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End Fed 1/2 wave antenna

Discussion in 'CB Antennas' started by HomerBB, May 5, 2012.

  1. HomerBB

    HomerBB Well-Known Member

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    I've been tinkering around with an inductor, and a variable capacitor, and my dipole rejoined end-to-end for a 1/2 wave monopole fed from one end.

    I tried this coil/capacitor parallel coupling first and could get no where near where I wanted for a match, etc:
    [​IMG]

    Then I made this up:

    [​IMG]

    and paired it up with the capacitor:

    [​IMG]

    I was able to get very close to what I was looking for with a 1.1 SWR R60 X0 @ 27.567



    27.400 was a 1.3 SWR and I neither wrote down nor photographed the MFJ readings so will have to do it again.

    [​IMG]
    #1
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  2. Marconi

    Marconi Supporting Member

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    Damn Homer, your really gettin' good at this antenna stuff. I want to see you get that odd looking diversity type antenna working right. I'm impressed again, but unlike it was with our First Lady, this is not the first time.

    Of course ignorance is bliss so they tell me.
    #2
  3. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Good job you are definitely learning how antenna theory can be applied.

    Nice pictures, the tower and yagi in the background look fantastic also.
    #3
  4. HomerBB

    HomerBB Well-Known Member

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    I'm not sure how much time I will spend on this one as I am still flying the .64 with my thoughts running to it. Also, Marconi's AP/NTO thread has renewed my interest in building an all metal AP.
    This one is really just because I've wanted to feed a 1/2^ monopole antenna.
    #4
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  5. Marconi

    Marconi Supporting Member

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    You go right on big guy, I got your back.
    #5
  6. Robb

    Robb Yup

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    Q: Since this 1/2 wl has no ground planes; are you experiencing TVI? A Big Stick antenna is nothing more than a tuned length radiator and a cap and coil. We all know that is a noisy antenna - much like an A99 or an Imax w/o ground planes - after all . . .
    #6
  7. Marconi

    Marconi Supporting Member

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    You can't be wrong about the A99/Imax being troublesome, too many have complained. But IMO, if the A99 is constructed as it should be I don't think it is particularly bad acting. I found that inside of one I had that was really bad, neither the coax or the coil was pre-tinned and there was a poor solder job to boot. Shield wires were sticking out in all directions, some were soldered and some were not.

    It was good enough for the thing to work, but inside the base it must have be broadcasting RF in all directions, and that is what caused the TVI. The proof is when I fixed the troubled area and hung the wire in a tree and worked it, the TVI virtually disappeared. Good construction always makes a better antenna installation. You can even test the idea at you location without much trouble.

    BTW Homer, I just saw a video the other day where a guy was using a variable cap to tune some antenna, and he recommend using a plastic diddle tool to connect to his tuning knob on the cap...so his hand or adjusting tool did not react with the work of the cap. Just for consideration.
    #7
  8. sp5it

    sp5it Master of puppets

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    1/2 does not need GP because it is 1/2 wl :)
    TVI are coming from transmitter not because of used antenna.
    Mike
    #8
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Actually Mike the 1/2 wave works without a GP because the feed impedance is so high that ground losses are insignificant. The antenna is actually voltage fed. Even a 1/2 wave antenna will benefit from the addition of a decent GP which at the very least will decouple RF from the feedline.
    #9
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  10. HomerBB

    HomerBB Well-Known Member

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    Lots of replies since I last had time to check.
    First, no, Robb, I haven't. I've only had it on the analyzer.

    Second, I can neither affirm nor deny the noisyness of this one yet. No matter how closely I place my ear to the analyzer it sounds the same . . . ;)

    Third, I also believe the 1/2^ will benefit from a set of GPs. I just wanted to put a no-GP monopole up because I has only done that in the sleeved dipole configuration before and that not an actual end fed system.
    #10
  11. Marconi

    Marconi Supporting Member

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    CK, here are some simple models of an 18' EF 1/2 wave dipole with 8.3' foot radials and without. This is intended to show that radials do help decouple the antenna from the mast/feed line.

    For you guys that might not believe what CK is saying, these models do show the radials decoupling RF from the feed line, but what is really the benefit in the cases of these models?

    You can see the current flow in red on the antenna view, and you'll also notice the gain and maximum angle of gain, but which likely has the advantage?

    View attachment .50 wave radial affect.pdf
    #11
  12. HomerBB

    HomerBB Well-Known Member

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    Looks like slightly more gain, slightly less TOA, much less nulls between the lones.
    #12
  13. Marconi

    Marconi Supporting Member

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    Pretty good report. Here is an overlay to show how close the models report the difference.

    Again, I ask when folks say there is a difference, I say how much difference. The model with the radials does look a bit better.

    View attachment Homer's overlay.pdf
    #13
  14. HomerBB

    HomerBB Well-Known Member

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    It seemed the cap had too much capacitance, so I made a simple one to try.

    2 plastic discs and aluminum tape
    [​IMG]

    tape on discs
    [​IMG]

    testing capacitance
    [​IMG]

    low and high ends
    [​IMG] [​IMG]

    Installed in antenna network
    [​IMG]

    Results:
    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG]
    #14
    4 people like this.
  15. HomerBB

    HomerBB Well-Known Member

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    Fooling with the antennas gives me a reason to justify brewing up things, then putting them to some usefulness. An example of which would be the inductor I put together from some strange parts:

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    No place to use it, yet, but I may run across a use on an antenna of some sort . . .
    #15
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