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Modified Vector 4000

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

  1. nav2010

    nav2010 Active Member

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    ...And the reason I know the choke is lowering the impedance rather than increasing it is because when it rained the vswr went up from 1.3:1 to 1.7:1 @ 50mhz. But it then tuned at 1.3:1 @ 47mhz which is an increase in electrical length. The upper radiator cannot change because its aluminum so the electrical lengthening had to have come from the lower leg. If the impedance went up then the antenna would have been resonant at an higher frequency not a lower one. For it to be resonant when wet then the new electrical length has to terminate somewhere in the choke itself probably two inches or so into the first winding of the choke after the rain dropped the choke impedance probably from one mega ohm to maybe 0.75 mega ohms. That's just a rough guess. Why is it doing it? probably because air has a different dielectric property than water.


     

  2. HomerBB

    HomerBB Sr. Member

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    @nav2010
    Can you post a photo of your antenna.
    I've never had an issue with an 11m version changing due to rain whether I used a choke or not. The only 6m verticdipole I've used were mounted from the side in a fan type setup with a 11m alongside it. Worked fine, too.
     
  3. nav2010

    nav2010 Active Member

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    I would but its raining really bad, the swr has gone to hell again so the tape didn't work. My 11m dipole worked fine even in rain but had two aluminum radiators, this has a coaxial lower element and its shockingly poor in the rain.
     
  4. HomerBB

    HomerBB Sr. Member

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    I see.
    All mine were tubing lower elements, too.
    Some had tubing upper elements, others had fishing poles with wire for upper.
     
    nav2010 likes this.
  5. bob85

    bob85 Supporting Member

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    Eddie
    That's about the same gain as your 27mhz model which it should be imho,

    I would be happy with it working like the 27mhz version, I want to change the radials & calibrate the analyzer before I get back to tuning,

    it looks like the upper 1/2wave is a little over 1/2wave electrically, I expected it to be a little long @178"
    what tube diameters are you using ?

    My monopole is using the lower 5 sections of vector 4000,
    I can't do anything about the radials until I make smaller spreaders,

    I must be getting old, bad weather never stopped me in my youth,
    not sure if I lost enthusiasm for antennas or just got soft sitting too close to the fire.
     
    AudioShockwav likes this.
  6. Marconi

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

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    Bob I didn't have all your dimensions, so I scaled my NV4K model at 27.205 to 50 MHz. That changed the radial bracket dimensions at the bottom too, and in the model above it is set at 1.36" inches. That too will change my tuning of the model.

    The model also has a simulated gamma also, and with a physical gamma added to the model...it may have shown a bit more gain.

    The FS model shows the Average Gain results, and that shows us the gain for the model above is understated...needing an error correction of 0.02 dbi. This error needs to be added to the reported gain to be correct.

    I posted 2 images of the antenna, so you could see the dimensions I used for the radiator and radials, noted at the bottom left of the images.

    The radials are a bit short too, but again that is due to a little geometry error in scaling the model to 50 MHz.

    When I have a model set in inches or feet and I scale...the scaling changes the dimensions. I can change all the dimensions to wire gauge values and those values will remain the same when using the scaling feature. I didn't bother figuring all that out.

    So, I just guessed at some dimensions I was missing.

    If I had the gamma dimension I noted in my post above, I would have done the model using a physical gamma match.
    Bob, you're not old yet. Wait until you get to 83 and have to watch all the details when making an Eznec model. To bad you didn't follow me on this journey back in the days when I started modeling.

    With your understanding, you'd have be good at it.
     
    #666 Marconi, Dec 13, 2020
    Last edited: Dec 13, 2020
  7. bob85

    bob85 Supporting Member

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    I don't have any of the final measurements until its tuned Eddie, the ones i gave Homer were a starting point,
     
  8. bob85

    bob85 Supporting Member

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    I got a bit more done on the 6mtr vector,
    drilled a spare vector hub to take sirio 827 radials, i will find out if 4x 1/4wave radials help or hurt the vector,
    just need to change the old rusty bolts & modify some spreaders so i can move them & adjust the cone length,

    it will be isolated from the mast on one of my fiberglass isolators.

     
  9. HomerBB

    HomerBB Sr. Member

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    Take a look at this model by Marconi.
     

    Attached Files:

  10. bob85

    bob85 Supporting Member

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    Homer we have conflicting models of what effect radials have on the vector,

    Eddies model shows radials cause a drop in gain & radiation wasted at high angles,
    Donald claims radials spoil the cake too,

    Henry's model shows radials give an increase in signal strength that becomes less of an advantage the higher the antenna is above ground,
    w8ji also says this type of antenna needs radials,

    Best way to find out is put 1/4wave radials on a vector & make them quick/easy to add & remove,

    My buddy 6 miles away has chopped a sirio tornado for 50mhz so i will have somebody for signal checks,

    rx will be the RSP1a, its the only calibrated s-meter i have.
     
  11. HomerBB

    HomerBB Sr. Member

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    Anxious for your results.
     
  12. Marconi

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

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    Homer, I have a better model that has a physical gamma match. The model you posted as an example above uses a simulated gamma which is OK, but I don't think it is as technically correct as my models with a real gamma match included in the model.

    Bob, maybe you could give me the exact dimensions for your new 6 meter antenna, and I'll scale my NV4K with a gamma match and I can check how close Eznec scale feature is to your 6m dimensions.

    Then I can fix the model dimensions to exactly what your 6 meter uses and we can address these ideas about different radial length head on.

    If I missed the point of your test Bob or my idea here is screwballed, let me know.
     
    HomerBB likes this.
  13. bob85

    bob85 Supporting Member

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    When its tuned Eddie,

    i have to mod some spreaders, i can only go longer on the radials with the 27mhz spreaders when i want to shorten them,

    i need to swap the gamma tube for a longer one to get a bit more capacitance.
     
  14. HomerBB

    HomerBB Sr. Member

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    A quick and easy spreaders solution:

    20200427_190316-01-01.jpeg
    Cable ties.
     
  15. Marconi

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

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    Take your time, but don't wait too long, I think I've got Covid again...with somewhat milder symptoms, probably due to antibodies.
     

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