I have built a J pole based on the link posted above, middle of airband is 122.5 so i put that into the calculator
I did add a mounting leg as per another calculator i found online.
My method of attaching the coax will need improving, i used self tappers and crimp on forks, im thinking bonding clamps would be good as they are purpose designed to join cable to pipes
It is an improvement over the dipole on airband, LBA ATIS is very faint with squelch on 1 on the dipole, but much clearer, with some interference with the J pole and the squelch on 2.
I made it from 15mm copper as i havent got any 22 pipe, its currently strapped to an old tripod and poking out of the Velux in the loft...
If I do decide to mount it externally i used tee's instead of elbows and left both ends open as I don't have any cap ends either
I did find a length of 28mm copper, would the extra diameter be any benefit?
Im also looking at making a small one as per m0slx with an N type chassis plug and a BNC to N adapter, but i need some suitable copper wire