By David - K3DAV (6/19/2012)
The J-Pole antenna is a simple antenna to build if you have a little background in soldering copper pipes together. All you need is some 1/2 inch ridged copper pipe, a 1/2 inch copper "T" connector, a 1/2 inch copper right angle elbow connector, and two 1/2 inch copper caps. The tools needed are a saw capable of cutting copper or a copper pipe cutter, a blow-torch, some soldering flux, sandpaper, and some good pipe solder. The rest is your own knowledge of soldering copper pipe together.
First let's get into what a J-Pole is and how they work. The actual design of a J-Pole is as a 1/2 wave antenna. But the poor design flaws make it perform like a 1/4 wave antenna.The length of the main radiator is equal to a 1/2 wave antenna. But the "J" section is measured out as a 1/4 wave ground radial, running up over 1/3 the length of the main element just a few inches away. This causes the main element to radiate from only the top half, just above where the "J" section is no longer next to it. The "J" section being only inches from the main element, acts like a shield that prevents the RF from the main element from radiating in that area.
The BLUE area in Figure 1 at left, represents the radiating RF pattern. Notice it is radiating from only the top half of the main element, above the "J" section. You will also notice that the pattern on the "J" side is slightly larger than the other side. This is because the "J" section acts like a ground radial and a director at the same time. The ground radial effect, lifts the signal slightly in that direction only. The director effect expands the signal pattern just a little in that direction only. This effect does not happen in any other direction except the side where the "J" section is. This in turn, makes the J-Pole very slightly directional to the side with the "J" section.
You will also notice that the signal pattern is shaped more like circles rather than large ovals as seen with most ground plane antennas. This is due to two things in the J-Pole design that are very different from typical ground planes. The first difference is, the J-Pole has no horitontal ground radials. The only ground radial is vertical like the antenna, and blocks the signal from the lower half on the main radiator. This alone makes the radiated RF pattern a lot smaller more resembling a 1/4 wave antenna. The other difference is that without horizontal ground radials, there is nothing to keep the signal pattern raised up and more towards the horizon where it should be. These 2 things combined keep the pattern close and smaller with a lot of the signal lost up in the air, and down into the ground.
Overall, the J-Pole is a fair antenna that was not designed with distance in mind. Because of the poor design using a vertical ground radial, the J-Pole is a 1/2 wave that works exactly like a 1/4 wave with no ground radials. Many J-Pole designers and builders on the internet try to make you believe their J-Poles have a +dB gain factor, but this is just not true. The J-Pole has no gain factor to speak of. In many cases, it may have a negetive dB loss (Below 0 dB gain). But the J-Pole should be considered a "Unity Gain" antenna over a dipole. This makes the J-Pole a good local antenna for 6 meters, 2 meters, and 70cm bands. It is good enough for getting into repeaters and even local FM Simplex talking. But real distance on these bands with a J-Pole will depend on special atmospheric conditions.
Keeping all of these facts in mind, a J-Pole can be designed for any frequency or band, but because of the poor 1/4 wave performance and no dB gain, it would not be practical for HF bands. Not even 10 meters. A basic store bought CB antenna would work much better on 10 meters than any J-Pole. So if you really want to build a J-Pole, just remember that it is best when used on 6 meters, 2 meters, and 70cm bands, and used for local repeaters and FM simplex. Otherwise, if you are looking to hit repeaters or simplex many miles away or even DX, do not waste your time and efforts. The J-Pole will disappoint you.
Building A J-Pole Antenna
Here are the simple instructions for building a J-Pole antenna for any band. We will include the calculating formulas for cutting the antenna for any band or frequency. But for our example, we are going to show the design figures for a 2 meter J-Pole.
Figure 2 at left, highlights the main radiating element. The simple formula to determine the length of this element is 705 ÷ by the frequency. So to build our 2 meter J-Pole, we first enter 705 in the calculator then the Divides button, then enter 146.000 as the center frequency of 2 meters. Then hit the Equals button. The answer should be 4.8287671. Now multiply that times 12 to get the number in inches. The answer should be 57.945205 inches. This is so close to 58 inches, that we will use 58 inches as the length. So once again the simple formula to determine the length of the main element is....
705 ÷ by frequency in MHz, then =, then X 12. The answer is the total length in inches. Our 2 meter main element is now determined to be 58 inches.
The next measurement to determine is the length of the "J" section as shown in Figure 3 at left. For this element, we use the formula of a 1/4 wave element. And that is 234 ÷ by the same center frequency of 146, then =. The answer should be 1.6027397. Then multiply that times 12 then =. The final figure should read 19.232876. This answer is close enough to the typical 19 inches that is always a 1/4 wave of 2 meters. So we will use 19 inches for the "J" section element. Once again the formula to determine the length of the "J" element is.........
234 ÷ frequency in MHz, then =, then X 12 to get the length in inches. Our 2 meter "J" element is determined to be 19 inches.
