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how effective is a j pole antenna on 11 meters ?

Discussion in 'CB Antennas' started by BOOTY MONSTER, Aug 20, 2010.

  1. how effective is a j pole antenna compared to a 5/8wgp like a maco 5/8 ? how effective is it compared to a 5/8 vertical like a imax 2000 ? since it only has one ground element does the radiation pattern have a lobe or null to its sides ? going by the online calculators it would be a little under 26 feet tall on the cb band , but it would have a much smaller footprint due to not using/needing horizontal or sloped ground elements . it would even be a smaller footprint than a sigma 4 style , but of course bigger than a imax or the new sirio .

    whats your thoughts , experiences or comments on the j pole antenna ?

    J POLE ANTENNA DESIGN CALCULATOR by K4ABT



    [​IMG]
     
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  2. Carl

    Carl Technologicaly Challenged

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    it would not work as well, there really only a 1/4 wave vertical dipole with a fancy stub to lower the impedance. if you want to build one go ahead, you would be better off building a cheap yagi.
     
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  3. Robb

    Robb Yup

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    Just as a guess; but the "A" dimension might make this impractical if are going to use copper for 11m. Isn't the Vector 4000 re-issue basically a J-pole? That design supports the upper vertical better than a 'simple' J-pole. What do you think - BootyMon? If using aluminum; then the velocity factor is different and the lengths must also change...
     
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  4. Marconi

    Marconi Supporting Member

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    BM, based on the idea presented, IMO the main problem with a J-pole for 11 meters is using 25' feet of vertical 1/2" copper. Seems to me there will be a severe lack of lateral support even using rigged tubing. The idea may work well for 2 to 6 meters and higher however.

    Carl, can you show us a link where you get the idea a typical J-pole is only a 1/4 wave radiator?
     
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  5. i remember freecell being very adamant that the sigma 4 (and its copies) were not j poles and i think theres several comments in the sigma 4 threads saying the same thing . if i tried it i would use aluminium and copper (i think) for the materials . your velocity factor comment reminds me that i forgot to ask how to tune a j pole . is it tuned by element length or shield/hot location on the elements ..... or both ?
     
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  6. Marconi

    Marconi Supporting Member

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    I've never built one BM, but Bob has. I would guess the best swr would be found by moving the coax pigtail up and down on the two lower 1/4 wave elements and if it was not resonant where you wish, then trim or add length to the 1/4 wave and re-tune. If that didn't work then try the same with the longer element. I have seen them designed with tunable tips that allowed for some adjustment without having to mess with cutting or adding length.

    You might want to read up on what K6MHE published some years ago in an article titled "The J-Pole Revisited". It's on the Internet and he considered a balun and insulating the J-Pole from the mast in his work.

    Have you put your antenna up yet?
     
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  7. W5LZ

    W5LZ Crotchety Old Bastard

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    A 'J'-pole should work about like any other 1/2 wave antenna (that's what it is, a 1/2 wave). There are practical aspects that determine what sort of metal should be used for an antenna. Strength is one of the things that determines that. Doesn't make a huge difference if it's copper, aluminum, steel, cast iron, or gold.
    got a 50 foot pole? Put that impedance matching 'stub' the right distance down from the top of that pole and feed it as with any 'J'-pole, and you got an antenna almost 25 feet high. Ought'a work just fine. With a little adjusting of the various lengths/distances, you can do the same thing with a tower, or flag pole, or water tower, or grain silo.
    'J'-poles on HF are sort of large and impractical, but, they would work as well on HF as they do on VHF/UHF.
    - 'Doc
     
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  8. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    A J-pole is not a 1/4 wave. It is a 1/2 wave fed thru a 1/4 wave stub at one end. The overall length of a J-pole is 3/4 wavelengths high. As for not needing a groundplane versus not benefiting from a groundplane, well that is a different matter. The J-pole would benefit from a groundplane just like any other vertical antenna but it does not require one to obtain a good match. The adjustable 1/4 wave stub takes care of that.
     
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  9. HomerBB

    HomerBB Well-Known Member

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    I asked this on the QS site and got some replies, so here goes:

    I understand that electrical lengths are not the same as physical lengths, however, with the supplied formula, this radiator works out to 25.93' (25' 11.2") in addition to a 20" apparently conducting piece beneath where the stub adjoins the main radial. The question that comes to mind is how an antenna of this length could possibly be classed in performance with an 18' half wave antenna? I'm having a hard time with that one.
     
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  10. packrat

    packrat Active Member

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    You have to adjust both wires at the same time and to the same position. Also, j-poles CAN be a bitch to tune. A copper 11m j-pole will be very heavy and hard to support.
     
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  11. Marconi

    Marconi Supporting Member

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    BM, if you have anything definitive from the Vector threads that supports the words that you claim freecell stated, then please point them out. My intention is NOT to be disrespectful to the guys that worked hard to make models, but the best we learned was that a few minimal efforts in modeling did not provide the results we hoped for, so we are back to words and opinions again.

    Conclusions were made regarding the bottom and top of the antenna being in phase---turned out to still be in question unless we accept assumptions as facts. Except for Dxer's modeling, Henry and Shockwave's models used thin wires with no taper for the elements and I think everyone agrees that doesn't make for good modeling conventions.

    Freecell presented us no proof, just words, and the rest of us are just speculating based on our personal experiences. We haven't gained an ounce of better understanding for what we hoped would be accomplished after almost a year of discussion.

    The one thing that I personally believe seems to be irrefutable, the Sigma4 works surprisingly well even though it has a 3/4 wavelength radiator which CB BS tells us---should not work well under any circumstance.
     
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  12. W5LZ

    W5LZ Crotchety Old Bastard

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    HomerBB,
    That 25.93 foot length is three 1/4 wave's long, you're right. But, one of those three 1/4 waves is part of the impedance matching section with that 1/4 wave 'stub' off to the side. That means that the radiating element, is 2/4 waves, or 1/2 wave long. So, it's perfectly normal to compare one 1/2 wave antenna with another 1/2 wave antenna.
    So why do 'they' do it that way? One reason is that it's a fairly simple way of doing the impedance matching. It's a lot more practical than some of the other means of doing that matching.
    If the support pole for a 'J'-pole antenna is metal, it's also a pretty good way of doing a safety ground, the tallest antenna element is directly connected to ground. That's certainly not enough for a really good safety ground, but it does help.
    - 'Doc
     
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  13. HomerBB

    HomerBB Well-Known Member

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    Thanks, Doc.

    Very good and simple answer. My question is because I was wondering about the antenna in relation to that end fed closed sleeve antenna I put up, about how it might have some theoretical relationship to how my sleeve works. I am still thinking . . .

    Homer
     
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  14. W5LZ

    W5LZ Crotchety Old Bastard

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    If you're asking how it would perform in relation to your 'sleeve' antenna, I honestly don't know. If they are both 1/2 wave antennas, I think they would be at least comparable in a very broad way.
    The biggest difference would be in relation to how the 'matching' is done, and how 'resonant' each antenna is.
    Would it be worth changing from one antenna to the other? All things considered, I can only think of one way of finding out...
    - 'Doc
     
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  15. HomerBB

    HomerBB Well-Known Member

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    I suppose you're right, Doc. I didn't expect a direct answer from this thread, as it belongs to BM re the j-pole. I was interested because I read somewhere that the open sleeved monopole was considered to be similar to a j-pole.
    Thanks again.
     
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