Why use a J at all? I'd just use a 1/2 wave radiator,
and feed it using a gamma loop setup. ,For 20m,
a 20 inch single turn loop should work ok. Can be
made from copper, or aluminum tubing. If you need
a capacitor, it can be made from coax. For 20m, about
100-140 pf should be pretty close... BTW.. this design
would be the same as any VHF ringo...MK