On Nov 27, 4:44*am, Jeff Liebermann wrote:
On Sat, 26 Nov 2011 14:22:52 -0800 (PST), Tom Horne

wrote:
Can anyone direct me to the dimensions to build a six meter J-pole
that is patterned after the Arrow Antenna two meter design that had
the feed point at the bottom of one of the elements rather than at a
point part way up the height of the matching stub.

The 144/440 antenna can be found at:
http://www.arrowantennas.com/inst/OSJ146440.pdf
with dimensions included. *You probably won't need the short element
in the middle which seems to do something for 440Mhz operation.

Two ways to do this:
1. *Plug the dimensions (in wavelengths) into an EZNec or 4NEC2
antenna model, and scale it for 52MHz.
2. *Scale the antenna dimensions linearly from 146 to 52MHz and
optimize with cut-n-try. *The driven element is 1/4 wavelength long,
and the parasitic element is 3/4 wavelength long.

The reason that I would like to use that design is that I would like
to use Rigid Aluminum Conduit to build the antenna. *There not being a
lot of simple inexpensive fittings for Conduit the way there is for
copper pipe a mechanically simpler design would be easier to build.

Maybe, but the 3/4 wavelength element is going to end up about 15ft
long, which is rather long for tubing without a top support. *The
"rigid" aluminum conduit is made to be bent to shape. *I suspect this
15ft long tube might deform in the wind.

Let me say that I would rather
not be subjected to any tirades about the inferiority of the J-pole
approach or the superiority of some other antenna design.

Ok. *I'll just ignore my experience with J-poles and pretend that
they're suitable for your unstated purpose.

That does
not mean that I am not open to suggestions for a better approach as
long as it is civilly stated and I am spared the application of Rant
Mode.

Approach for what application and mounting? *If you want an antenna
with a rather high takeoff angle, an end fed antenna, such as the
J-pole will send most of your RF towards the birds instead of the
horizon. *If you want an antenna with a radiating coax cable (by
lacking a balun), the J-pole is quite appropriate. *If you want an
antenna that produces impressive cloverleaf horizontal gain patterns
when side mounted on a tower, the J-pole is the hot ticket. *The
J-pole does have one big advantage. *It was originally called a Zepp
antenna, after the Zepplin airship, where the antenna was suspended
downward. *Because the low voltage point is at the base of the
antenna, there would be no sparks to ignite the hydrogen gas. However,
if you don't plan on flying your j-pole on a Zepplin, this advantage
might not be very useful.
http://en.wikipedia.org/wiki/J-pole_antenna (first paragraph)

Please feel free to offer undying praise and monetary renumeration for
the advice. *I'm always open to being paid. *There are probably better
designs but since you've already decided on a J-pole, it's probably
futile to suggest alternatives. *Besides, it might precipitate a
debate or arguement. *Oddly, I learn more from such debates and
arguments, than from pontifications.

5/8 wave 6m ground plane:
http://802.11junk.com/jeffl/antennas/gnd-plane-02/index.html
5/8 + 5/8 wave 6m ground plane:
http://802.11junk.com/jeffl/antennas/gnd-plane-05/index.html

I would normally offer to build you a NEC2 model of your proposed
antenna, but since I don't know what diameter aluminum tubing you have
in mind, and since it's 1:30AM, I'll pass for tonite.

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558

All hands

Let me start over. I want a transportable, six meter, vertically
polarized antenna. The parts have to be readily assembled &
disassembled. I would like it to have a somewhat higher than normally
attempted radiation pattern for use between buildings, hills,
ridges... [I had thought that a J-pole might be a good compromise
instead of antennas with a more horizontal gain pattern.] It would be
really golden if the adding of additional parts would change the
pattern back towards the horizontal to use the antenna when it is
located above local horizon. Parts should be short enough to fit in
the foot well of the back seat of an ordinary sedan. It's a stretch
but If it did not need guying that would contribute to ease of
deployment especially in an urban operating location were the
anchoring possibilities for guy lines may be limited. [I'm trying to
devise an X frame tripod. It would be made out of fiberglass
structural channel in order to lower the weight of the tripod assembly
but that is not an antenna design problem.] I may need the antenna to
be elevated when operating near pedestrian traffic to reduce the risk
of RF contact or excessive exposure. Shielding the feed point from
physical damage; such as by having it inside a piece of Box channel
would be quite desirable. I'll leave it there and see what folks can
think of that might work.

--
Tom Horne, W3TDH