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Base Marconi's1/2 wave J-Pole an alternative viewpoint

Discussion in 'CB Antennas' started by Marconi, Mar 26, 2018.

  1. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Here are two model of the J-Pole with the Average Gain showing 1 in Free Space.

    These model are over real Earth using Eznec Average soil condition so the model does not represent the situation for every installation. So, you mileage may vary.

    Note: I added am image of the tabular currents for wire #8, the mast. You will notice all the current for each segment in this wire is a very low magnitude suggesting very little CMC flowing.

    I added a choke to the top of the mast so I could connect the mast directly to the antenna in this case. The J-Pole antenna act very badly if Common Mode Currents on the mast or feed line. I these currents are not minimized, the skewing is terrrible and this is the worst case for the skewing that folks tend to attribute to the J-Pole in general. That said for this antenna...this bad situation should never be tolerated. Later I will post an image of the pattern with the choke removed to demonstrate the problem.

    For convenience in my modeling only, I consider the mast as the feed line in this case and if there were even marginal currents flowing on the mast...it could easily be seen as a red line...similar to the currents noted on the radiator in the Antenna image.

    I also added an image of page #40, from Henry HPSD, entitled "The Avanti Sigma4." My highlighted marks show his conclusion for the J-Pole compared to the Sigma4.



    Excuse my typo's and other errors if any.
     

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  2. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    No words to describe the model, except as noted in a previous post where I promised such a model.

    From now on, my models do not represent any antenna anywhere on Earth. That as always been the case, understood or not. You might also find this true of all antenna models present. posted, and published. It also makes a very poor argument, if that's all you have to support your opinion.
     

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    #2 Marconi, Mar 28, 2018
    Last edited: Mar 28, 2018
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  3. HomerBB

    HomerBB Sr. Member

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    Help me understand what happened differently in the last model. It is a classic cloud warmer!
    It just has the choke removed? WOW.
     
  4. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    The new QST Mag has a niffty little 2m Vert made from 2 coat hangers...like 4ft tall...modeled shows as much gain as J-pole...
    Homer right up your alley...think I'll build one for field day...mabe stacked pair :)
    All the Best
    Gary

    Homer: it looks like the old Bow-Tie UHF TV antenna...using the coat hangers as over/under Delta Loop set-up fed directly with coax :)
     
  5. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Yes! But just to be clear the antenna was also directly connected to the mast.

    Can you imagine guys getting a J-Pole and having to deal with the consequences of this?

    Might this possibly help demonstrate why this design has the bad reputation. This was the point of my thread, and why I thought a little added clarification might suggest to a few operators they might re-consider the J-Pole if they could minimize the CMC this one looks to have when the mast is connected directly to the antenna. That is assuming the real antenna responds even close to the mode in this regardl.

    BTW, I asked you if you could detect skewing on your 2-meter J-Pole. I'm just curious.

    Did you see my other J-Pole models with different ways I posted for solving this problem. Do you think folks that tried the J-Pole and had terrible success...were blaming the antenna for acting bad?

    I also did some comparison models using the NV4K with and without isolation (ISO) and with this antenna...connecting the antenna directly to the mast, changed the pattern a little and dropped the gain some...but no where near this J-Pole model.

    Even though these are only models and don't represent a real antenna anywhere, could such a demonstration possibly cause some operators to wonder?

    I wasn't trying to be deceptive and pull the wool over anyone's eyes Homer... even if it looked that way.

    Thanks for asking.
     
    #5 Marconi, Mar 28, 2018
    Last edited: Mar 28, 2018
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  6. HomerBB

    HomerBB Sr. Member

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    Post a picture?
     
  7. HomerBB

    HomerBB Sr. Member

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    @Marconi
    Never imagined you being deceptive, just focused on one aspect and moving by something else. I'm bad about that.
    I haven't had it operational long enough to notice, but I will be adding a choke to it soon!
    So far it seems to work equally well close up in town, and out a few miles.
     
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  8. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Well I tried...Copyrighted material of course...SRI
    Gary
     
  9. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    April 2018 QST magazine pages 37-38 if you can find a copy...tried to load from Digital Edition...can't load HTML pic hr...blocked it
     
  10. HomerBB

    HomerBB Sr. Member

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    Rats...
     
  11. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Well Homer, can you tell a little bit by looking at the model and the title...how the model is constructed?

    Original model showing a fair pattern but is showing workable skewing that may be hard to detect:
    wM = with mast
    wISO = the mast is ISO from the antenna
    L = Load inside of a square box is a simulation of a choke at the top of the mast. This device is set to 1pF of capacitance in this case.

    Worst case model showing a very bad pattern after I removed the choke from the mast:
    wM = with mast
    nISO = no isolation

    I did not code the title to show no load (nL) but I mentioned that I removed the object that simulates a choke.

    I understand your confusion. This was not your fault. I did not explain well enough.

    I know words get to be a problem sometimes, but this shows why words are necessary, and I do forget sometimes to describe something...and it might seem I take you guys for granted.

    If your setup is different from this model, which is not isolated and has no choke...then this model surely does not even come close to your 2-meter J-Pole.

    That aside, however, if I understand this antenna...nothing you can do will fix the skewing...that is the question I asked you, just to be clear...can you detect skewing on your 2-meter J-Pole?

    Could I have said it in 2 dozen words? It may be possible, but not on my best day this late in life.
     
    #11 Marconi, Mar 28, 2018
    Last edited: Mar 29, 2018
  12. HomerBB

    HomerBB Sr. Member

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    My 2 meter jpole is isolated from the mast, but has no choke on the coax.
    About the only way I'd likely be able to notice skewing due to the 1/4^ stub is to rotate the antenna toward and away from a single transmission source.
    Not sure it would be detectable. My Icom has led bar readout, not a needle . . .
     
  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I had a feeling something was different. I could not imagine you using your radio if the match was as bad as my model with the very bad pattern shown, with no mast and no isolation.

    BTW my codes in some titles use a n=no, and the larger code like nM might mean nMast or nMatcher. Steve put me on to this idea. The reason I use this code is that Eznec limits the number of characters, numbers, and punctuations that can be used for a title.
     
    #13 Marconi, Mar 28, 2018
    Last edited: Mar 29, 2018
  14. HomerBB

    HomerBB Sr. Member

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    Ahh.
     
  15. Henry HPSD

    Henry HPSD 19DX348

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    Good to see how critical things can be.
    Perhaps u realise...but just to have it said... isolated doesnt always mean no wanted or unwanted currents...

    Imagine a yagi... with isolated elements..
    That thing works... ...just a couple metal pieces isolated from each other....
    The same is true for other antennas.
    (That influence while isolated)

    It is possible to "isolated" the j pole from the mast while the mast will still have "currents" and skewing.. keep up the good work..
    Kind regards H.
     
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