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Base Have a look at this Antenna

Discussion in 'CB Antennas' started by AudioShockwav, Jan 15, 2020.

  1. HomerBB

    HomerBB Sr. Member

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    There are significant changes between the two. TOA goes from 10 degrees to 15 degrees and the max gain goes down when the radial spacing is reduced from 18” apart to 10” apart. This would seem to lend credit to the claim of a straight narrow radial shape effecting a steeper TOA.


     
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  2. Marconi

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

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    Homer, I think you're specifically referring to the patent column #3 at line 67 and ending at column #4 at line 6. You make a good point and you may be right.

    It is always good to have different points of view in discussions.(y)

    I have no dimensions for this idea.:rolleyes: It looks like this will make a big difference at the hub or at the radial loop in order to make conductors #16,#18 parallel with the mast #14.

    I understand the patent images are not to scale, but they do seem to suggest some differences in construction among Fig. 2,3 vs 5,6. Fig. 5.6 the radials look much more parallel.

    Do I to set conductors #16 and #18 parallel based on the width of the top mounting bracket at level "A" that is about 6" inches wide in Fig. 2,3? This would make the loop at level "B" about 6" inches in diameter.

    Or, do do I set the conductors to the width in Fig. 5,6, at levels A,B that looks almost parallel with the loop at level B. This will make the mounting bracket 30" at level A?

    Homer, I added 2 models below that you may remember. It is your vertical CFHW with a simulated gamma match. I think you built this one at some point. I should have added a feed line, but if you choke it right you should have little CMC on the FL/mast.

    You can compare these to the Astro Plane.

    Thanks for watching close.

    How say you?
     

    Attached Files:

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  3. HomerBB

    HomerBB Sr. Member

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    I did build the center fed dipole with a gamma. I've also built several center fed coaxial dipoles with the coax fed upward through a metal tube.
    All work well, but, as you can see, the have less gain than the Astroplane. They do have a good rf pattern. The model over earth at 32' shows all the rf where it is useful for DX.
    As for the width of the straight radials I don't know. I assumed they meant the width of the upper bracket. . .
     
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  4. Marconi

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

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    This is what I will try.
     
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  5. Marconi

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

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    Homer, here is a Free Space model that uses my idea for the feed point location. It shows the typical maximum gain = <>2.19 dbi for a vertical CFHW. It reports a near perfect Average Gain Results (AG), not the overstated high maximum gain reported above in some of my example models that show unacceptable AG and error corrections results for the gain like Avanti reported in their ads.

    I made the radials parallel at 6" and created a 6" diameter loop at the bottom. This model is what these changes produce as a result. No other changes were made. I was surprised this model was this good with this drastic change in construction.

    IMO, this model will need new dimensions in order to work with a good match.
    It is way too short and is resonant at 30.4 MHz as a result of simply making the radials parallel.

    Homer, can you remind us what is supposed to happen to the TOA when the radials are made parallel?


    The Free Space maximum looks good near what we might expect for such an antenna. I just sent you a Homer 1/2 wave model and it also shows similar maximum gain, not the overstated gain that Avanti published.

    The Average Gain results for this modified model are no so bad, so the feed point seems to be located correctly with these dimensions. However on changing dimensions, who knows what the model will require to show better results.

    That said, I still don't see any change in the TOA. :eek:

    I haven't given up though.
     

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    #65 Marconi, Jan 29, 2020
    Last edited: Jan 29, 2020
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  6. HomerBB

    HomerBB Sr. Member

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    According to this, the parallel radials should produce downward tilt. Either the length of the radials must be changed, or the distance out from the mast, or both.
     
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  7. Marconi

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

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    AP Patent said:
    "... When the flare was omitted and the conductors 16 and 18 were tested parallel to the first conductor 14 it was found that the takeoff angle at maximum signal strength was as much as to below the horizontal. Although for certain uses, such a downward tilt may be desirable..."


