Today I clipped out the connection I had made at the bottom of the radiator,
and the SWR bandwidth remained 2MHz as is was with the connection. The group
of SWR measurements moved up an average of 150 KHz from my readings with the
connection, but otherwise the performance of the "Slim Jim" configuration is
identical. The length of the gap may now serve as a tuning mechanism, which
wasn't a feature with the bottom connection, and the archives show some
builders made good use of that facility. Thanks for spotting the similarity,
Jeff. My 6M antenna will no doubt be better for it.

73,

Chuck

"Jeff" wrote in message ...
Have you not just re-invented the 'Slim Jim'??

73
Jeff

"Chuck Olson" wrote in message
. ..
I've been working on a ladder line J-Pole design for 6m, but have had
very
little luck until today. The problem has been critical tuning and narrow
SWR
bandwidth. The improvement came today when I implemented an idea I had
concerning the severed piece of wire that "just goes along for the ride"
in
the ladder line after cutting the 1/4" gap for the 1/4-wave shorted
stub.
I
figured that many successful 50 MHz J-Poles are made from copper tubing
because the thicker elements give it good bandwidth. My idea was to make
use
of the extra wire and connect it at top and at the gap to the radiator
side,
making the half-wave radiator act as a much thicker element. My 2:1
bandwidth went from 300 KHz to 2 MHz.

The resulting antenna design is very straightforward, using the Velocity
Factor of 0.91 for the 1/4-wave stub and 0.95 for the radiator. This
essentially sets the radiator length equal to the standard 468/F dipole
length. If you analyze the operation of the very successful "Open Stub
J-Pole" that Arrow makes, you will find they use the 0.95 FV for both
the
radiator and the stub, which is appropriate in their open design. So
that
pointed me in that direction as far as cutting lengths are concerned.
The
only remaining question was the location of the feed tap for 50-ohm
cable.
I
used alligator clips on the coax to find the best position, and that
turned
out to be 4 3/8" up from the shorted bottom end, with the shield going
to
the gap side. My rig sees a 1:1 SWR from 50.0 to 51.2 MHz, and it gets
to
1.6:1 at 52.1 MHz. With this information, it should be easy to design
one
that takes full advantage of the antenna's bandwidth to provide
operation
over the widest segment of the 6M band.

My intuition told me that there should be some advantage to using
450-ohm
ladder line compared to 300-ohm twinlead. Maybe this extra bandwidth is
it.

73,

Chuck, W6PKP