On Thu, 14 Jul 2005 22:01:35 -0000, (Dave Platt)
wrote:


I have a copper tube J pole for 2m. Is there a good way to convert it
to 2M/440?

Try using it as is. I have a 2m J I made many years ago & it has a
decent SWR on 440 & seems to work fine.

They'll usually work as-is on 440, but aren't entirely efficient. SWR
is higher, and so is the radiation angle - a good deal of power ends
up warming the nearby treetops and hilltops rather than going out
towards the horizon.

The take-off angle is about 35degs!


I've seen at least three ways suggested to construct a more-effective
dualband J-pole:

[1] Separate matching arm and feedline for 440 (the "copper cactus"
design)

[2] Adding a downwards-pointing, shorted-at-the-top quarter-wavelength
stub (coaxial or arm) partway up the radiator, leaving the distance
between the open end of the stub and the top of the main matching arm
equal to 1/2 wavelength at 440. The stub presents a high
impedance to the 440 power, constraining its current to remain
mostly in the 1/20-wavelength-at-440 part of the radiator.

Edison Fong WB6IQN published a design in QST (February 2003),
which uses this approach to create an improved 2-meter/440
twinlead J-pole design. The same trick ought to work with a
copper pipe J-pole, I imagine. [There are some publishing errors
in the QST version of Ed's paper... he's got a corrected version
available, I believe.]

Basicly an isolated piece of copper/alu pipe in the middle of the
radiator will convert the 2m j-pole into a 70cm collinear.
This trick will give you extra gain because it downs the angle of
radiation for that band considerable.

G3JAM (I'm told) tried to integrate a J-pole into a piece of PVC pipe,
back in 1981 via Wireles World. He was forced to find his solution
into a hairpin loop and variable coax in order to achieve easy and
correct matching.

I made one of flat alu strip placed in a pvc tube based on the G3JAM
design, you find this drawing he
http://radiogaga.e-dentify.nl/pa3abk/Townsman_uk.gif
To be honest the drawing is from a Dutch hammagazine Electron jan 1991
and is based on the principle which was outlined by G3JAM in 1981.

Also have a look at the page of pa0fbk
http://www.qsl.net/pa0fbk/hampage_uk.htm (scroll down) he is using
coax and ends up into a slim design.

View it's easy to slide a piece of pipe over the radiator it's worth
to try it out. Just isolate it from the radiator. It will not disturb
the SWR on 2M.
Before you go, note down the signal report of a remote repeater (avoid
that one which is at a angle of 35 degs on a hilltop), so you can see
the difference.
Jan/pa3abk