I need to proofread more. "At the junctions between the *ANTENNA* wire
and the *LOADING* coil there is a transfer of energy..." would have
been a bit clearer. Also very possibly inaccurate. The energy
transfer may happen somewhere else in space as the fields around the
antenna wire do not have the exponentially decaying radial behavior
that the coil surface wave fields have.

I expect, though, that the current at the antenna wire/coil junction is
what does the exciting of the surface wave modes on the helix.

Also should have said "a few turns over a few inches for a *TWT
OPERATING* at microwave frequencies".

-Dan