On 13 sep, 16:07, Owen Duffy wrote:
lu6etj wrote :

I agree with Cecil, Owen, I do not see nothing strange on have common
mode current in one part of the line and not in the other, ideal choke
it is an open circuit to RF currents.

Miguel,

There are two issues.

Firstly, if the common mode choke (or isolator) is physically small with
respect to wavelength, and there is common mode current on the line
immediately adjacent on one side of it, why is there not an almost equal
common mode current on the other side... or explain the current path that
allows the first mentioned current AND that complies with Kirchoff's
Current Law.

Secondly, the common mode 'conductors' are coupled conductors. I see that
Roy has dealt with that, so I will leave it at that.

For these reasons, it is naive to think that a practical common mode
choke has such extremely high impedance that the common mode current
through the choke is zero, or even near to it. Even if it did drive
common mode current to zero or near zero at that point, that does not
mean there is no common mode standing wave, just that a node exists at
that point.

Ask yourself how whether the use of a common mode choke (isolator) to
effectively reduce common mode current between the common mode choke and
the tx don't also reduce common mode current between the common mode
choke and the dipole feed point.

Owen

Owen

It is obvious, we are talking about different things. External fields
always will induce common mode currents in conductors, then it is
virtually impossible completely remove it. We will have commond mode
currents in our line if our ham neigbourhood turn on his TX or our
wife talk near with her celluar phone too.

When I talk about interrupt common mode current with choke I am not
thinking in prevent all possibles external induction fields over the
section of line isolated with the choke but in interrupt the current
flow that would have in this point if not were the device placed. In
that sense no puntual devices can avoid induction laws. Also, a good
isolator it is a capacitor and always will allow displacement
currents.

From electromagnetic point of view the inductor field beyond the
isolator or ideal choke it is the same that any other external field.
Not work either if as isolating device I installed five meters of
optical fiber and a pair of transducers :)
Do you search for a TL incapable to induce external fields?, then we
are not talking about baluns, perhaps we could talk about EM shieding
of the TL.
Similar situation if we install a perfect balun and run the TL paralel
to the antenna, common mode current not depends.

73

Miguel LU6ETJ