A slightly different way of looking at it from what Gary wrote...but
quite similar.

It's better to think of a two-wire transmission line, probably. If
you want to think of the ground plane, just realize that it's
identical to the situation with two conductors driven out of phase:
you can insert the ground plane without any effect on the fields at
all.

Then each wire does radiate, but to the extent that their currents are
coincident in space and in opposite directions, those radiations
cancel. Net field at any point in space is the linear combination of
all the fields arriving at that point, at that instant in time. As
the wires become more separated, the radiations observed at a distance
no longer cancel. You're not the same distance from each wire, and
more importantly, the phase you see differs. Consider what you see if
the wires are separated by half a wavelength, and you are in the plane
the wires are in...and what you see if you are in a plane
perpendicular to the plane the wires are in and passing between them.
If you observe the fields close to one of the wires, of course the
cancellation is not good there, either, though that's energy
propagating in the direction of the line. Note that there's no
radiation from coaxial line, so long as the net currents in the inner
and outer are exactly out of phase and the current distribution is
uniform around the outer conductor (assuming the conductors are
exactly coaxial), even if the outer conductor is not very thick.

I'll (once again) recommend the "Antennas" chapter of King, Mimno and
Wing, "Transmission Lines, Antennas and Waveguides." You'll find it
in the antennas chapter rather than the transmission lines chapter
because it's radiation rather than energy propagation along the line,
I suppose. The introductory material in that chapter bears on this
topic, and later in the chapter there's very specific mention of
radiation from transmission lines, including what seem some
non-intuitive results about amount of radiation versus line length.

Cheers,
Tom

Ron wrote in message .com...
Can someone explain how a transmission line starts radiating as the separation
between the center conductor and ground plane becomes greater and greater.
Assume you out start with a wire over an infinite copper ground plane that forms
a 50 ohm Zo transmission line. Then increase the distance between the wire and
the ground plane until the wire becomes an end fed antenna. What happens to
cause radiation to begin?

Ron