Building the J-Pole Antenna
Center Frequency ~146 Mhz
Why is this great antenna so "fussy?" Well, it is a free space radiator, meaning that it wants to couple into literally any nearby object, metal or not! If you use covered wire or bare wire there will be a significant tuning difference; if you use wires of different thicknesses, there will be widely varying resonance points. In short, no matter what you do with this antenna, it will effect its resonance point!
So you might be saying, "What else is new, this is a vhf antenna, right?" And you are correct; all vhf antennas are subject to the list of above "influences." But what makes this one different is that there are three (3) tuning lengths that must be exact in order for this to work at all! These lengths are very "tightly coupled" and if you cut one of them too short, you may never get the thing to tune up properly, and you really can't conveniently add back to the wire lengths once they've been cut!
Commentary : Some antenna enthusiasts consider the J-Pole to be easy to build, while others cannot seem to get it to work no matter what they do. I built one that tuned up fairly quickly, but attempted a second one which took me much longer to get working. If you are new to antenna construction, I would proceed with caution on this project. Even if you are a seasoned veteran, this is no "day at the beach!" However, if you stay with it, you will soon be on the air with your new homebrew J-Pole.
So, why do I like J-Poles? First is the low angle of radiation! It's about as low as you can get from a VHF homebrew antenna! Second is the db gain which is quite respectable, at least a "decibel" over a ground plane in terms of efficiency. Third is the slim profile and ease of mounting on a high pole or tower. A possible forth is the low cost of the parts which can be "scrounged" from any "junk" box. J-Poles really do make great antennas; you just have to be patient with them and be flexible! Flexibility is the key!
Quick Headings Reference List
- Citations and Credits
- Construction Approach
- Materials Required
- Where to Begin
- Finding the Resonance Feedpoint
- Choosing Your Final Feedpoint Connection
- Enclosing the Antenna
- Setting the Antenna up Outside
- Final Observations
- Citations and Credits:
The inspiration for this homebrew project came from the ARRL magazine QST, the July 1995 Edition. The article was entitled "Build a Weatherproof PVC J-Pole Antenna," pages 62-64, written by Dennis Blanchard, K1YPP. This is a very well written and detailed article about how to actually build a J-Pole, one of the most comprehensive popular treatments of the subject I had encountered in my research. If you can obtain a "paper" copy, or read it over the net, I strongly suggest that you do so. It is full of tips and ideas about how to build antennas and particularly the "tempermental" J-Pole!
- Construction Approach:
Rather than present a design of fixed measurements, I am going to offer a strategy for working with the J-Pole in place, so to speak, on the work bench or test bench. Measurements, where specified, are suggested beginning points only! I have seen so many articles on the net bemoaning failed construction attempts that I have become suspect of "build-antenna-quick" schemes. This is one of the most difficult antennas to build! So approach this project with a degree of caution and skepticism, and in the spirit of curious experimentation. It is probably going to take more than one try to get this one right!
- Materials Required:
Here is a summary list of the basic requirements...
- Heavy Gauge Solid Copper Wire
- PVC Pipe, 7 feet or more in length
- Antenna Analyzer, or a VHF radio and SWR meter
You should have on hand a good supply of the type of wire that you are going to use since the antenna is best built from one piece of uniform wire, not piecing odd bits together, although this might possibly work. For example, I have been using insulated #14 house wiring in most of my experimental builds. I bought a 25 foot roll of this wire at an electrical supply house. Since it has three (3) wires inside the "skin," I am getting 75 feet worth of antennas! (A smaller roll would probably be a better choice.) I also have found that the more rigid the wire is the better. Flimsy small wire does not hold its shape very well and therefore does not tune well. (Often J-Poles are built from copper tubing.)
Other materials that you will need are a 7-10 foot length of PVC 1 1/2" diameter pipe with a cap end and perhaps couplers to attach to an existing pole or pipe, hot water pipe foam insulation, and some small blocks of wood or plastic. You will also need a soldering gun, solder, and some electrician's insulation tape.
For test instruments, you will need an antenna analyzer, or a VHF radio and an SWR meter, and a short length (6 feet) of 50 Ohm coaxial cable with appropriate connectors. You may also need an elbow VHF coupler, and a SO-239 coax connector, the solderable type.
- Where to Begin:
You will be...
- Cutting the Wire Stock
- Making the Tuning Stub
- Cutting the Radiator Section
- Preparing for the Tune...
