I seem to remember writing a post about how I make J-poles on here but can't seem to find it. It might have been the old forum...
If you make it out of 22 mm pipe, it becomes a bit less wobbly in the wind and you can then do a few things to make tuning easier.
Get yourself a square flanged SO-239, like this:
Three brass 22 mm clamps like this:
http://www.screwfix.com/p/22mm-pipe-cli ... pack/2401j
And a 22 mm compression coupling:
http://www.screwfix.com/p/straight-coup ... pack/66818
Cut the three brass clamps on one side of the screw so that you're left with the screw and two semicircles. Like this:
You can clamp the flanged SO-239 directly to two of the clamps with nuts and bolts, or perhaps the screws supplied.
Get a small length (6" should do it) thick (Ø2-3 mm) enamelled copper wire and tin one end (your iron must be at over 400 °C to do this properly - the Weller iron I suggested will do the job) and solder it to the centre conductor of the SO-239 (with a smaller iron).
Make sure your enamelled copper wire fits into the solder bucket of your SO-239.
You should now have a SO-239 bolted to two circles made of brass (the clamps) with a 6" piece of enamelled copper wire soldered to the centre of the SO-239.
Get some steel wool and rub the J-pole elements with it like that scene out of Ali G.
Now slide the SO-239 brass clamp assembly onto the short element of your J-pole. Or undo the screws, or do it in a slightly different order, it's up to you.
Take your remaining cut up 22 mm brass pipe clamp and slide it onto the long element of your J-pole. Line it up in an arbitrary position along the elements, but in line with the SO-239 assembly and tin the enamelled copper wire in the position above the lone 22 mm brass clamp. The wire should be fairly straight in all directions but with a small amount of slack, so that you can move the SO-239 up and down by about 5 mm in each direction relative to the other brass clamp.
Tin the brass clamp on the long element, but do not get any solder on the element itself, and solder the enamelled copper wire to it.
You now have a J-pole that you can tune up and down by tightening and loosening three screws.
When you're happy, solder the brass clamps to the J-pole. You should do this as soon as possible, but realistically within a few hours of doing your Ali G impression.
Be careful to not melt the dielectric in the SO-239, especially if you decide to solder the flange to the brass clamps.
This is where the blowtorch comes in handy to get it hot as quickly as possible.