On Sep 3, 7:18*am, Cecil Moore wrote:
wrote:
For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.

Well, there's your problem. In a properly functioning
transmission line, whether balanced or unbalanced, the
currents in the two conductors are differential, not
common-mode. The two currents are equal in magnitude
and 180 degrees out of phase and their fields tend
to cancel which minimizes radiation.

Common-mode currents only occur when the two currents
are not perfectly differential, i.e. not equal amplitudes
and/or not 180 degrees out of phase.
--
73, Cecil *http://www.w5dxp.com

Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase. If they weren't, the CM
choke could not act to cancel them and you would need differential
mode chokes to get rid of them. On your balanced feedline, the voltage
at any point is delta between the amplitude of line 1 and 2.

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

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

So that means I get 0 volts when I touch one side of the 120V in my
electrical service to the 120V other side? I was afraid I might see
240V.

wrote:
So that means I get 0 volts when I touch one side of the 120V in my
electrical service to the 120V other side? I was afraid I might see
240V.

If those signals are equal in amplitude and 180 degrees out
of phase, it means they are *differential*, you will see 240v,
and you had better not short them together. OTOH, if they are
common-mode signals, they are in-phase and you can short them
together to your heart's content - no current will flow.

Again, you seem to have the IEEE definitions of "differential"
and "common-mode" exactly reversed in your head.
--
73, Cecil http://www.w5dxp.com

On Sep 3, 7:25*pm, Cecil Moore wrote:
wrote:
So that means I get 0 volts when I touch one side of the 120V in my
electrical service to the 120V other side? I was afraid I might see
240V.

If those signals are equal in amplitude and 180 degrees out
of phase, it means they are *differential*, you will see 240v,
and you had better not short them together.

OTOH, if they are
common-mode signals, they are in-phase and you can short them
together to your heart's content - no current will flow.

Again, you seem to have the IEEE definitions of "differential"
and "common-mode" exactly reversed in your head.
--
73, Cecil *http://www.w5dxp.com

Really? Do you believe the currents in a resonant 1/2 wave dipole are
common mode or differential mode?

wrote:
Again, you seem to have the IEEE definitions of "differential"
and "common-mode" exactly reversed in your head.

Really? Do you believe the currents in a resonant 1/2 wave dipole are
common mode or differential mode?

Assume it is a 1/2WL wire in free space. Where
is the common reference?

I've heard antenna currents called "common-mode"
currents but neither Kraus nor Balanis call those
currents "common-mode". They are usually called
"antenna currents" (in phase, radiating) vs
"transmission line currents" (out of phase, non-
radiating).

In any case, differential currents on a transmission
line are 180 degrees out of phase and ideally, non-
radiating. Common-mode currents on a transmission
line are in phase and radiate. The currents in a
folded dipole are in phase and radiate.

Within a ferrite toroid wired in a 1:1 current-choke-
balun configuration, common-mode current induces flux
in the toroid with virtually none from differential mode.
If the device is made out of turns of coax, the differential
currents never see the choking impedance.
--
73, Cecil http://www.w5dxp.com

On Sep 4, 7:51*am, Cecil Moore wrote:
wrote:
Again, you seem to have the IEEE definitions of "differential"
and "common-mode" exactly reversed in your head.

Really? Do you believe the currents in a resonant 1/2 wave dipole are
common mode or differential mode?

Assume it is a 1/2WL wire in free space. Where
is the common reference?

There is none unless you choose to define one. You could go halfway
down the output choke and call it "common refrerence" or "isolated
ground" or whatever you want to call it. Since there usually no need
to reference it axcept perhaps for tutorial purposes, I wouldn't call
it anything. You could also reference it to earth ground but only if
you physically connected it to the aforementioned centertap.

I've heard antenna currents called "common-mode"
currents but neither Kraus nor Balanis call those
currents "common-mode". They are usually called
"antenna currents" (in phase, radiating) vs
"transmission line currents" (out of phase, non-
radiating).

In any case, differential currents on a transmission
line are 180 degrees out of phase and ideally, non-
radiating. Common-mode currents on a transmission
line are in phase and radiate. The currents in a
folded dipole are in phase and radiate.

The ideal transmission line is common mode and does not radiate
because the fields cancel as you said earlier. The dipole antenna is
ALSO common mode but the fileds do NOT cancel because the conductors
are physically 180 degrees apart from each other so they cannot
interfere with each other; instead the fields radiate into free space
rather than cancel each other out. It is rather simple really. It is
correct to call antenna currents "common mode currents". If the
currents on a transmission line are differential, how would (COULD)
they be converted to common mode currents on the antenna? We would
need a 180 degree phase shift somewhere. Answer: the currents on both
the transmission line and the antenna are common mode.

Within a ferrite toroid wired in a 1:1 current-choke-
balun configuration, common-mode current induces flux
in the toroid with virtually none from differential mode.
If the device is made out of turns of coax, the differential
currents never see the choking impedance.

Which explains why you should call it a balun and not a choke. If the
balun and source impedances match there should be no choking
impedance; maximum power trnasfer should occur.

--
73, Cecil *http://www.w5dxp.com

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.


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

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!

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.