We will be making these cuts for an approximate center frequency of 146.00 Mhz. Cut a length of wire from your stock about 70 inches long, one solid piece. It could be an inch or so longer if you need a little extra margin for error. We'll make the "J" part first. From either end measure off 16 1/8 inches and carefully bend this length back on itself in parallel with the main part of the wire, leaving about an inch as the "thickness" of the "J". The total length should be 16 1/8 inches for the "J". Remember to allow for the "curve" in the "J". This curve is a semi-circle with a diameter of an inch. (You could even square it off if you like as long as the overall length is 16 1/8 inches.) Remember, this length is just a starting point. You could make it up to one (1) inch longer if you like since it can be shortened with a shorting bar, see below for details.
Now make up 3 or 4 little block separators (1/2 to 3/4 inch long) to keep the wires firmly apart by a separating space. This space may range from a 1/2 inch to nearly an inch, depending on how much room you have in the tube when the foam is inside. You want a snug fit, but moveable. A good compromise is about 5/8 of an inch, but you choose what works for you. Then with electrical tape tightly wrap these blocks in place with the wires on the outside of the blocks about every 3 or 4 inches. Now carefully skin the wire, if you used the insulated type, from the bottom of the "J" for about 4 or 5 inches up the antenna, or to the first separating block. (You may leave insulation on the very bottom of the "J" where you may be handling it.) You have just made the 1/4 wave tuning stub!
This photo shows the basic pieces of the antenna: the PVC pipe on the far left, the short piece of black insulation extending out from the pipe, the "J" portion of the antenna, or 1/4 wave tuning stub, with its wood block spacers held in place using electrician's tape, the feed point coax connection, the shorting bar, and the coil of the remaining coax, or choke, partially visible on the far right.
Note: Connect the shield side of the coax to the short part of the antenna, and connect the pin or center conductor to the long length of wire, the radiator.
Note: The connection to the coaxial cable is not soldered at this point, but remains loose so that it can be moved along the skinned portion of the wire. With needle nose pliers, you can make small hooks that almost wrap around the wire "bars" and will thus permit easy movement along it. You don't always have to use solder on the braid, but be sure it is twisted enough to make a good contact.
Note: I used four (4) separating blocks to keep the "J" wires apart. You could get away with three (3) or perhaps even use five (5). Just be sure to leave enough room at the bottom of the antenna to skin the wire for about 4 to 6 inches so that you can find the resonance point.
From the top of the 1/4 wave tuning stub, measure 38 inches to the end of the wire and cut at that point. This may still be a tad too long, but better to be able to trim than to be too short. The correct length is 37 1/2 inches. You have just made the 1/2 wave part of the antenna! (Again, we are looking at approximate lengths which will need to be adjusted when tuning up. If you are brave, you may cut it to the exact length and forget about it.) :)
Note: The free space length of this radiator section is actually 38.5 inches; but remember, this section is surrounded by foam and by the PVC pipe, both of which have a velocity factor tending to make the antenna "look" longer than it really is. If you were to build this without any foam or pipe from the tuning stub up, you would need the longer measurement for the radiator! (So the foam and the PVC pipe tend to slow the wave down.)
Take the PVC pipe, which does not have to be cut yet, and insert the pipe foam insulation at one end. You might have to trim the foam along its length to make it fit into the pipe. (You can buy various diameter sizes and foam thicknesses.) The original article emphasized that the 1/2 wave length part of the antenna should be in the center of the pipe; the foam helps to do this, as well as the placing of small bits of foam wrapped around the wire, held with tape, to make centering standoffs. I place these standoffs about every foot or so. The more you can center the wire, the better! Insert the entire antenna into the pipe except for the last 4 or 5 inches of exposed wire.
- Finding the Resonance Feedpoint:
"Getting down to brass tacks" ...
- Making the Coax Connector
- Testing, Tuning, and Tweaking the Antenna
- Further Tuning Using a Shorting Bar
- Extra Tuning Procedures for the Radiator
You need to make up a connector that will slide back and forth over the bottom part of the "J". This is the feedpoint location for tuning the stub. I took a short piece of coax and cut off the end connector, leaving just bare wires. Wrap these wires around the bottom of the stub by making little "hooks" out of them. You may have to solder the braid side lightly to make it stiff enough to work around the #14 gauge wire. You want a snug contact, but slidable! (You may want to buff the wire with steel wool to get the best electrical contact.) And, most importantly, the braid of the coax connects to the short length and the "pin" connects to the long length, or the radiator.
