J-Pole antenna is an end-fed omnidirectional antenna that should be fed with 50-ohm impedance balanced line. When fed with coax (whether it is an RG-58 or RG-8) cable, it should be connected with air-choked balun.
There are several variation of J-Pole antenna, including the Slim Jim and also the ladder-line J-pole suitable to be used with hand held transceiver.
Here is the plan for a typical J-Pole antenna (measuremant in inches):
Copper tubing is usually used as the construction material for J-Pole antenna.
A mounted J-Pole antenna:
Alternatively, you can buy ready-made J-Pole antenna for hamradio transmission from KB9VBR J-Pole Antennas.