J-Pole Problem

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4XKE, Apr 21, 2012.

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  1. W4OP

    W4OP Ham Member QRZ Page

    I'd be interested to know why there is a theoretical point that would obviate the need for a balun- thanks,

    Dale W4OP
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    When you build an antenna like that, the velocity factor of the PVC pipe will play a big factor. The lengths will be off by a considerable measure, perhaps 20% or more.

    That may account for the original SWR of 3 to 1. Your antenna may have been resonant down around 120 MHz or so.

    If you have an antenna analyzer, build the antenna as described, then check to see where it actually tunes up, will be lower than the 2M band. Calculate the percentage difference, then trim the stub and radiator by that percent. Recheck and trim again, if needed. A J-Pole is a simple antenna to make, but anytime you change any parameter, the dimensions will change also.

    Joe
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I like camping. But I bring an HF rig and prefabricated wire dipoles, because there are many great camping spots where there's absolutely nothing to work on 2m.

    Ironically, at the Grand Canyon south rim (elevation 7000 feet above sea level), there's pretty much nothing to work on 2m. I tried, with 100W and a very good antenna. It sucks, there's just nothing around there. But with 5W on 40m CW to an inverted vee at 20 feet, I made over 100 contacts just fine.

    2m FM would be almost the last thing I'd turn to for camping, unless I was camping on a mountaintop overlooking a city.
     
  4. W0BTU

    W0BTU Ham Member QRZ Page

    You're a wise man. :)

    Like the Double Bazooka dipole, the J-pole is simply a terrible antenna design that just refuses to die like it ought to. There are FAR better antennas than the J-pole out there, that don't radiate from the coax shield.

    The ARRL should do everyone a huge favor and do a big QST cover article talking everyone out of the J-pole once and for all. Better yet, you should have to swear that you will never promote the J-pole antenna as a pre-condition of joining the ARRL. :rolleyes:

    Try a ground plane, or perhaps a 3-element beam. Properly built, either antenna is superior and there's little or no RF on the outside of the shield.
     
  5. W4OP

    W4OP Ham Member QRZ Page

    I couldn't agree more Mike. The double bazooka, like the dipole is quite easy to make, and users assume that a good match means it is performing well.
    As long as amateurs keep wanting J poles and double bazookas, manufacturers will continue offering them.


    Dale W4OP
     
  6. W0BTU

    W0BTU Ham Member QRZ Page

    You're right, Dale! :)
     
  7. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    Having built several J-pole antennas I can say without hesitation that there are better designs. The J-pole is simply and end fed half wave antenna fed by a linear quarter wave matching section at one end. It is by definition a balanced antenna and always needs a Balun. It is also a votage fed antenna and so is effected by everything around it including you when you try to match it.

    A quarter wave ground plane is much easier to build and match plus it is oblivious to stuff a wavelength away from it. J-poles are a fun experiment but they are not at all practical.
     
  8. W0BTU

    W0BTU Ham Member QRZ Page

    Interesting comment here: http://forums.qrz.com/showthread.php?168073-Does-a-J-Pole-Need-a-Balun&p=1275333#post1275333

    "The J-pole is one antenna that probably should not have a balun for the following reasons.

    A J-pole needs radials, a counterpoise or ground plane to work properly. Most J-poles use the outside of the coax shield for that purpose. If you deprive it of the coax shield to obtain balance then it will use the mast or tower. A balun or ferrite will block or limit the current that is allowed to flow on the outside of the coax shield.

    One previous poster suggested that the choke balun (ferrite) be placed about 19 inches below the J-pole. That would allow a 1/4 wave counterpoise for the antenna and would be acceptable.

    I cover this and other antenna material here:
    http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf

    73,
    Terry, K7FE"

    I fully agree with Terry. And he's by no means the only person to have figured that out.
     
  9. W4OP

    W4OP Ham Member QRZ Page

    Wouldn't that bottom 1/4 wave be out of phase though, thus altering the main lobe off the horizon?

    Dale W4OP
     
  10. W0BTU

    W0BTU Ham Member QRZ Page

    I don't know. When I said I 'fully agreed', I wasn't thinking about that part. :)

    Just the part about the balun.
     
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