    I agree this is what the patent says. I wasn't sure if setting the radials parallel to the mast would do as the patent suggests, but this is why I wanted to see what might happen if I modeled the idea. If this was easy to prove in the real world I would think somebody would have reported such results or maybe I miss it.

    The model is not right and I plan to try and fix that. I'm not finished Homer.

    What I see is this mod added capacitance and that makes the antenna short.

    1. I plan to change the length for the top hat, the radiator length, and the radial lengths to see if I can get the model balanced and show a reasonable match at 27.205, and not at 30.000 MHz as it is now.
    2. Try and get the Average Gain under control.

    Then see if the Free Space pattern shows a lowered TOA and maybe even below the horizon.

    Homer I have seen some of my models show a maximum gain pattern below the horizon, but I never tried to figure out how and why it happened. If I had, that would make this task a lot earier.
     
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  8. HomerBB

    HomerBB Sr. Member

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    Sounds like you're going in the right direction.
    It would be a very interesting result if you pull this off.
    I might even build one scaled to 6m or 2m if you can get an 11m one figured out...

    BTW, I put the experimental 2m V4k back up for the fun of it. Weather's too bad to work on it, but it is working. I want to start a new one that is better. I added 5" to the length to 63" long, but my radials are 1/4" shorter than I want, as is the ring. I want them 19-1/4 to center me on 146.5 Mhz. It is a little low right now close to 144 Mhz.
     
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  9. Marconi

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

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    Homer, I want to be careful here and show a couple of steps with this model as I try to set it close to what we might agree the patent says about lowering the TOA at maximum signal and showing 10*-15* degrees below the horizon. See this idea presented in patent that your posted earlier.

    Quote:
    "...It has been found, however, that the flare affects not only the impedance, but also influences the takeoff angle at maximum signal strength. The flare of the construction described hereinafter provides a takeoff angle that is about the maximum reasonably allowable for most efficient CB use of this antenna. When the flare was omitted and the conductors 16 and 18 were tested parallel to the first conductor 14 it was found that the takeoff angle at maximum signal strength was as much as to below the horizontal..."

    Below is the PDF file showing the A/P model I modified the radial flare to resemble what we think the patent describes. I did not change anything else, but the radial wires might have changed their lengths a little bit on making them parallel. This a geometry thing going on. I made this model with "nTune" included in the model's title.

    The next model will be another model that I try to get tuned at 27.205 MHz and I will change its title to show, in part, "wT1" in the title.

    Here is what I did to get a modest match close to resonance @ 27.300 MHz and the Average Gain = 0.993 = -0.03 db which is understated and is close to perfect = 1.

    Homer, I'm going to wait before I proceeded further and see if DB can do an A/P model in FS using his 4NEC2, and show us steering and a TOA at or below the horizon or not.
     

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  10. HomerBB

    HomerBB Sr. Member

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    At least you've gotten a match. It is now 18' tall!
    The basket segments look to have the same shape as the Big Mac...
     
  11. Marconi

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

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    No Homer, the antenna is maybe 8" -10" taller at 12.61' feet tall.

    What you measured is wire #19 the support mast inside the radials. That is the 1/2 wave mast length that Bob talks about and DB says in the mast length that produces the maximum gain.

    I admit it is easy to miss, but look again.
     
    #71 Marconi, Jan 30, 2020
    Last edited: Jan 30, 2020
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  12. Marconi

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

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    If so, then this antenna might turn out to be a true collinear with the gain a little better than a vertical center fed 1/2 wave.
     
    #72 Marconi, Jan 30, 2020
    Last edited: Jan 30, 2020
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  13. HomerBB

    HomerBB Sr. Member

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    Ok. I see it
     
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  14. Marconi

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

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    Thanks.

    Imagine looking at all these wires, segments, a many other details trying to construct a decent working model...like you do when building a homebrew antenna.
     
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  15. Marconi

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

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    BTW, I had to make this Big Mack feature to get the impedance up near 50 ohms.
     
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