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ASTROPLANE best vertical antenna ever?

Discussion in 'CB Antennas' started by nosepc, Oct 6, 2013.

  1. freecell

    freecell Member

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    if you connect a metal structure to the bottom of the mast (element #14) then it no longer constitutes the 1/2 wavelength section of the antenna radiator below level "A." in this condition antenna currents will flow up and down a conductive support structure as though it were part of the antenna radiator, hence requiring isolation at this point to prevent the previously mentioned condition.



    this is not a problem in conventional elevated radial vertical monopoles (ground plane antennas) as the antenna radiators in these designs are always insulated from the ground radial mounting bracket and u-bolt assemblies where the support structure is attached.
     
    #556 freecell, Feb 26, 2021
    Last edited: Feb 26, 2021

  2. The DB

    The DB Sr. Member

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    Ahh, the mast (or conductor 14 in Freecell jargon because he apparently doesn't know how to speak english). I could write a document longer than the patent itself on the effects of just the mast of this antenna. There is really that much to be said, and the patent mentions almost none of it. It is one of the areas that if you truly want to understand this antenna you really have to go beyond the patent and get more information.

    As far as the patent is concerned, it talks somewhat about shorter mast lengths, but says almost nothing about longer mast lengths. It doesn't say that using them is bad, or that you will have any problems. Its literally just not there.

    Most of the below is based on extensive modeling, but I am going to give the short version, even so, forgive the length. It really isn't something that is easy to explain.

    A month or three ago (or perhaps a little longer), earlier in this thread, bob85 and Marconi were discussing one of the aspects (oddities) of this antenna's mast.

    Effects of mast length on the "take off angle". The patent does mention a change with said angle with changing the length of the mast, however, it doesn't talk about how much of a change changing the mast length causes. The answer to this is about 2 degrees at its extremes. You get more of a difference when changing the dimensions of the basket area of the antenna, and together, you won't see a change of more than 5 degrees, which takes drastic changes to achieve. I find it odd that this minor change is even mentioned in the patent when imho several more important things were left out. I mean seriously, a change this small will not be noticeable to this antenna's intended market. To make matters worse, this change in "take off angle" only shows up in free space. Once the antenna is above an earth, any earth, this change disappears completely, so from my point of view, it is pointless to even mention.

    Effects of the mast length on feed point impedance. For most mast lengths of 1/2 wavelength or longer, there is virtually no difference when it comes to feed point impedance. There is an oddity with something that happens on this mast, that combined with another oddity that happens elsewhere on this antenna design that combined has a stabilizing effect on the feed point impedance of this antenna. It really is a long story to explain what is going on, but the short version is, most mast lengths of over 1/2 wavelengths will have very little effect on the feed point impedance (or SWR) of this antenna.

    Effect of mast length on gain, well, this is more the same than anything, for the same reasons. The effects of a longer mast length on gain are, for the most part, very small. You will never notice these differences.

    According to modeling, the optimum mast length for all of these is in fact not physically 1/2 wavelength long, but closer to a foot or two beyond that. Its close enough that you can use that length and not have any problems, but if your a perfectionist, slightly longer is slightly better.

    Now there is a caveat to this. There are certain lengths where all three of these go haywire. These are close to odd 1/4 wavelength multiples (actually slightly past) of mast lengths. There is a set of lengths near each of these points of about a foot in range that the take off angle goes almost straight up, and the impedance goes sky high. Then, once you get past this small area all is good again. The balancing aspect I mentioned before with this antenna is simply overloaded in these small regions of mast length. Although, typically, this doesn't end up being an issue.

    The reason this doesn't end up being an issue for most people is there is another conductor connected almost directly to the mast electrically near the feed point, and that is the shield of the coax. All of the same things that affect the mast length also affect the coax length, but, the antenna does something good here. If either the mast or the coax lengths put this antenna in its "haywire mode" for lack of a better description, the antenna uses the other one. If you get the very rare chance that both are in said region of lengths, you simply put a choke on the feed line 18 or 19 feet below the feed point and the problem is again solved. Optionally, you can simply put a choke at this point on the feed line and not have to worry about it at all, but it is unlikely that you will have this problem to begin with.

    Again, sorry for the length, what is amazing here is how much I actually left out...


