J-Pole Problem

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

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  1. KM5FL

    KM5FL Ham Member QRZ Page

    Johnny,
    As others have stated, a J-Pole has less gain that a well designed 1/4 wave groundplane. But if you have your heart set on building a J-Pole that is pretty durable and easy to build, here's a link to Arrow Antennas website.

    http://www.arrowantennas.com/osj/j-pole.html

    Actually it's a dual band antenna for 2m/440. If you build it according to the instructions, no tuning is necessary.. All the parts are avaliable from Radio Shack and your local hardware store..


    KM5FL
     
  2. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    I understand the RF ground issue Mike, the mast can be grounded and the base of the matching unit can be grounded connected directly to the mast like the copper cactus J-pole. I guess what I was saying is that a half wave antenna doesn't need radials per se since the currrent is maximum at the center of the antenna.

    Anyway one thing I forgot to mention to Johnny is that 300 ohm window line is easier to match to the stub. He can then connect a 4:1 Balun and coax where he want's to enter the shack. I did that and it worked pretty well, it also keeps common mode off of the coax. Only thing is he will have to build a 4:1 Balun but it is easy to do using coaxial cable. I also agree that as long as your having fun that's all that counts. I built four of them because I wanted to experiment. That's how I know there are better antenna designs.
     
  3. KB0YYO

    KB0YYO Ham Member QRZ Page

    I built a J Pole several years ago.
    When I was a tech. I made it out of copper tubing.
    I have it mounted on a 20 foot pipe mast . I get out well over 20 miles on low power.
    The swr is fine, the signals are good and it works.
    I did have a problem at first on the bench. the problem with the swr was not the antenna but me being too close to it. I put longer coax on and moved away and it improved. I then moved it off the out side work bench as the garage is metal sided and it improved immensely. Very good swr. I have not had a problem so I leave it alone.
    It does the job I built it for.
     
  4. W0BTU

    W0BTU Ham Member QRZ Page

    I don't understand what you're trying to explain. (But that's OK. :) A grounded (or not) mast at VHF would have to be pretty darn short to have the same RF potential at whatever point on the mast we measured it.

    Yes, we agree that the current maximum is at the center. But in the real world, the current isn't zero at the ends. I stand by what I said above about the need for RF ground/radial(s)/counterpoise/whatever-you-want-to-call-it for any end-fed antenna.

    This discussion has been repeated so many times over the years on QRZ and eHam that there ought to be a sticky thread or article about J-poles. Anyway, I don't think anything would be gained by prolonging this discussion. Thanks for your viewpoints.
     
  5. W9XMT

    W9XMT Ham Member QRZ Page

    To support Mike's point, note that AM broadcast stations using base-fed, 1/2-wave towers drive them against a set of 120 x 1/4-wave (or longer) buried radials.

    They certainly would save that cost if 1/2-wave radiators did not require them for best radiation efficiency from the antenna system.

    WLS in Chicago (50 kW on 890 kHz) uses 240 x 1/2-wave buried radials with a tower a bit taller than 1/2-wave.
     
  6. W4XKE

    W4XKE Ham Member QRZ Page

    Okay, a question on the Ground Plane antenna. All those I see described are 1/4 WL. That translates to a radiating element that's only 20" tall. (Seems like it'd work better with more length.) What about a 5/8 WL Ground Plane instead?

    Is there some reason we don't see this configuration? I'd tend to believe that a 50" radiating element would produce a stronger signal - unless 5/8 would somehow spoil the theory of the design?

    (PS - keeping in mind this is intended to use without a mast - just hanging.)

    http://www.hamuniverse.com/2metergp.html
     
    Last edited: Apr 24, 2012
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Longer isn't necessarily better. Two small problems arise with a 5/8-wavelength ground plane (they can both be overcome, but they are obstacles to deal with):

    -It isn't a 50 Ohm feedpoint antenna anymore, like a 1/4-wave is; it has capacitive reactance that must be tuned out, and normally that's done using a series inductor having the same but opposite reactance. That inductor is critical and also adds electrical length to the antenna, so the physical radiator must be shortened a bit. That's why 5/8-wave 2m whips aren't really 50" long, they're always shorter. 50" would be too long to make this work, and resonate in the 2m ham band.

    -In order to get any effective gain from it, the radials need to be longer, thus making the antenna not only taller but also wider.

    After conquering those two small obstacles, you end up with an antenna that has only very marginally more gain than an ordinary 1/4-wave ground plane and is more complicated to build and keep in the air.

    I'd go with the 1/4-wave ground plane.
     
  8. W0BTU

    W0BTU Ham Member QRZ Page

    I think Terry, K7FE covers the 5/8λ vs 1/4λ question (in addition to the no-radials-needed myth) in his excellent Antenna Myths document at http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf. In short, there's not much difference.

    As he mentions there, a collinear or an array of stacked vertical dipoles are a much better choice than either the 5/8λ or 1/4λ.
     
  9. KO6WB

    KO6WB Premium Subscriber QRZ Page

    I built a J-Pole from 300 ohm twin lead. It was for my hikes in the wilderness areas of Arizona. I knew my HT with a rubber duckie just wasn't going to cut it. The J-Pole worked very well, was small and light weight. It made the rubber duckie look like a dummy load (it is you know). Would a J-Pole make a good home station antenna? I don't think so. There are so many other choices that work so much better.
    Most amateurs rate their antenna performance to the other antenna they are using. So if that antenna is a rubber duckie then yes the J-Pole is the cats meow. You would do well to experiment and see the difference. At 2 meters it's fairly cheap.
    73
    Gary
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    KO6WB, that's right.

    I've been on 2m since 1966 and have tried hundreds of antennas over 46 years. Wrote up a lot of that, with data and photographs in my CQ "VHF" column in the 1980s (you can download that stuff).

    Everything works great when you make contacts, but without standards of comparison, there's no way to know how great or possibly terrible it really is. Install ten 2m antennas spaced several wavelengths apart on the same property and switch between them rapidly during contacts, and that provides some insight.

    I've done that. The j-pole is not a good performer compared to many simpler antennas. I haven't a clue why hams latched onto it, other than perhaps to use copper tubing and their Bernz-O-Matics to make something.:eek:
     
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