CP,
It's actually called a 'Double Bazooka' antenna and has
been around for ever. While it appears to be broaderbanded
than a 1/2 wave dipole, the problem is that the radiation
efficiency tapers off as you get to the extremes of the
frequency coverage. It will still show a low SWR, but because
of several things (reactances mostly), the radiation level
decreases so that you actually aren't radiating any more than
if you were using a 1/2 wave dipole with a high SWR. The
net gain over a common 1/2 wave dipole is zero.
It is a fairly 'quiet' antenna, not quite as much noise as
the
common dipole. At HF the diameter of the conductors make no
difference in the antenna's being broadbanded. The diameter
has to be increased to a sizable percentage of a wave length
before it really makes any appreciable difference. Something
like 6 feet diameter at 80 meters, and around 2 feet at 10/11
meters.
There is a formula for this type of antenna. You need to
know the velocity factor for the kind of coax you use. Cut the
total length of the dipole for (468/f{in Mhz}) = L1. Find the
electrical 1/2 wave length by multiplying this L1 by the coax's
velocity factor = L2. Subtract L2 from L1, divide it by two,
and that's how far from each end to short the inner conductor
and braid. The rest of the antenna is done as in the example.
There's a pretty good examination of coaxial dipoles in one
of
the ARRL's Antenna Compendium books.
'Doc

PS - The 'CCD' or Controled Capacitance Distribution antenna is
a whole 'nuther animal.