Archive for August, 2007
A j-pole is a very inexpensive homebrew antenna that is quite easy to assemble from several pieces of half-inch copper pipe. There are countless articles on the Internet on how to assemble a j-pole, but this article will show you my first experience in assembling a home-built j-pole. Essentially, assembling a j-pole is more akin to a basic plumbing project than it is a radio project.
Please be aware of the hazards of assembling your own antenna. Tiny pieces of copper will be prevailant (during the cutting), the pipe will get extremely hot (while soldering the pieces together), and never touch an antenna while transmitting. Work in a well ventilated area so that you don’t inhale fumes from soldering.
First, it’s necessary to buy several lengths of half-inch copper pipe from your local hardware store. Depending on how good the store is, they may pre-cut the pipe into the lengths you need, or they may not. For a j-pole to work on the two meter band (144mhz to 148mhz in Canada,) you will need at least 230cm of copper pipe. Also obtain 2 end caps, 1 ninety degree ‘elbow’, and 1 ‘tee’ connector. These are all common plumbing parts.
Cut the copper into pieces following these exact measurements: 147.20cm for the radiating element, 48.84cm for the matching section (or ‘stub’), and 4.59cm for the center piece which will separate the radiating element and the stub. These measurements are based on a center operating frequency of 146mhz. If you need measurements for an alternate frequency, this site has a very useful j-pole measurement calculator.
I purchased a hacksaw to cut the copper into correct lenths. However, I learned that you can get a convenient copper pipe cutting tool that does the job in a fraction of the time (and costs a fraction of what a hacksaw costs.) The leftover pipe will be used to put at the bottom of the antenna, as a support pipe.
Next, sand the ends of the copper pipe with plumber’s sandpaper, which will help make the solder bind to the copper better. Fit the ends of the pipe together with end-caps on top of the radiating element and the stub. Fit the end of the radiating element into the top of the tee, the center section into the side of the tee, and the support pipe into the bottom. Take this time to measure the pipe from end to end again, as the measurement will affect the operating frequency.
Using a butane torch, heat up the tee connector (do not apply heat directly to the copper pipe itself — only to the connector) until it is hot enough to apply regular electrical solder. The solder should melt right into the joint upon contact, binding to the copper pipe and connector. I’ve been told not to use plumber’s solder, that regular electrical solder is best for this job. Do the same for soldering the matching section to the angle joint, and then again for the angle joint to the center piece. Allow the pieces to cool between soldering — copper retains heat for a long period of time.
After that, it’s a matter of attaching a connector to the matching section by way of soldering it. I used three different methods and connectors. Any method works well, but the easiest is to solder the wire directly on to the antenna. A more professional way to do it would be to solder a PL259 female connector to the matching section. The point of first contact for the wire or connector must be exactly 4.8cm up from the top edge of the piece of center pipe.
Soldering on connectors and wires was by far the most difficult part of assembly. Try to have a SWR meter handy to measure the RF power coming back down the transmission line. After some work, all four of my j-poles get 1.5:1 SWR or less from 144mhz through 148mhz.