Hobbies And Interests
Things You'll Need
Instructions
Cut the main vertical section -- the long end of the J -- about 15 1/2 feet long. This is 2 feet longer than needed for the radiating section; the extra lengtht will be used to mount the antenna on a mast. Slide the T fitting along the pipe until it is 13 feet, 6 3/4 inches from the end. Apply flux and solder it in place.
Cut the short vertical of the J to a length of 4 1/2 feet. Cut a small section of pipe to 5 1/2 inches. This will be the bottom of the J and it will be trimmed to fit. Place it in the T fitting and put the L fitting on the other end. The goal is to have the two vertical sections 5 1/16 inches apart at the tube walls -- not at the centers. Trim the short pipe as necessary, then flux and solder it.
Solder the short vertical pipe into the L fitting. Solder pipe caps on both vertical sections. This is now a half-wave antenna with a tuning stub on one end. Prepare the coaxial cable by removing the outer sheath to expose the shield braid. Push the braid back and slide the inner conductor out of the braid. You'll end up with a Y-shaped cable with the center conductor and shield forming the upper end. Solder a terminal on each end.
Attach the coaxial cable to feed points located 5 1/4 inches above the bottom of the J. Remember to measure from the pipe wall, not the center. Use hose clamps to hold the terminals against the pipe. The center conductor attaches to the long vertical and the braided shield attaches to the short vertical. Hose clamps allow you to slide the feed points up and down to achieve the best antenna tuning.
Cover the feed points, braided shield and coaxial cable end with silicone sealer made for electronics. If desired, spray the copper parts with clear acrylic to prevent corrosion. Radio waves travel by skin effect on copper tubing, so corrosion eventually degrades performance.