How does a J-pole work
I'm going to build a 2 meter j-pole out of 1/2 copper tubing today. Can someone explain to me in very simple terms why the coax is not a direct short through the copper tubing? It appears to me that the coax conductor and shield are common through a short piece of copper. How does this contraption radiate?
The abbreviated answer to your question is that the path through the tubing is not a direct short because it is a significant fraction of a wavelength long at 2m. An extreme example of this is a quarter wavelength transmission line - it can have a short-circuit at one end and yet present a very high impedance at the other end.
Originally Posted by wb4dlr
There's a detailed description of how the J-pole works on Cebik's website:
Some people say its works like an overly complicated end fed half wave antenna with 1/4 wave impedance matching section, that works equally as well as a simple 1/4 wave ground plane.
You may have trouble seeing what appears to be a direct short because of the difference between Alternating Current and Direct Current. If you already know the difference and why the J-pole would not have a short, read no further.
If you apply DC current to the coax, you will indeed have a short circuit.
When applying AC, at the frequencies we use, the path that appears to be a short actually has a very large amount of impedance to the AC.
Let us look at one cycle of the AC (in our case, the polarity of the voltage is changing once every 1/146,000,000 second).
As the voltage starts to rise from zero, it forces electrons into the parts of the antenna. As the voltage continues to increase, the electrons are accelerated.
Now (very important), accelerating electrons build both a magnetic field and a electric filed around the antena parts. These fields move off the parts in waves. This takes work to build these fields and waves. Just like pulling a weight with a rope, there is resistance to getting it moving, and so do the electrons offer resistance to getting moving. We call this resistance "impedance".
Looking at the shape of a J-pole, one can see that the waves will overlap eachother. The waves will interact differently in different places around the J-pole. In some places they will add and double their strength. This make is very easy for the electorns to move at that point on the antenna part. In other places they will subtract and cancel eachother out. And there can be all kinds of other combinations; adding only 1/2 strength, loosing only 1/10 strength, etc. At these points, the waves are pushing back on the electrons in the antenna parts. All this can also add and subtract to the "impedance" the AC current is seeing as it moves along the J-pole parts.
All the different impedances on different places on the J-pole add up to a total impedance seen at the point where the signal (the signal is our AC current changing polarity at 146,000,000 times per second) enters the antenna.
Therefore, the end of the coax sees a total impedance value, rather than the simple DC short.
In a J-pole, the feed point is at the place where it appears there is a short. But remember, the AC is building fields, going to zeor, then building fields in the opposite direction, ect. The attachment point of the coax sees all the resistnace (remember, we call it "impedance") to accelerating and decelerating those electrons, and making and collapsing those fields.
And another point of AC current. Just like it takes work to build those fields, when they collaps as the voltage returns to zero, the work it took to build those fields gets dumped back into the antenna parts. This is another source of impedance to the flow of the AC current.
The amount of impedance in the antenna is very dependent on the frequency. If we apply 60 Hz current (which is our household current), the J-pole would act like a short. The time that 60 Hz (changing voltage 60 times per second) has to go before it returns to zero voltage (that is, the time it takes to start at zero, go to +117 and return to zero) is so long that a whole lot of current can pass through the short before a lot of impedance can build up. There is enouh time that you would blow a fuse if you connected the J-pole to your wall outlet -- but it offers a lot of resistance (impedance) to VHF radio frequencies (146,000,000 Hz).
Another way to look at it is at household current, the antenna offers 60 bursts of impedance per second, while at your radio's frequency, the antenna offers 140,000,000 bursts of impedance per second.
Just to tie up some possible losse ends; when the switch is first turned on in a DC circuit, there is a rise in voltage from zero to the value of the source (let us say a battery). During that time, the battery sees the same "impedance" that an AC current would see. But (here is the BIG difference), once the DC current comes up to the max voltage of the battery, the electrons are no longer being accelerated. They are just running along at a steady speed. There is no more "impedance" in the circuit for DC current. Now the battery can send everything it has through that circuit. Ditto for the moment the switch is turned off. The circuit presents "impedance" due to the electrons slowing down to rest. It is the exact same impedance except in the opposite value, and it only lasts for that instant after the switch is turned off.
So, a short for DC current (and slow AC current) can actually be a resistance to AC current.
Man kind has observed and measured enough of the various ways AC current and radiating elements of antennas interact, they can predict things about it. This is what antenna designing is about.
Summary: AC current acts very differently from DC current, because of the rising and falling of voltage, and acceleration/deceleration of electrons.
The next step: Your radio is designed and built to work properly with a 50 ohm impedance. We can design an antenna to develope that 50 ohms impedance, but as you can imagine, it is 50 ohms only at a specific frequency. If we change the frequency, then the impedance offered by the antenna will change. This is why our antennas have to have specific shapes and lengths of its parts. A shape and length at one frequency may not work good at another (i.e., it would not provide 50 ohms impedance at the frequency we want).
