ve2pid wrote:
We can use EZNEC to simulate the radiation of a coax with a wire as
suggested in the help file "The radiation properties of a coaxial
feedline can be modeled by connecting a wire of the coax shield's
diameter to the point on the antenna where the shield connects."

We then see that the current has a maximum somewhere along that wire.

My question: Why do we have a maximum? Is is because some current is
also going up from the source outside the coax?, In that case, some
kind of constructive and destructive interference could happen on the
outside of the coax due to phases differences between the two
currents, creating maximum and also nodes.

Or is there another reason explaining that maximum?

The current can be caused by either of two mechanisms, or both:
conduction or induction.

Conducted current occurs when, for example, a coaxial line is connected
to a dipole. Current from the inside of the shield splits and part goes
into the antenna and part goes down along the outside of the shield.
This can be effectively stopped with a current balun (common mode choke).

Induced current will be zero if the transmission line is symmetrically
placed with respect to a symmetrical antenna which has equal currents in
the two halves. But in other cases, the fields from various parts of the
antenna won't sum to zero at the location of the transmission line, so
current will be induced on it. Current baluns can reduce this current
also, but it generally takes at least two, spaced a quarter wavelength
apart.

Now for the question. You have a maximum for the same reason you have
maxima and minima along an antenna -- in fact, a transmission line with
common mode current *is* an antenna, and radiates the same as any other
one. The maxima and minima can be explained as the envelope of traveling
current waves reflecting from an open end (or current balun or other
discontinuity) and interfering with each other. These are known as
standing waves, because the positions of the maxima and minima stay in
the same places and don't move. A lookup of "standing waves" will bring
you a lot more information, and I'm sure some good graphics, to further
explain it.

Roy Lewallen, W7EL