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.

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.]

[3] Add a 1/2-wavelength 440 parasitic radiator, mounted next to the
bottom of the 2-meter radiator on standoffs.

This one is a bit speculative. There's a published design on the
ARRL website which uses this approach to improve the radiation
angle for a center-fed vertical 2-meter/440 dipole, and I think
the same trick ought to work for a copper-pipe J-pole if you put
the parasitic radiator on the opposite side of the antenna from
the matching arm.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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