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Modified Vector 4000

Discussion in 'CB Antennas' started by bob85, Oct 3, 2009.

  1. davev8

    davev8 Gold Star/Marvin Award Member

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    HO......that's not very helpful...especially when a big percentage of your own previous posts are asking for help ...so i think unless you have something useful to add to this thread i suggest you don't
    ...... i am thinking with the very basic mistakes you have made with gain etc that maybe you did not build it. ....the end


     
    #466 davev8, Jan 4, 2020
    Last edited: Jan 4, 2020
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  2. davev8

    davev8 Gold Star/Marvin Award Member

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    A very big thanks for that. hoop and radials are easy to remember at 1/4 wave...is the radiator a little shorter than early on in the tread ?? this works out for 27.5mhz to be less than 28 feet ....i will still use what you say as you are the one with decades of field testing ...is this with the antenna isolated from the mast ? or don't it make any difference...i see modeling 5/8 waves the TOA go up when you model a mast as well
     
  3. Shockwave

    Shockwave Sr. Member

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    Different manufacturers have experimented with the length of this design, building models anywhere from 27 to about 30 feet in length on 11 meters. They have obviously noticed the same characteristics I see in field testing, that the length of this radiator is has a bigger impact on focusing the pattern on the distant horizon than other antennas with a single radiation point.

    With both the cone and the top section radiating, the length of the whip and cone also controls the phase delay to the top section. Altering that phase delay between the two sections, allows you to introduce "electronic beam tilt". On 11 meters, our antennas are relatively close to the ground in terms of wavelength and these variables in height also determine what length provides the most effective beam tilt and target area for your location.

    On VHF, most antennas are usually mounted many wavelengths above ground and that reduces the effect that changes in height have on the take off angle. That also allows one length of antenna to fit more applications than we might see on 11 meters where antenna height is typically much closer to the wavelength in use and therefore, has a larger impact on TOA.

    I've honestly never tested the design in the field without a mast or with an isolated mast. What I can tell you is when the antenna is not properly tuned, it will show CMC on the coax and mast. Changes in the length of either one can show changes in the amount of reflected power at that point. Once the antenna is tuned to 50 ohms, none of this is noticeable anymore.
     
    #468 Shockwave, Jan 4, 2020
    Last edited: Jan 4, 2020
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  4. Shockwave

    Shockwave Sr. Member

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    Another interesting clue as to how the cone functions with RF currents can be noticed in the CST computer model I posted years back. Pay particular attention to the shape of the current node around the 1/4 wave cone. The 1/4 wave radiator always shows maximum current at its base feedpoint with the current steadily reducing over that length.

    That is not at all what the computer model is showing in the 1/4 wave cone section of this antenna. Here we have a 1/4 wave radiator that manages to show maximum current right in its center at though it was a 1/2 wave! I can only replicate this current node shape in modeling when the 1/4 wave element is excited with RF from both ends... I have long suspected that this second current is folding over the top of the cone and radiating on its outside surface, in a constructive phase.
     
  5. HomerBB

    HomerBB Sr. Member

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    Shockwave,
    At the risk of sounding thick headed, I ask, "Is this antenna you refer to above the VHF that you install with the 1/4 wave cone and Mhz/9212 scale radiator? "
    Just clarifying...
    Homer
     
  6. Jay Mojave

    Jay Mojave Well-Known Member

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    Hello All: Quit a few postings here, good deal.

    Going thru extremes to achieve a accurate electromagnetic measurement is not overdoing it, if gets you a accurate repeatable measurement. Then it can be compared to a analytical computer generated electromagnetic measurement.

    Yes I have a few piles of antenna parts and such, Masts, Booms, Elements, and such in storage in junk piles 1 thru 4. Attached is a picture of a old bottom 4 element hoop Sigma 4 type antenna that I replaced with better and stronger stuff. It doesn't have to be new and shinny but represent the real thing in size and electrically. Yeah its seen better days but I will clean it up and test it.

    Jay in the Great Mojave Desert
     

    Attached Files:

  7. Shockwave

    Shockwave Sr. Member

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    Yes.
     
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  8. Marconi

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

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    Donald I agree with you in the part, where I made bold type above, and especially with the word "always." However, I'm not sure I can agree that what we see here really is as you suggest...that the CST model is showing us maximum current gain right in the center of the 2 x 1/4 wave radials shown as though the cone radials were 1/2 wave radiating elements. Donald there has never been enough CST information on this subject, so I don't know for sure, but I disagree.

    I use to think this list of values were (A/m) amperage per meter values like it shows in the image. But, then I got to thinking that 2.37 A/m is a lot of amperage, and instead my Eznec models show the red lines as the values for gain. Understanding, that the farther away the red line is from the radiator the more gain we have and visa-versa.

