Ron, when the conductors in a transmission line are close, much much
less than a wavelength, the currents, which are out of phase, create
plus and minus EM fields that effectively cancel in all directions.

As the spacing becomes greater and becomes a significant portion of a
wavelength, the fields cancel in some directions but the spacing causes
the phase to add in other directions. Under this class of conditions the
line starts to become more like an antenna than a transmission line.

As a point of further confusion, coaxial cables that have woven
overbraid have a leakage inductance that allows some of the rf to leak
out. So, even coax has some radiation due to leakage.



Ron wrote:
Can someone explain how a transmission line starts radiating as the
separation between the center conductor and ground plane becomes greater
and greater.

SNIP