6 Meter J-Pole Antenna
An Amateur Radio Project
Having recently expanded my amateur radio collection to include 6-meters, I found myself needing something more than the wire dipole antenna I originally installed. Most of the big boys around my QTH were regularly working a repeater which is located roughly 70 miles away. Sadly, my 5 watt QRP FM unit into the wire dipole was not cutting the mustard.
My original plan was to purchase an after market base vertical. After seeing the prices and comparing gain figures, it became obvious that I could do just as well myself for around $20 in material! I had already successfully constructed a 2-meter J-Pole, so why not apply the same concept to a 6-meter version? Being one who enjoys a challenge, I set out to try.
Theory of Operation
The J-Pole antenna is an omnidirectional, half-wave antenna fed via a quarter-wave matching stub. The result is essentially an end-fed dipole which exhibits more gain and a lower take off angle than the traditional half-wave dipole. The original idea comes from the Zepp antenna which was developed for use on the German (Led) Zeppelin airships. The quarter-wave matching stub acts as a transformer and transforms the high impedance of the antenna to match the 50 ohm impedance of standard coaxial cable. This makes the antenna not only economical to build, but also fairly easy to interface with standard coaxial transmission lines. Along with these advantages, the silly looking thing actually talks pretty good too! The math and my 2-meter field testing show between 3 and 5 dB gain over the simple half-wave dipole (depending on frequency SWR). Not bad for a homebrew job, eh?
Unlike many other large radio-related undertakings, the materials for this project can all be purchased through your local hardware or plumbing supply store. In fact, aside from the coax, this antenna is nothing but a big plumbing project.
Item Description Qty.
10-foot, 3/4" Copper Pipe 2
4-foot, 1/2" Copper Pipe 1
3/4" Copper Tee 1
3/4" 90 degree Elbow 1
3/4" to 1/2" Coupler, Copper 1
1/2" Copper Cap 1
3/4" Copper Cap 1
3/4" Hose Clamp 2
2" Hose Clamp (Mounting) 3
A Few Words About Constructing This Antenna
This project is a study in accurate use of measures and copper pipe soldering techniques. If you have never soldered copper pipe before, you might want to consider measuring, cutting, and dry fitting the antenna before taking the parts to a local plumber for final assembly. That said, soldering copper pipe is not difficult if a few rules are followed. Prior to actually assembling the antenna, it is a good idea to take a few scrap pieces of copper and a coupler or two and practice your soldering technique. Once you have gained the confidence of actually doing this, your project will go much smoother. Here are a few pointers:
Make Sure the Pipe Joints are Clean
The ends of the pipe to be joined must be clean and free of oxidation. Plumbers sandpaper is the best way to accomplish this. Thoroughly clean each pipe end using sandpaper just prior to soldering. Also, make sure to clean the inside of each coupling joint. The pipe surfaces should look rough and shiny like a new penny. Once a surface has been cleaned, avoid any contact with your hands. The oil on your skin will prohibit good solder adherence.
Use Plumbers Solder Flux
Before fitting the two pipe portions together, apply a thin coating of plumbers flux to both the pipe and the inside of the joint coupler. The flux heats up and provides a path for the solder to flow into the joint.
A Quick Soldering Tutorial
Here is my method for soldering copper pipe joints.
This method will work for this antenna project as well as any other plumbing
projects you might have around the house.
Construction Notes for the J-Pole
There's not much in the way of step-by-step instructions for this antenna. A picture is worth a thousand words. Using the simplified mechanical drawing below, you will be able to measure and construct your J-pole for the desired frequency. I found a good SWR (2:1) across 2 MHz on either side of the design frequency. Of course, if there is one frequency you use
most often, you should measure and cut the J-pole for that specific frequency in order to get a 1:1 SWR. I obtained a flat 1:1 match at 52.525 MHz using the dimensions outlined in the drawing. This puts my match at no more than 2:1 across the entire 6-meter phone band.
In order to save weight and lower wind resistance, 1/2" copper pipe was used for the top 45-inches of the antenna. This also gives it a much nicer look. The length of 1/2" copper is arbitrary which is why it is not noted on the drawing. I actually used 45-1/4 inches because it made the measurement easier. The rough measurement of 45-inches was chosen more for maximum weight savings as well as making the antenna look more balanced to the eye. I have a thing for aesthetics I guess.
Also note that all exposed pipe ends require a cap to keep the elements out. Be sure to figure the caps into your measurements. I strongly urge dry fitting the entire antenna prior to soldering so that all measurements can be checked and verified.
You will notice from the drawing that a 1:1 coaxial decoupling balun has been specified. Since 50 Ohm coax is unbalanced and being fed into a balanced antenna, this decoupling loop is necessary to keep unwanted RF from being absorbed from the radiator and straying down the shield of the transmission line. We form the decoupling loop by turning RG-8 coax 5 times in a 5-inch diameter. I used a roll of masking tape as my guide and it yielded a perfect 5-inch diameter loop. Secure the loop to itself with tie wraps. Once the coax has been attached to the antenna and the SWR adjusted, the loop should be further secured to the antenna with tie wraps as shown in the illustration.
Feeding the antenna is done by soldering the coax ends to pipe clamps. This makes SWR adjustment easier than soldering the coax directly to the copper pipe. You simply slide the pipe clamps up and down in tandem until you obtain the best match. I was able to obtain a 1:1 match at 52.525. Additionally, future adjustments and coax changes are much easier this way.
Have fun with this. It's a great antenna that can be made in a day for a fraction of the cost of an aftermarket antenna. If you decide to build one, please post your experience/results. We would love to hear your thoughts.
73 de W8CWE