"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



It looks like I left out the design frequency that resulted in the 2:1 SWR
bandwidth from 49.54 to 51.53 MHz, and that was 51.13 MHz. It looks like
there's an offset of +600 KHz between the frequency used to determine
cutting length and the actual center of SWR bandwidth. But once that's
known, the design can be calculated for precise frequency coverage.

Maybe we should call it a "Fat-Wire J-pole" - - feel free to offer
alternatives - -

73,

Chuck, W6PKP