k1drw wrote:
I just cannot understand how I2 gets coupled to the dipole antenna
element since the coax gets coupled at the PL-259 via the outside
surface of the braid.

It is the physics of the coaxial cable that causes I1
and I2 to be equal in magnitude and opposite in phase.
The internal coax fields force that condition. At the
antenna, I1 has only one choice of paths - into the
antenna (if matched). But I2 has two choices - into
the antenna (if matched) or down the outside of the
coax. It will divide at that point according to Ohm's
law most of it taking the path of least impedance.
A choke increases the impedance down the outside of
the coax making the antenna impedance more attractive
to the current.
--
73, Cecil http://www.w5dxp.com