Now we need to figure out the spacing between the main and "J" elements. This little piece of pipe is shown in Figure 4 at left. Of course there is another formula to determine the length of this pipe also. Here is how we do it.
On your handy calculator enter 22, then ÷, then the same center frequency of 146, then =. The answer should read 0.1506849. Multiply that time 12, then =. The final answer should be 1.8082188 inches. This is close enough to 2 inches, so this piece of pipe will be cut to 2 inches. Once again the formula to determine the "J" spacer is.......
22 ÷ frequency in MHz, then =. Then X 12 to see the answer in inches.
The final piece of pipe is the section that comes down from the copper "T" connector and mounts the J-Pole to the mast (As shown in photo below). It is always a good practice to make this section about 20 to 24 inches, no matter what band the antenna is made for. always use the same 20 to 24 inches for the mounting section. Not any longer and not any shorter.
Assemble the sections as indicated in the photo at left. Make sure all solder joints are properly prepaired by sanding the ends of each piece of copper pipe, and sanding the inside sleeve of the "T" and elbow connectors to make them clean and have a rough surface. Then insert each pipe and heat it with the blow-torch until the solder melts when it touches the copper. Allow some of the solder to slip down between the pipe and connector to make a good solid solder joint. Do this all the way around the pipe connection so it is all soldered and sealed tightly. When the solder is in place, remove the torch fire from the pipe, and allow the area to cool. It is always best to have the pipes and connections mounted on a surface that allows you to do the soldering, but not disturb the pipes when they are cooling. Do not move the pipe until it cools down enough to make the solder solid and cool to the touch. And don't forget to put the two 1/2 inch copper caps on. One on top of each of the main and "J" elements.
Connecting The Coax Feedline
Now that the antenna is fully constructed and cooled down, inspect all of your solder points to be sure they are good and solid connections. And be sure the "J" element is straight and even with the main element all the way up. You should have a J-Pole antenna that looks like the photo at the top of this article. Now it is time to connect the coax feedline.
Connecting the coax to the J-Pole takes another formula. You must determine how far up the main and "J" elements from the "J" spacer pipe do you mount the coax feed points. We do this by entering 23, then ÷ by the same center frequency of 146, then =. The answer should be 0.1575342 feet. Then multiply times 12 then =. The final answer should be 1.8904104 inches. This is again close enough to 2 inches. Once again the formula to determine how far up the elements from the "J" spacer the coax feed points should be is.....
23 ÷ by frequency in MHz, then =, then X 12. For our 2 meter J-Pole, the coax feed points should be 2 inches up on the main elements from the "J" spacer pipe. As shown in Yellow squares in the photo above, connect the center conductor of the coax to the main element, and the shield conductor of the coax to the "J" element directly across from each other.
Prepairing The Coax First
There are a few ways to connect the coax to a J-Pole. First strip off about 3 inches of the black jacket on the coax. Un-braid the shield wire and then twist the lose wires together to make one shield connection. At the top of the center conductor, remove about 1½ inches of the foam insulation to expose the copper center conductor. It is a good idea to always coat the ends of both wires with a light coating of solder to bind the strands of wires of the shield together as one wire, and help protect both wires from the weather. As an option, you can attach a bananna jack post to each wire as the connector instead of just bare wires.
Another good idea is to tightly wrap the center conductor, and the shield wires separately with a couple wraps of electrical tape. Leave only the soldered tips exposed. This is especially necessary if you use a good coax with a foil shield that wraps around the foam insulator. You do not want that foil to touch the main element. It will be a direct short and the antenna will not work. You also do not want any bare shield wire to touch the "J" element below where the wire is connected to this element. It will change the SWR by a lot, and once again the antenna will not work. All bare wires should be covered well except where connections are made.
Attach the wires to their respective element with a 1 inch hose clamp. Some people like to drill a hole into the elements and tap a sheet metal screw to hold the wires. But this is not a good idea. Adjusting the SWR with this antenna is done by moving the attached wires with the hose clamps, up or down on the elements. It is much easier to just losen the clamps to move the feed point up or down, then drilling new holes each time. It also makes it easier to replace the coax if necessary in the future.
And by the way, when adjusting SWR, both the center conductor and shield connections on their elements must be moved up or down together evenly. If you move one wire by 1/4 inch, then you must move the other wire 1/4 inch in the same direction. Both wires should always be directly parallel to each other at all times. Moving the coax feed points up increases the resonant frequency. Likewise, moving them down lowers the resonant frequency.
At this point, it is a good idea to make a choke coil to prevent stray RF from feeding back down the shield of the coax line into your home. There is a detailed article on how to build a quick choke coil that takes just a few minutes to make using the same coax feedline that will attach to the J-Pole. You can CLICK HERE to read the article.
Mounting the J-Pole can be done by using 3 or 4 heavy duty hose clamps or U-Bolts on the 20 inch copper section to any kind of mast.
If you have any questions or comments about this article, please feel free to email me at firstname.lastname@example.org