I made a new Collinear J pole today with 4 elements. It performed OK but still not as good as my 9 element CoCo.
I then made another 4 element one but with the elements 137.5mm rather than 132mm. Still kept the phasing stubs at 16 turns and 53mm height. All made out of 1mm copper wire. 137.5 is half wavelength I calculated
Worked a treat !!
The max range of the J pole is about 20nm less than the CoCo. 190nm vs 170nm
Tomorrow I'll remake the bottom end of the antenna with the quarter wave stub at 68mm and the three quarter stub at 206mm keeping the tap at 6mm.