OK, you are ready to test! Hook up your test device, either an analyzer or a vhf radio set to the lowest power along with its SWR meter, set the radio for 146.00 Mhz. (NOTE: Use a Handi-Talkie radio if you can, and don't use more power than 1 watt!) Be sure you make a choke out of the feed coaxial cable by winding it up into a coil of about 4 or 5 turns, 6 inches in diameter; this is important. Locate the sliding feeder about 1 inch from the bottom of the "J". Make sure you have a snug connection. Don't use clamps since they will become part of the antenna and skew your readings. Pinch with needle nose pliers for a good fit. Push the TX button and observe the SWR meter.
Perhaps with someone helping you, keep the TX going and slide the feedpoint connector up or down while watching the SWR readings. Find the lowest point you can. You want to "dip" through the tune point, noting SWR's of about 1.4 or 1.5 at the high end (148) and the same for the low end (144), thus achieving bandwidth symmetry. At 146Mhz you should be near 1:1. Keep tweaking until you find the best tuning range. When you find resonance, you will know it. It suddenly all comes together! (Remember to take you hands way from the antenna if you think you have a good match and watch for any effects, i.e., changes in resonance.) Mark the location on the wires with a pencil or a scratch mark.
Hints: Use the "information" coming back from your radio and SWR meter, or analyzer, to find out what is going on with the item under test. Is it near resonance or very far away? As you change frequencies, what happens to the SWR? Can this give you a clue as to whether it is too long or too short? (This part can actually be fun!) If your resonance point is too high, then the antenna is too short. If it is too low, then it is too long. If it is too long, you are in luck, you can change it. (If too high, you will need to begin again with a new piece of wire... )
This photo shows the final test condition of the antenna. The foam insulation has been pushed almost all the way into the PVC pipe. The antenna itself has been pushed almost all the way into the PVC pipe as well. Only the last few inches of the tuning stub are left outside so that the coax connector can be moved as required.
Note: The antenna is in an almost fully assembled condition, just as though it were going to be put up outside. This means that it has all the foam insulation in place inside the pipe for the entire length of the antenna.
Note: Although not shown here, the choke or coil of about 4 or 5 turns is located as close to the feedpoint as possible. If you don't do this, you will get a very different reading when you use the choke outside on the pole. You must replicate as much as you can the actual conditions that the antenna will "see" in real life!
If you can't seem to find a good match, take a short piece of copper wire and make a shorting bar by curling the ends and pinching it near the very bottom of the "J" tuning stub. This will shorten the stub. Repeat the above tuning process and move this shorting bar up by fractions (1/8") at a time. This is why it is a good idea to make the initial lengths longer than you think you are going to need. This shorting bar can then vary the tuning stub length. When you have a match solder this bar in place.
This close up photo shows the most critical tuning section of the antenna: the skinned wires at the bottom of the "J", the coax connection, the shorting bar soldered in place, and the wire insulation left on the very bottom of the "J" which may help in isolating your hand in the tuning process.
Note: Connect the shield side of the coax to the short part of the antenna, and connect the pin or center conductor to the long length of wire, the radiator.
Note: The connection for the coaxial cable is not soldered at this point. It has been trimmed back about one inch for both center conductor and shield, and both have been curled over to make sliding connectors.
Note: The shorting bar has been soldered in this photo. You may also leave this unsoldered since it may have to be moved too. Again, I made small "hooks" curving around the stub wire and pinched them fairly tight. These two tuning points, the feedpoint and the shorting bar, will have to be adjusted until you find the resonance point.
Another option at this point is to tweak the 1/4 wave stub by "pinching" the wires together just slightly. Between each separating block, you may gently pull the wires a bit closer together thus changing the impedance of the stub. I only recommend this approach as a way to discover the resonance point and not as a permanent solution. Once you have found the resonant "dip" through the target frequency, go back and try to straighten out the wires and compensate back to resonance by changing the feedpoint or tuning bar. Only as a last resort would I suggest using tape to keep the stub in its "new" configuration.
If you still can't seem to find a good match, shut everything off and pull the antenna out of the pipe. Very carefully cut 1/8 of an inch off from the top and try it again. See if the SWR improves generally. Keep doing this trimming until you have found resonance. Be care, if you trim too much you will have to throw this antenna out and begin fresh with another piece of wire. (Don't cut shorter than the known resonance length for the 1/2 wave section, 37 1/2 inches.)
