Xmit loop antenna for all bands.Dbl helix cont"d

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Nov 21, 2009.

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  1. K4SAV

    K4SAV Ham Member QRZ Page

    For a verification of what I just stated, I ran that system thru EZNEC. First putting the mesh antenna on the top of a 100 ft tower, fed the way you described. I assumed a fairly good radial system at the base of the tower (10 ohms ground loss). The gain on 1.8 MHz was -0.05 dBi.

    Removing the mesh antenna, leaving only the coax, and repeating the analysis, the gain on 1.8 MHz was -0.54 dBi. That 0.46 dB of difference is due to the current distribution on the coax shield changing because of the change in top loading. If I replace the mesh antenna with a wire to simulate the same amount of top loading there will be no change in gain. So the antenna is the coax shield and the "mesh" does essentially no radiating, it only slightly alters the current on the outer shield of the coax.

    Jerry, K4SAV
     
  2. K4SAV

    K4SAV Ham Member QRZ Page

    Actually it comes from a career of 53 years in electronics as well as 38 years of designing electronics as a profession. The concepts used in this antenna however are not complicated and don't require a lot of expertise to understand. To understand why the antenna you built works, you only have to understand how a coax works. I was looking on-line to see if I could find a reference that was already written rather than trying to explain it in these short notes. I haven't found one yet.

    Maybe a very abbreviated answer will suffice. To keep a coax from radiating it is necessary that the currents in the inner layer of the shield be the same value, and 180 degrees out of phase with the currents in the center conductor. That REQUIRES that all center current be returned via the shield so the shield HAS to be connected directly to the load and any loops used in making that connection have to be insignificantly small. Also for that to happen the load has to be unbalanced (but not necessarily equal to coax Zo). So if the coax load is a balanced antenna, there must be a choke added of sufficient impedance to prevent currents from flowing on the outside of the coax.

    Because you violated all those rules with the use of a coax with the shield floating, the coax radiates. And since the "mesh antenna" has insignificantly small radiation resistance on the low bands, the coax is the only significant radiator.

    Jerry, K4SAV
     
  3. KB9MZ

    KB9MZ Ham Member QRZ Page

    Jerry, a final word with respect to your last post. Anybody in the hobby has the ability to measure mesh to determine its point of resonance or to read up to determine that a Faraday shield radiates. I illustrated such a measurement on my page. If a mesh is determined to be resonant at a particular frequency most hams would question your statement that it is the coax that was radiating! Because it was "you" who said that does not imply that credability is automatically assigned. It certainly requires a modicom of logic to be assigned when any ham can measure the resonance of a piece of mesh for himself, which is then a denial of your statement. Providing this to the professor in 101
    as an explanation for your contrary point of view would most certainly prevent graduation at examination time.
     
  4. KB9MZ

    KB9MZ Ham Member QRZ Page

    Oh Jerry, I explained the reasons both for and against disconnecting the braid. One would only do this if the feeder interfeared with the radiation pattern.
    It is doubtful if you would notice any difference in hearing what ever choice that you make. Either way it does nothing to prove that the mesh doesn't radiate. I would imagine at this very moment that statement is being proved in error. Why not borrow four pieces of window mesh and connect them to make a tube style antenna instead of relying on your couch intuition. I recognize now that you cannot accept change and thus to continue would be fruitless.
    Thanks any way for your comments which you have shared with all for consideration.
    Regards
    Art
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    I never said a piece of mesh doesn't have a point of resonance. As a matter of fact I gave you that resonant frequency. The radiating coax is not a matter of opinion. "Couch intuition" was not necessary. It is the result of a simple analysis. As far as graduating from class 101, I no longer worry about that. I am many years beyond that point.

    If you wish to determine for yourself if the coax radiates, it is very simple. Just measure it. A simple RF current meter like this one is all you need.
    http://www.w8ji.com/building_a_current_meter.htm

    Antenna design is not just a matter of dreaming up ideas to explain their operation. You have to verify those ideas through analysis and testing, preferable both. I gave you the analysis. It would be to your benefit to repeat that analysis for yourself and to also do some testing to either confirm or deny that analysis. It would be a giant step forward in improving your understanding of antennas. That is the reason I made any posts to this topic.

    Jerry, K4SAV
     
  6. K8JD

    K8JD Ham Member QRZ Page

    I had an aquaintance tell me that his burned out 500 w lamp, covered in tinfoil would outperform a Yagi, and he had theoretical calculations to prove it. (But no QSL cards)
     
  7. KB9MZ

    KB9MZ Ham Member QRZ Page

    Jerry I supplied al in this thread from having such an antenna and the logic or theorem that agreed with my findings. I shared them because as an engineer I look for logic to the contrary. In your one but last posting you made a few statements which led up to your statement that a mesh had a low impedance therefore it was in efficient or some thing like that. That statement has no merit or assisting logic. Surely you know that the impedance of a radiator consists of radiation resistance plus the conductor resistance. If one improves the radiation resistance which is the object of a radiator then one must reduce the losses. This can only be done by reducing the resistance losses. If you are familiar with computer programs you cannot help not to see that as gain increases the overall impedance reduces such that it makes it difficult to feed. Thus if a radiators loss resistance begins to drop radiation increases (Isqd R).I don't think there is a need to discuss the statements that led you up to your final point except that the bottom line statement you made is inherently flawed. To continue such a line of reasoning would be an insult to all hams following this thread and harmfull to the newbies
    Thanks anyway for your participation.
    Regards
    Art
     
  8. K4WGE

    K4WGE Ham Member QRZ Page

    Art just needs attention

    Without fail, every so often Art will post some incoherent garbage to draw in some well-meaning forum participants in what they think is a technical discussion. Every time it ends with Art's neurotic-psychotic behavior taking over.
     
  9. KA5S

    KA5S Ham Member QRZ Page

    It's hard to understand what he means,but it almost sounds like a self resonant cylinder in Johnson and Jasik .04 wavelength long by .04 wavelength in diameter.

    It's supposed to be 50 percent efficent and was used in a radio-proximity artillery fuse.

    Of course the temptation exists to make one out of mesh; .04 wavelength at 80 meters is still ten feet across and long, and sheet metal would be heavy. No to mention one heck of a wind load!

    Cortland
    KA5S
     
    Last edited: Nov 22, 2009
  10. KB9MZ

    KB9MZ Ham Member QRZ Page

    Just took hold of my own suggestion.Got four window screens and stuck them together using adhesive foil. No screws because they have to go back.
    Placed on lawn and hooked the feed on.Fired up top band and it was 1:1
    I suppose I will have to throw that away since it doesn't match the Eznec computer files so called output!
    Such is life
    Next time I will make another one but without the aluminum frames to see if that matches Eznec. I will put the window screens back another day.
    Next time it will be a smaller one so I can climb the tower with it in one hand and drop it into the rotator.
     
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