A Stacked 3-Element J-Pole
By Richard Morrow, K5CNF
fter reading the J-pole article written in an earlier issue for antenneX by Joel, KB1EG and the experiments run by Old George, KC5MU, I decided to try the Stacked J-Pole as an experiment to see how it would work, using the measurements that George arrived at through his many experiments with J-Poles. I went to a local building supply store and bought two 10 foot, (3.05m) long 1/2-inch (1.27cm) diameter hard drawn copper pipes and nine 90-degree elbows, a T connector and two pipe caps.
The pipes were cut into three 39-inch (99.06cm) lengths to make up the vertical radiators. Then the phasing lines were cut to lengths of 19.5 inches (49.53cm.). When cutting these sections, the long sections were measured with an elbow on one end. The cut length for the copper pipe was determined by measuring from the outside of the elbow to a point 39 inches down the pipe. I subtracted the length of the elbow from 39 inches and cut at that point so when the elbow is soldered to the pipe, the total length will be correct. The top element must have a pipe cap on it to keep water and bugs out.
Cutting the phasing lines is a little more difficult because the total length of the line must be 39 inches long including the pipe at the bend at the middle of the line. The 1/4 wavelength matching section was cut to the 20 inches (50.8cm) length. Spacing from the radiator is 3 inches (7.62cm).
Figure 1 shows the construction of the antenna. The components are color-coded. Red designates the vertical radiators, light blues represent the 90-degree elbows and dark blue is the phasing lines. The matching section is green. At the bottom of the antenna, the T-connector is purple and is where the mast will go. For a mast I will use a 15-foot (4.6m) length of 3/4-inch (1.91cm) diameter copper pipe that will be driven into the ground about 3 feet (.9m) deep. This should ground the antenna and eliminate any static build up.
Figures 2 and 3 show how the measurements should be made.
Now this is one big antenna, in case you have not figured this out by now. So to assemble this one, at the office I made up the phasing lines first and then went outside to solder them together. I also added the elbows and the cap to the top radiator and the elbows to all of the other radiators. Next, I took all of these parts outside and did the final assembly of the antenna on the concrete sidewalk. I soldered the antenna together with my torch. Now as anyone who has soldered with a torch knows, when you get the torch adjusted correctly and the solder is flowing well, two things will happen: first, the wind will die and the torch will instantly overheat the joint and the solder will run all over the place, or Second, the torch will be blown out. Frustrating! Despite this, the antenna was built on a windy day which is a normal day in Corpus Christi, Texas USA.
Photo 1 is a picture of the radiator end of phasing line and shows the plastic brace that will be slid up to the connection to the radiators after the line is soldered into place. This is to give more mechanical strength to the antenna. Before soldering the line, it is necessary to slide the plastic all the way out to the joint of the phasing line to prevent melting. After the soldered joints are cool, the brace can be slid to the proper spot and glued in place with hot glue or some form of RTV sliastic. Painting the antenna will prevent corrosion and if a color gray or light blue is used, it will become invisible. Green will accomplish the same thing in an area with lots of trees. I mention this only if there is a need to disguise the antenna.
Photo 2 is the completed antenna, less the feed point connection.
After the photos were taken, the antenna was disassembled and taken home for testing. I plan to report about the test results in the next month issue for August 2000. My thanks to Old George for the recipe! I hope Old George will share some more of his many archived files with us! 30-
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Last modified: December 31, 2010