    The DB
     
    #557 The DB, Feb 26, 2021
    Last edited: Feb 26, 2021
    HomerBB, Handy Andy and Robb like this.
  3. bob85

    bob85 Supporting Member

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    DB
    why do you think the mast needs to be physically a little longer than 1/4wave below the hoop?

    i agree but why is that ?

    What i think is likely going with the astroplane,

    We know according to respected sources that when you move the radials on a sigma style antenna, open sleeve antenna & ssfm closer to the monopole electrical length decreases & resonant frequency goes up,

    physical length must be extended for the same electrical length,

    is the Astroplane electrically shortening its mast ?
     
    HomerBB likes this.
  4. HomerBB

    HomerBB Sr. Member

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    Lol. Ahh, ha, ha...
    Turn the AP upside down and it is a virtual twin to the S4/V4k. The S4/V4k skirt/cone/basket elements have long been called radials in discussions of their length and spread/ring diameter on rf pattern and gain in relationship to the vertical radiator.
    It is the AP skirt/cone elements in relationship to the mast length, etc of which Bob speaks. Thus "radials" in his question. And a "very good" A++ question at that.
     
  5. freecell

    freecell Member

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    http://www.radiomanual.info/schemi/ACC_antenna/Avanti_Astro-plane_AV-101_patent_US3587109_1971.pdf

    page 4 line 63 to page 5 line 10. see conductor or element #14 in figures 2, 5 and 6.

    "Indeed the antenna construction substantially counteracts the tendency of prior art antennas to tilt the radiated signals substantially upwardly. A polar diagram showing the radiation characteristics of the antenna of FIG. 2 is shown in FIG. 7. Performance tests resulting in the diagram of FIG. 7 were run. For one such test, an antenna scaled down to proportionate wavelength dimensions for operation at 146 megahertz was prepared. Thus the first conductor or mast 14 was 33.5 inches in length. The distance between levels A and B was 17.75 inches and the conductor 46 (unloaded) was 15.75 inches. Conductors 16 and 18 flared outwardly as described herein. The pattern was measured at a distance of about 100 feet from the antenna."

    "It is seen that the beam at maximum strength is tilted upwardly from horizontal only about 5° and that the radiation pattern lobes are of an optimum shape. In other tests of the same installation, but with shorter mast lengths of 21.5 inches, 24.5 inches and 27.5 inches it was found that the radiation angle at maximum beam strength tilted upwardly substantially more. The conclusion to be drawn, therefore, was that a mast having a length below level B of about one-quarter wavelength, i.e., about the length of the conductors 16, 18 gave an optimum takeoff angle."

    and again, page 6, lines 14-17: "An antenna constructed in accordance with the specific embodiment just described is 12 feet in length (except for the mast which desirably extends at least about 8 feet below level B)"

    and again, page 7, lines 13-16: "In the antenna of claim 3 in which said first conducting element extends beyond said second reference plane a distance at least equal to about the distance between said first and second planes." level "A" is the first plane, level "B" is the second plane."

    there is no such thing as antenna swr and there are no radials on the astroplane antenna.

    page 5, lines 56-60: "The clamping and proportioning provides a significant flare of the conductors 16, 18, (not the loop) which as previously described, influences both takeoff angle and impedance. The takeoff angle and impedance may be varied or altered, as described hereinbefore."

    elements 16, 18 and 20 form a quarter wavelength matching section in the form of a stub, they are not being used to collect antenna current and are not part of the antenna radiator. the active radiating antenna is composed of elements 14 and 47, which includes 46, 48 and 50.

    an antenna does not "have an swr" or standing wave ratio. standing waves IN A TRANSMISSION LINE are due to reflection from a mismatch at the antenna input IN ANY LINE that is not terminated at the antenna load in its characteristic impedance (Zc), which in this instance would be R=50 jX=0.

    if this condition exists then reflection between the source transmitter and the antenna load as well as subsequent standing waves in the line are non-existent, the length of the line has no effect on swr measurements and the line measures R=50 at every point along the line.

    if R=50 then the antenna load is "matched" to the line. if X=0 then the antenna load is "tuned" to resonance. swr=1.0: 1 and now you can "use whatever length of feedline that will reach from the source to the load." only under these conditions is this statement accurate.
     
    #560 freecell, Feb 26, 2021
    Last edited: Feb 26, 2021
  6. HomerBB

    HomerBB Sr. Member

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    @Marconi
    @The DB
    Could either of you model the AP with a typical set of 4 radials at the ~1\4ƛ down from the ring on the 36' mast and a choke on the feedline at that point for kicks and giggles?
     