And to tie up another loose end: unfortunately we use the same name for units of DC resistance as we do for AC impedance. The world would be better if a different name had been picked for impedance units. I believe this fact has caused unmeasureable confusion for those starting out in electronics.
Last edited by K0CMH; 11-30-2008 at 01:54 PM.
I built one out of copper several months ago. I was not happy with the results. The SWR varied greatly with height and surrounding objects. I could never get the SWR to a range that suited me (1.5:1 or less across the band). Every length of coax I used changed the SWR. I tried a choke and it didn't help.
I then built a ground plane antenna and it has a SWR of less than 1.2:1 across the band. I will stick with it.
I can offer that something may be wrong with the j-pole you built.
I have built any number of these j-poles, for both 2 meters and 70 cm, and they all work find. Friends of mine have also built many and they also work fine.
Possibly there is something wrong in the lengths of the parts of your j-pole. You may try downloading a design from the internet and double check all the measurements of your j-pole.
I know that I once had to take one of mine apart because of the same problem you mentioned, and found that I had purchased elbows with different lengths of the flared section, so that when I soldered them together to make the "U" part of the antenna, the two upright parts were to close together. A trip to a different hardware store found elbows that were like the ones I had used on my first j-pole, and reassembly resulted in another perfect j-pole at 146 MHz.
And also, yes, J-poles are very sensitive to metal objects that are close and at or above the bottom of the U. They tend to ignor metal objects below the bottom of the U.
Also, are you aware that a j-pole often has to be "tuned" by moving the attachment points on each of the two upright parts. Also, when tuning, one must move their body below the bottom of the U, or better, many feet away from the antenna, so that they do not interfer with the SWR measurements. Again, I found this out the hard way. The first design I down loaded said nothing about this. I found that me being within a foot of the antenna really threw off the swr reading. When I finally found a different design which mentioned this, I found that my SWR was just fine. I had to step down four steps on the ladder so that I was well below the bottom of the U.
Also, the j-pole is a "50 ohm antenna". If you accidently use 75 ohm TV coax, the SWR will be poor.
Hope this helps.
Originally Posted by KX0Z
I have to agree on that. A J-pole has a little gain over the ground plane but not much, about 1 db which is not significant. The quarter wave ground plane is easy to build and is much easier to match to coax than a J-pole. In fact, the ground plane is almost fool proof.
I'm sorry you don't have the experience or understanding to realize that others possess a skill set that you seem to dismiss as fantastical.
Mine work well - I built two of them, breakdowns for portable use - one slip joints one threaded joints. Expensive buggers with the price of copper, but it's a physically robust design in copper pipe.
But while it works OK, it's not a miracle antenna - some claim 3 db over a 1/4 wave vertical. I don't know myself. But mine work just fine for what they are.
The short is 1/4 wave from the open ends, and as such reflects an open circuit away from the short. It has to do with standing waves. You should read about them.
Originally Posted by wb4dlr
The J-pole is a pretty poor antenna design. The "antenna" is an end-fed 1/2 wave. It is UNbalanced.
The UNbalanced antenna connectes to a BALanced stub. Bad news for common mode currents.
The BALanced stub is fed by an UNbalanced line, the coax!!! Again that's bad news, because again we have an balabced to UNbalanced junction that is improperly treated.
So the truth is the antenna radiates, the matching stub radiates, and the feedline radiates with a J-pole. Anything conductive the J-pole is mounted on will also radiate. Sometimes you get lucky and the unwanted radiation isn't an issue, but most times it is an issue. Sometimes people don't notice it, but it always happens.
You can see how the J-pole evolved here:
One issue is that J-Pole users will say their antenna "works fine." But Compared to WHAT??? Have they compared it even to a standard ground plane?
The truth is, a J-Pole CAN be an acceptable antenna; it's easy to construct (if not always easy to adjust) and in some cases may be more easily installed mechanically thay other antennas, including those that require a proper ground plane. (I.e., the "real estate" for a J-Pole is almost nil, whereas a traditional 1/4 wavelength ground plane antenna will require radials.)
But the J-Pole is not a "miracle" antenna. If "works fine" means accessing the local repeaters, and nothing more, then that's all that's needed. And in most cases, a simple 1/4 Wave vertical or ground plane antenna might function as well.
Too many people think their antenna (ANY antenna) works well (or even spectacularly) because they have no reference for comparison.
If a J-Pole suits your needs, by all means, use it. But consider other alternatives, and don't claim "great" results without a meaningful comparison.
(JUst FYI, I DO use a couple of J-Poles of the "Copper Cactus" Variety; for 2 M and 1.25 M bands; but that's because I only operate FM on those bands to a very few, very local, repeaters for club net check-ins.)
Last edited by WA9SVD; 11-30-2008 at 07:22 PM.