    Eznec calculates amperage per segment for each wire and this information is found in the Tabular Currents Table Report. It is noted in the last image of the PDF file below.


    [​IMG]


    I have a vague recollection of you talking about this, but I never understood the point or the test with your two sources on a single wire.

    Posted below is a model of the V4K with no loop on top of the radials.

    Donald, do you have any idea why this CST model has no radial loop on the model?


    I had to add 9" inches to each 106.99" inch radial to set the antenna back to resonance. When I did that, it raised the TOA 1* degree with the same 2.37 dbi gain as my Free Space speck model with a radial loop showing maximum gain to the horizon.

    Examples for these modeling ideas are in the PDF file below.
    1. My model shows the currents on the radials, in red, and is similar to what you describe in your comments at the top.

    2. I noted the amperage for the tip of radial #3 at segment #46 showing the least amount of current at 0.01415 (A). I also noted at the base of the antenna for wire #3 segment #1 with the most amount of current at 0.34455 (A) and that is as a 1/4 wavelength should be,

    3. I posted the base of the V4K to show the wire #3 at the base of the antenna.

    4, Shows the pattern with maximum TOA of 2.37 dbi @ 6* degrees above the horizon.

    5. Is an image of wire #3 in the Tabular Currents Report for each of the 46 segments in that radial wire and it shows a current trend like a typical 1/4 wave element.
     

    Attached Files:

    #473 Marconi, Feb 20, 2020 at 12:23 PM
    Last edited: Feb 20, 2020 at 12:33 PM
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  9. bob85

    bob85 Supporting Member

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    Has anybody figured out how to model the cone with an unballanced load ?

    terminating a transmission-line with resistors of any value won't cause radiation, unballance is what causes cmc to flow and radiation to occur,

    try it with 2 parallel conductors terminated with any value resistor it does not need to match the line impedance, = no radiation

    then flare the conductors, = a tiny bit of radiation increasing the wider the flare


    now figure out how to unballance the line by only loading one conductor,
    = more radiation due to cmc cased by unballance.
     
  10. Marconi

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

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    I haven't figured out yet exactly what you really mean above, but Shockwave says he does not use any CMC mitigation features on his installations, if he can get the feed point match at or near perfect.

    I recently modeled his idea to check out what Eznec might predict. I also tryed his 9212 / frequency conversion factor that he mentioned to HomerBB...somewhere and a while back.

    The 3 overlays that I compared below were 2 models of the 0.75w version. One with and one without a choke. I did this because the model without the choke showed a small amount of common mode currents on the mast. So that suggested to me, if I had included a coaxial feed line in the model...the pattern would likely suffer and RFI could present some problems.

    The 3rd model was my idea of Donald's antenna using a tweaked version of his 9212 factor. It worked out to be 6" inches longer than my 0.75 wavelength version without a choke. His model showed me no CMC just like he told Homer.

    Maybe one day I will try making these models using a physical gamma match on the model. That way I can get the tap point much closer than using the simulated gamma idea I used here. Bob, I can use the specs you sent with your image and dimensions for the New Vector 4K long ago including the capacitor value you posted. That said, I have no idea what Donald's gamma dimensions are with his longer radiator and I won't be asking him anything about the specs for his production antenna.

    Below are these 3 overlays, and I noted the radiator lengths for each model including Donald's calculated length for his 0.82 wave antenna.

    Bob, get me a EZBob image of the idea you talked about above, OK? Pictures work better for me.
     

    Attached Files:

  11. bob85

    bob85 Supporting Member

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    Eddie,
    antenna match won't cause cmc or eliminate it, cmc is a ballance issue,

    i have no paint on this pc to draw Eddie, it would make explaining what i mean easier no doubt,


    what i mean above is you want to model the lower 1/4wave on its own without the vertical element above the cone to see if it really does radiate,

    if you put resistors across a transmission-line it won't radiate regardless of how bad the match is unless the conductors are NOT perfectly parallel,

    you can do a simple test yourself in the real world to prove it with coax or ladder line and resistors,

    so why does the j-pole leg radiate? because its unballanced,

    one side sees the end impedance of the upper 1/2wave while the other sees a higher impedance air,

    it will not radiate if you cut the upper 1/2wave off & terminating the top end with a resistor won't fix it,
    to make it radiate you need to make the conductors not perfectly parallel so the fields are not fully cancelled or unballance the load, or unballance the source placement,

    the same applys to the vector,
    you have some small radiation due to none parallel conductors,
    but thats not what we want to see,

    if you want to see the radiation caused by cmc on the outside of the cone rather than radiation due to none parallel conductors you must unballance the model by terminating the center conductor & leaving the hoop terminated with air,

    if you cannot do that i don't think you will ever see radiation due to cmc when modeling the lower 1/4wave with eznec.
     

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