Somewhere around the second or third wind of being a ham I learned about the illustrious J-Pole antenna. It has been around for a while but has been touted as having somewhere around 6 dB of gain. While this seems like an impressive amount of gain for such a simple antenna, it seems that it's really closer to 2-3 dB of gain. Still not bad for an antenna built from the plumbing section.
It's been about a decade since I first made my J-Pole antenna. If memory serves me correctly though, I followed the calculator on hamuniverse to build my J-Pole. A slightly more capable calculator can be found at MØUKD's website. While you could use a specific frequency on the calculator, say for your local repeater, using the center frequency (146.0 MHz) of the 2M band is also a good choice. Doing so should provide you with adequate VSWR across the entire band.
Parts & Tools
The parts list for a J-Pole is quite simple, the majority of which can be found in the plumbing section of your local hardware store.
- 1x 10' section of 1/2" copper pipe
- 1x 1/2" copper elbow
- 1x 1/2" copper tee
- 2x 1/2" copper cap
- 2x hose clamps or ring terminals and screws
The list of tools for construction is just as short as the parts you'll need.
- Copper pipe cutter
- Propane torch
- Solder & flux
Preparing the Parts
Start off by cutting your pipe into the various pieces you'll need. Cut the pipe to the lengths given by the calculator. Your 3/4 wave element should be about 57 inches (147.9 cm). The 1/4 wave element will be just under 20 inches (49.3 cm) but cut it a little bit long for tuning. Finally use a piece about 2 inches (4.5 cm) long to separate the 3/4 and 1/4 wave elements. This measurement is not critical.
Putting it all Together
Now that you've got all your copper pipe cut, solder one end of your 3/4 wave element to the tee. Apply flux to the pipe, put the tee on and heat the joint with the torch. Once it's hot apply the solder and it will wick itself in. Repeat this process with the elbow and the 1/4 wave element. Finally join the 3/4 wave and 1/4 wave elements with your short stub. To give your J-Pole a little elevation, you can use whatever pipe you have left and solder it to the bottom of the tee.
Strip a few inches off of the feed line and separate the center conductor the shielding. Be sure to un-braid the shielding to keep as many strands from falling off. The center conductor will be connected to the 3/4 wave element and the shielding will connect to the 1/4 wave element. There are two ways that you can connect the feed line to the antenna. One involves using hose clamps, the other is to use crimp on ring terminals. The feed line will need to be attached about 2 inches (4.9 cm) above the spacer.
Tuning
Tuning a J-Pole is fairly simple if you have a wattmeter and even easier if you have an antenna analyzer. Your VSWR can be adjusted by moving the center conductor and shield up and down the antenna. Just keep in mind that a very small adjustment will make a big difference. If you left your 1/4 wave element on the long side, you can also cut it down bit by bit to help get your VSWR closer to 1:1.
While you're tuning the antenna, I would suggest securing the center conductor and shield to the antenna with copious amounts of electrical tape while you go into the shack to check your VSWR. Once you have your VSWR as low as it'll go, place a mark on the antenna and permanently secure the feed line there. I ended up using ring terminals because I couldn't get a reliable connection with the hose clamps. To secure the ring terminals, I first used a nail to dimple the copper before drilling a hole for the screw.
The Choke
Having an RF choke is an important part of building an antenna. Without one, the shield of your coax can become part of the antenna. That is something that nobody wants. It can cause all sorts of issues that can be solved just by the choke. There are two options when it comes to making a choke. You can use some ferrite beads around the coax, or you can wrap the coax up and make a choke that way. I've seen various examples of ways to wrap the coax and make a choke. The directions I've used most recently said to use a 41mm piece of pipe and wrap the coax around it about 6 turns. To give me another excuse to use the 3D printer, I designed and printed a 41mm tube. After an hour or so for it to finish printing, I just wrapped the coax around the tube about 5 or 6 times. Finally I secured each end with a zip tie and wrapped everything with electrical tape. Wrapping the choke with "F4" tape or similar waterproofing tape would be a better option. Not that waterproofing is needed, but electrical tape gets kinda gummy and gross after time in heat.
The End
If you did everything right, which it would be hard not to, then you should now have a very capable antenna.