How does a J-pole work

Discussion in 'General Technical Questions and Answers' started by NC4CW, Nov 30, 2008.

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  1. NC4CW

    NC4CW Subscriber

    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?
    Thanks:confused:
     
  2. G3TXQ

    G3TXQ Ham Member

    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.

    There's a detailed description of how the J-pole works on Cebik's website:
    http://www.cebik.com/content/a10/vhf/jp1.html

    73.
    Steve
     
  3. KX0Z

    KX0Z Ham Member

    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.
     
  4. K0CMH

    K0CMH Ham Member

    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: Nov 30, 2008
  5. W5DWH

    W5DWH Ham Member

    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.
     
  6. K0CMH

    K0CMH Ham Member

    DWH:

    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.
     
  7. KA4DPO

    KA4DPO Ham Member


    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.
     
  8. NA0AA

    NA0AA Ham Member

    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.
     
  9. W8JI

    W8JI Ham Member

    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.

    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:

    http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

    73 Tom
     
  10. WA9SVD

    WA9SVD Ham Member

    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: Nov 30, 2008
  11. M3KXZ

    M3KXZ XML Subscriber

    I don't think it matters that the matching section radiates. But I do think it should have a 1:1 current balun at the feedpoint to choke off the common mode currents on the feedline.
     
    Last edited: Nov 30, 2008
  12. N3MQM

    N3MQM Ham Member

    With threads like this the "uneducated", like me, move up a step.

    Thanks
     
    Last edited: Dec 1, 2008
  13. KA9VQF

    KA9VQF Ham Member

    Yes, the ‘U’ part of your J-pole is really a transformer at the frequency it is used at. I know that’s a little hard to grasp when you can see it is a direct short at DC but radio waves are just that waves not direct current.

    I’ve built any number of J-poles and they have all worked to a degree. Some better than others even though to look at them side by side you can detect no difference. Its all in the placement of the finished antenna.

    Personally I like simple ¼th wave ground plains because they are easily made and adjusted.
     
  14. K7FE

    K7FE QRZ Lifetime Member #1

    When you use a choke on the feedline you limit one of the sources of "balance" that a typical J uses. It would probably perform worse. Most J designs count on the outer portion of the shield to act as a counterpoise to the end fed half wave.

    73,
    Terry, K7FE
     
  15. AE6ZM

    AE6ZM Ham Member

    J-Pole

    Whille I do not have any lab grade analysis, I do have some anecdotal observations on my J-pole performance. My antenna is the so-called "Super J" with a second half wave added with a matching stub, base at about 20 feet above ground level. Using my FT-60 set to 5 watts, I am full quieting into a 2M repeater located at about 4000 feet 80 miles away. So, while this may be a "poor design", mine seems to "work fine". It was easy to build, easy to mount, and easy to feed. In my book, that makes it pretty near perfect.

    Wes
    AE6ZM & VE7ELE
    ARRL Technical Specialist
    Lincoln, CA
    Placer County ARES
     
  16. WB2UAQ

    WB2UAQ Ham Member

    A useful characteristic of a J ant and other stub matched antennas: The stub matching arrangement can be quite freq selective and reduce intermodulation problems.

    In my case I live near a weather service transmitter (164.4 MHz) and a number of paging transmitters in the 152 - 153 MHz range. By making the spacing closer and closer beween the conductors in matching section, I was able to reduce the antenna's bandwidth considerably. Unfortunatly the adjustments became very critical BUT I was able to reduce the intermod considerably. I don't have any weather channel images at all and very little paging service issues.
    73, Pete
     
  17. W8JI

    W8JI Ham Member

    It only matters because the unwanted common mode cancels low angle radiation, makes the antenna sensitive to how it is mounted and what it is mounted on, and makes SWR vary with the environment around the feeder and what the antenna is mounted on.

    This is why we generally don't see commercial systems using J-poles, even though they are very popular for Hams. In ham radio the ease of construction and low cost outweighs the electrical disadvantages.

    73 Tom
     
  18. M3KXZ

    M3KXZ XML Subscriber

    Maybe I'm misunderstanding something, but I can't agree that the j-pole needs to use the outer shield of the coax as a counterpoise. Think about it, would you expect an off-centre fed dipole to require a counterpoise? No, I guess, as it has antenna element both sides of the feedpoint. But it would need a balun as it's an unbalanced antenna and would result in common mode currents on either a coax feeder or on a twin line feeder. Now, when I look at a j-pole, I see an OCF dipole that has been bent into a "j" shape, and has a shorted transmission line matching stub. Being nothing more than a bent OCF dipole, it doesn't need a separate counterpoise.

    I've attached a picture to illustrate! What have I missed?

    Pete
     
  19. AI6DX

    AI6DX Ham Member

    I agree with your statements above.... I built a nice 222mHZ copper J-pole. Tuning it was very simple by moving the attachment parts and watching the antenna analyzer bring me to a 1.2:1 match at 223.5 mHZ.

    They are really simple, dependable and durable antennas.
     
  20. WB2WIK

    WB2WIK Premium Subscriber

    ::Here's a good statement:

    It only matters because the unwanted common mode cancels low angle radiation, makes the antenna sensitive to how it is mounted and what it is mounted on, and makes SWR vary with the environment around the feeder and what the antenna is mounted on.

    This is why we generally don't see commercial systems using J-poles, even though they are very popular for Hams. In ham radio the ease of construction and low cost outweighs the electrical disadvantages.

    73 Tom

    ::I agree with this. The j-pole radiates, as does anything conductive. It's a lousy antenna design, and the only ones using it are amateurs. You'll never see a "commercial j-pole" because it's a terrible antenna design. A 1/4-wave ground plane with 4 radials, made from an SO-239 and five pieces of 19" copper wire (about $2 to build) is, in general, a better antenna.

    Of course those of us who think so have only tried everything there is over 40 years or more.

    Feel free to keep trying stuff.:p

    But when you come up with a new idea that works better, try to make sure it's actually a new idea. The j-pole's been around a long time and nobody has ever proven it to work better than a 1/4-wave ground plane -- ever.

    WB2WIK/6
     
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