As you have no doubt surmised, this tune up is a "tricky business." You are essentially adjusting three (3) lengths all at once. First there is the 1/2 wave length or the actual radiator, then the tuning stub, and finally the feedpoint on the stub. But, I have found that by doing all this "in place," that is, treating the antenna as a "live" test, there are not going to be any surprises when you get it up 30 feet and it doesn't work! We have all been down that road before! So I recommend getting it working on the bench and then taking it outside. The SWR may still be different outside too, since it may try to couple into other objects including its own feed line. That is why you must have a choke, a coil of 3 to 5 turns on the coax, very near the feedpoint of the antenna. This will help to decouple it from the antenna and suppress currents on the outside of the coax.
- Choosing Your Final Feedpoint Connection:
You do have some choices here...
- Feeding Your Antenna, Which Way?
- The Bottom Feed Approach
- The Side Mount Feed
- "Playing a 'Tune' on the Trombone"
Assuming all has gone well, you have a resonant antenna on the test bench and you have marked the locations where this proved good. Now, you have to decide how you are going to bring your feedline into the antenna. You have two (2) choices, and maybe you can devise more. You can solder the coax just as it is and make a smaller coiled choke inside the PVC pipe, bringing the coax out of the bottom of the open pipe, as in a chimney mount; or, you can cut a hole in the pipe and come out the side with an elbow connector. (I used the latter on the J-Pole shown above.)
If you are going to come through the side of the pipe, you need to solder an SO-239 connector to the stub. I took short pieces of solid wire, about an inch long, and soldered them to the connector first. Then, curling the other ends of the wires into small hooks, I soldered them to the marked locations on the stub. (They should be as short as you can make them without mis-aligning the stub part of the antenna.) This piece will probably just emerge from the hole drilled into the PVC pipe, due to the curvature of the pipe and the flat mounting bracket of the SO-239 connector. Make the hole just wide enough to pass the connector through it. It should be a snug fit. You can use a washer if you like. The final outside connector will be the elbow. You may have to file the face of the opening you have made, thus flattening it a bit. When assembled properly, the two connectors will clamp themselves together on either side of the pipe, securing the entire side mount assembly in place.
You may also use this same technique to tune the antenna when using the side mount. In fact, I would strongly recommend it. Follow the process described above but don't solder the wires in place yet on the stub. Be sure to make the "hooks" as complete and snug as you can, nearly encircling the wires on the stub. To tune, with the side mounted connectors in place, carefully push or pull the stub from the end to find the resonance point. (You will be moving the entire antenna inside the pipe, so it might be a bit stiff. This process looks like you are playing a trombone.) Again when you have found the points, mark them as best you can. You may be able to solder them in place, but it is more likely that you will have to disassemble the side mount connectors, pull the stub out of the pipe, solder the SO-239 to the stub, then push it back into the pipe, and finaly re-connect the elbow on the outside, clamping it all together again.
- Enclosing the Antenna:
At this point, you may want to cut the PVC pipe. Its overall length is approximately 5 feet, depending on how much extra the tuning stub took up. Make sure you have "centered" the antenna on the inside with any extra lengths of foam which can be taped together. I added an end cap to the top of my PVC pipe and needed a coupler at the bottom to attach to another section of PVC pipe. I also wound a coax coil of about 4 turns, diameter 8-9 inches, to be used as a choke which I taped to the PVC pipe. (My antenna is only up about 25 feet.)
- Setting the Antenna up Outside:
Set up your J-Pole as high as is practical, remember it has a very low angle of radiation making it an excellent long distance antenna! However, as has been noted, it likes to couple into things nearby, including trees! Try to keep it in the clear as much as possible. This may even be more important than the height. And don't forget the choke, 4 to 5 turns on the coax feedline about 8-9 inches in diameter as close as is practical to the feedpoint. I used electrician's tape to bind it to the pole and bind the choke together into one circular shape. (It should be noted that the J-Pole is not different from any other VHF antenna in regard to height and object clearance.)
- Final Observations:
My emphasis in this article has been on how to set up a test bench situation and "tweak" this antenna until you get it working. That is why I kept presenting measurements as approximations and erring on the long side. I have tried soldering back wire I had cut off and it really doesn't work. So, my approach has been to "design" the antenna in place and under test until you see resonance. Then, close it up and consider it done!
There are so many variables that can influence the outcome of this design from the type of foam, to the type of wire, to the type of pipe, to the wire spacing in the stub, etc, that a theoretical discussion becomes nearly meaningless! This is a hands on project!
Don't give up on the J-Pole. They are difficult, but you may never be able to buy an antenna as good as this one, the one you are making!