The J-Pole Antenna

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The J-Pole is a half-wave antenna that is end-fed at its bottom. Since the radiator is longer than that of a 1/4-wave ground-plane antenna, the vertical lobe is compressed down toward the horizon and it has about 1.5 dB of gain compared to the ground-plane configuration.

The stub-matching section used to transform the high impedance seen looking into a half-wave to 50 Ω coax is shorted at the bottom, making the antenna look like the letter “J,” and giving the antenna its name.  Rigid copper tubing, fittings and assorted hardware can be used to make a really rugged J-pole antenna for 2 meters. When copper tubing is used, the entire assembly can be soldered together, ensuring electrical integrity, and making the whole antenna weatherproof.

No special hardware or machined parts are used in this antenna, nor are insulating materials needed, since the antenna is always at dc ground. Best of all, even if the parts aren’t on sale, the antenna can be built for less than $15. If you only build one antenna, you’ll have enough tubing left over to make most of a second antenna.

Construction
Copper and brass is used exclusively in this antenna. These metals get along together, so dissimilar metal corrosion is eliminated. Both metals solder well, too.

Cut the copper tubing to the lengths indicated. Item 9 is a 11/4-inch nipple cut from the 20-inch length of 1/2-inch tubing. This leaves 183/4 inches for the 1/4-matching stub. Item 10 is a 31/4-inch long nipple cut from the 60-inch length of 3/4-inch tubing. The 3/4-wave element should measure 563/4-inches long.

Remove burrs from the ends of the tubing after cutting, and clean the mating surfaces with sandpaper, steel wool, or emery cloth. After cleaning, apply a very thin coat of flux to the mating elements and assemble the tubing, elbow, tee, end caps and stubs. Solder the assembled parts with a propane torch and rosin-core solder. Wipe off excess solder with a damp cloth, being careful not to burn yourself.

The copper tubing will hold heat for a long time after you’ve finished soldering. After soldering, set the assembly aside to cool. Flatten one each of the 1/2-inch and 3/4-inch pipe clamps. Drill a hole in the flattened clamp as shown. Assemble the clamps and cut off the excess metal from the flattened clamp using the unmodified clamp as a template. Disassemble the clamps. Assemble the 1/2-inch clamp around the 1/4-wave element and secure with two of the screws, washers, and nuts as shown. Do the same with the 3/4-inch clamp around the 3/4-wave element. Set the clamps initially to a spot about 4 inches above the bottom of the “J” on their respective elements. Tighten the clamps only finger tight, since you’ll need to move them when tuning.

Tuning
The J-Pole can be fed directly from 50-ohm coax through a choke balun (3 turns of the feed coax rolled into a coil about 8 inches in diameter and held together with electrical tape). Before tuning, mount the antenna vertically, about 5 to 10 feet from the ground. A short TV mast on a tripod works well for this purpose.

When tuning VHF antennas, keep in mind that they are sensitive to nearby objects—such as your body. Attach the feed line to the clamps on the antenna, and make sure all the nuts and screws are at least finger tight. It really doesn’t matter to which element (¾-wave element or stub) you attach the coaxial center lead.

Tune the antenna by moving the two feed-point clamps equal distances a small amount each time until the SWR is minimum at the desired frequency. The SWR will be close to 1:1.

Final Assembly
The final assembly of the antenna will determine its long-term survivability. Perform the following steps with care. After adjusting the clamps for minimum SWR, mark the clamp positions with a pencil and then remove the feed line and clamps. Apply a very thin coating of flux to the inside of the clamp and the corresponding surface of the antenna element where the clamp attaches. Install the clamps and tighten the clamp screws.

Solder the feed line clamps where they are attached to the antenna elements. Now, apply a small amount of solder around the screw heads and nuts where they contact the clamps. Don’t get solder on the screw threads! Clean away excess flux with a non-corrosive solvent.

After final assembly and erecting/mounting the antenna in the desired location, attach the feed line and secure with the remaining washer and nut. Weather-seal this joint with RTV.

Source: The ARRL Antenna Handbook