Use a 54” piece of 450 ohm Ladder line. Cut out 3 inches of the center part of the ladder line. Solder a PL-259 on the bottom of the Ladder Line (both side of the ladder line will solder on the ground side of the PL-259 - this is used for the shorted part of the J-Pole). Cut a 1” PVC pipe 1 1/2” long. Cut a 36” piece of RG-174 coax. Wrap the coax around the PVC pipe. One end will attach to the PL-259, the center of the coax will connect to the center of the PL-259 and the shield of the coax will connect to the outer portion of the PL-259. Strip the insulation off lh ladder line 2 1/2” above the PL-259. Attach the center of the coax to the side that will have the long wire attached, the shield side of the coax will connect to the short side of the J- Pole. Connect the Long wire to the ladder line.
Check the SWR, if it is not less than 2:1 in the operating portion you want to operate, you can tune the J-Pole by moving the tape of RG-174 up or down. Do not move it more than ~ 1/4” at a time.