  7. Marconi

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

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    Homer, I'm here but I have "The Hives."
     
  8. HomerBB

    HomerBB Sr. Member

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    Oh, no! You're turning into a bee?
    I'm sorry, Marconi. I have poor timing sometimes.
    Get better, soon.
     
  9. The DB

    The DB Sr. Member

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    I have mentioned before (and you and Marconi have also discussed this to some extent earlier in this thread I think) about the odd current distribution on the mast. Even though it is near physically 1/2 wavelength long, the current distribution is that of a 1/4 wavelength element. I think it is capacitance with the basket area of the antenna that causes this. If this is the case, the basket area of this antenna adds enough capacitance to the mast that it takes more than a physical 1/4 wavelength to compensate. If this thought is correct, then changing the dimensions of the basket area of the antenna would also effect how long the mast has to be.

    I have done modeling tests in the past where I modeled various ideas of how this antenna operates, and was able to simulate many aspects of this antenna outside of this design. The one thing that I was unable to simulate was the mast being this long. If there was no basket with a mast inside, the mast element was always physically close to 1/4 wavelengths long. This is also why I see the basket area being the cause of the mast needing to be this length.

    I disagree with this. The reason is the basket on the V4k and the like is all connected to one side of the feed point, acting in some ways as a set of radials. The basket on the Astroplane, however, is connected to both sides of the feed point, and their is a current path that actually passes through from one side to the other. I would agree that in many ways the baskets of these antennas have some similarities, but they are definitely not the same thing.


    The DB
     
    bob85 likes this.
  10. HomerBB

    HomerBB Sr. Member

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    Aw shucks. You hurt my feelin's.
     
  11. bob85

    bob85 Supporting Member

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    The 1/2wave mast does have an unusual current distribution that looks like its electrically only 1/4wave long, I have never seen that effect before,

    How does a vector look when you cut the monopole down so it extends about 1/4wave above the hoop ?
     
  12. HomerBB

    HomerBB Sr. Member

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    Bob, The DB says it don't matter 'cause the vector cone is fed from one side an' the AP cone is fed from two sides so he don't b'leive it an he ain't gonna do no more models to see 'cause it's done been talked about before but nobody remembers it.:mad:
    When Marconi gets better he'll do it for us.:)
    One of these days I'm gonna make a wee little AP and hang it upside down an' .... and a wee little vector an'....:confused:
    Besides all that, I don't know. Probably won't tune in band if at all because it's too short.(n)
     
  13. bob85

    bob85 Supporting Member

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    We have talked about it before Homer, remember the blue line models DB did.

    Theres so much been posted that looking back for something is hard work,
    I can't think of another antenna that has a 1/4wave current distribution on a 1/2wave conductor,

    The rise in frequency & impedance drop you get when moving the 1/4wave elements closer to the monopole of a sigma & ssfm is not the same thing we are seeing with the Astroplane models strange distribution.
     
  14. HomerBB

    HomerBB Sr. Member

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    Thanks, Bob.
    I was just being cute.
    Just trying to ride The DB a little.

    I am curious about all this.
     
  15. The DB

    The DB Sr. Member

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    The Vector models didn't have this effect. I am not sure why the Vectors don't have this effect, at least to the extent of the AP. The baskets both should add capacitance to whatever is inside them, yet when I model the Vectors the voltage node is a few feet past the basket, not about 9 feet past it like what happens on the AP mast. If something like this did happen on the Vector, we would be seeing a radiator that is something like a full wavelength long I think, and we definitely don't see that.

    I remember that @Shockwave pushed the .82 wavelength radiator on the Vector for a while (I guess he still does) based on real world testing on the FM version of the Vector. I recall modeling suggesting at that time that it should be .8 wavelengths in length for said optimal length, although I think we were looking at two different sets of data. He was looking at data as it was being received at a certain point in some distance away from the antenna on the ground, while the model I used was specifically looking at gain on the horizon, so in his case his design was likely shifting the main lobe of the atenna further down towards the earth.

    I am not sure why this electrically shortening effect is so much greater in the AP than the Vector. My current thought is something is different between how the baskets of these antennas are operating, but I'm not entirely sure on what that is.


    The DB
     
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