
radio frequencies, principally because the length of conductors at power
frequencies is usually a very small part of a wavelength and can be
approximated well using a simpler analytical model than the traditional
'Telegrapher's' equations applied to transmission lines.

Those who resort to explaining radio frequency transmission lines and
transformers using 60Hz models need to justify the accuracy / limits of
the approximation. The low frequency model does not explain high
frequency roll-off in RF transformers and inductors, so it is clearly
incomplete.

Almost all of this discussion is using lumped circuit analysis (a low
frequency model) to explain not just transmission lines, but conductors
with significant mutual coupling (the nominal radiator and its feedline).

Owen

On Sep 3, 10:56*pm, John Smith wrote:
Cecil Moore wrote:
wrote:
Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase.

You are 100% wrong. From "The IEEE Dictionary":
"common-mode (1)(general) The instantaneous algebraic
average of two signals applied to a balanced circuit,
both signals referred to a common reference."

The "instantaneous algebraic average of two signals"
180 degrees out of phase is ZERO.

Cecil:

You must be getting old ... stop man! *:-)

Regards,
JS

--
It is like a nightmare where the public servants are the people which
the police are supposed to protect us from!

Yes, ZERO with respect to the isolated reference point. They are
measured with respect to each other, not ground. No current should
flow through the ground line if the feed is perfectly balanced.

Common and Differential Modalities

"Owen Duffy" wrote in message
...
"Jerry" wrote in
news:s5Ivk.448$sq3.441@trnddc07:
...
I realize that you have asked this question to Richard, who is far
better
prepared to answer than I am. But, it seems that you are confused
about how two sine waves add. Maybe I am wrong, and you do know how
two sine wave voltages generated at different times and are connected
in series combine to being other than 180 degrees from terminal to
terminal.
All the 208 power lines I am familiar with *are* 208 from terminal
to
terminal when each leg is 120 "terminal to center". Where did you
get the "240"?

Jerry KD6JDJ

Jerry, in the three wire system with 180° phase difference between Line1
and Line2 wrt Neutral (the centre wire) the voltage Line1 wrt Line2 is
exactly twice the Line1 to Neutral voltage.

You probably don't use the word Neutral over there, it is the word used
for four wire three phase systems and it is equally applicable for a
three wire two phase system where the Neutral wire carries the
"imbalance" current.

None of this is a good analogy to a two wire open transmission line at
radio frequencies, principally because the length of conductors at power
frequencies is usually a very small part of a wavelength and can be
approximated well using a simpler analytical model than the traditional
'Telegrapher's' equations applied to transmission lines.

Those who resort to explaining radio frequency transmission lines and
transformers using 60Hz models need to justify the accuracy / limits of
the approximation. The low frequency model does not explain high
frequency roll-off in RF transformers and inductors, so it is clearly
incomplete.

Almost all of this discussion is using lumped circuit analysis (a low
frequency model) to explain not just transmission lines, but conductors
with significant mutual coupling (the nominal radiator and its feedline).

Owen

Hi Owen

My post to the other guy evidently read as though I was asking a question.
Actually I read the original post as to have errors and was expecting some
lanswer to justify how 120 degrees can be associated with a set of only 2
terminals. There is *no* 240 volt pair of terminals available in a 208/120
power system.
Here in "the states" 208/120 refers to a 3 phase system Y circuit. That
means there are 3 sets of single phase 120 volt circuits available at the
panel. Also available is a set of three terminals that are 208 volts 3
phase.

Jerry

Common and Differential Modalities

On Sep 3, 11:30*pm, Owen Duffy wrote:
"Jerry" wrote innews:s5Ivk.448$sq3.441@trnddc07:
...

* I realize that you have asked this question to Richard, who is far
* better
prepared to answer than I am. * But, it seems that you are confused
about how two sine waves add. *Maybe I am wrong, and you do know how
two sine wave voltages generated at different times and are connected
in series combine to being other than 180 degrees from terminal to
terminal.
* All the 208 power lines I am familiar with *are* 208 from terminal
* to
terminal when each leg is 120 "terminal to center". * Where did you
get the "240"?

* * * * * * * * * * * *Jerry * KD6JDJ

Jerry, in the three wire system with 180° phase difference between Line1
and Line2 wrt Neutral (the centre wire) the voltage Line1 wrt Line2 is
exactly twice the Line1 to Neutral voltage.

You probably don't use the word Neutral over there, it is the word used
for four wire three phase systems and it is equally applicable for a
three wire two phase system where the Neutral wire carries the
"imbalance" current.

None of this is a good analogy to a two wire open transmission line at
radio frequencies, principally because the length of conductors at power
frequencies is usually a very small part of a wavelength and can be
approximated well using a simpler analytical model than the traditional
'Telegrapher's' equations applied to transmission lines.

Those who resort to explaining radio frequency transmission lines and
transformers using 60Hz models need to justify the accuracy / limits of
the approximation. The low frequency model does not explain high
frequency roll-off in RF transformers and inductors, so it is clearly
incomplete.

Almost all of this discussion is using lumped circuit analysis (a low
frequency model) to explain not just transmission lines, but conductors
with significant mutual coupling (the nominal radiator and its feedline).

Owen

- Hide quoted text -

- Show quoted text -

I found it difficult to go beyond the simplistic 60 Hz analogy in this
discussion to best decribe my concept (when writing on usenet). All
one can hope for is that the general idea comes out and hope people
realize that the variables you mention do exist as well and do not
detract from the very fundamental concepts.

On Sep 3, 7:12*pm, Cecil Moore wrote:
wrote:
Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase.

You are 100% wrong. From "The IEEE Dictionary":
"common-mode (1)(general) The instantaneous algebraic
average of two signals applied to a balanced circuit,
both signals referred to a common reference."

The "instantaneous algebraic average of two signals"
180 degrees out of phase is ZERO.
--
73, Cecil *http://www.w5dxp.com

You reference line 1 to line 2. You put your RF voltmeter acroos the
two conductors. When doing so you measure two times the voltage that
line 1 or line 2 reads with respect to an imaginary isolated grounbd
at the centertap the line 1 or 2 measures separately with respect to
centertap. OF COURSE the summation of all points on the common
reference is zero at all locations at all times, that is why it is
called an isolated "ground"! That is why I said earlier you could
connect the centertap of the isolated CM side to real ground with no
effect on the circuit in an earlier post.

On Sep 3, 7:18*pm, Cecil Moore wrote:
wrote:
I agree that a balun operating at 14 MHz would be a choke when
operating at 70MHz.

You would do well to agree that a well-designed W2DU balun
operating at 14 MHz is choking the heck out of the 14 MHz
common-mode current in order to achieve the balun function.

You seem to have the IEEE definitions of differential
signals and common-mode signals exactly reversed. Because
of that misconception, might you be the one who doesn't
understand how baluns work?
--
73, Cecil *http://www.w5dxp.com

Cecil, if a transmission line operated in the way you think, it would
be radiating fields all along its length transmitting RF all along the
length of the line. Nothing would get the antenna. The conductors in
the lines MUST carry mirror image currents and voltages to indeed
cause the cancellation of fileds you speak about. THAT is how energy
gets to the antenna. Ever hear of a ground loop?