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questions: long line antenna

Discussion in 'Home Brew' started by RustDemon, Nov 10, 2011.

  1. RustDemon

    RustDemon Active Member

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    if I divide 468 by 28.4 I get 16.71 is that feet? if so convert to inches at 200.57


    using a random wave length calculator on line I resolved that 1 full wavelength at 28.400 Mhz is 421.7 inches so half that would be a half wave length correct? for a total of 210.3 inches for a half length dipole.




    so I wind up with almost a foot difference using that calculator and the formula you recommended... what am I missing?
     

  2. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    you are missing the velocity factor of the wire:unsure:

    KWARC Dipole Calculator

    Here online calculator. plug the numbers in and the script will figure out the lengths.

    A dipole is not hard, the formula will get you in the ballpark, cut the wires a little longer install it at the height and location you will be using it at, then start cutting each side equally, to get it to resonance. Inverted V or flat top also makes a difference in the vswr,

    Height above ground also effects the vswr. MANY useful resources online, google is your friend.
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    None of these "magic" numbers or calculators are ever going to be 'exact' except by accident. There are just too many variables that affect resonance, and it's seldom that any two installation situations/circumstances will be the same. Those calculated lengths for resonance are almost always going to be a bit long. That gives you the advantage of being able to adjust things without having to add wire to the antenna.
    Then it just depends on how you do the 'measuring' of resonance. SWR is never an indication of resonance. It only tells you about impedance matching. There are always two steps in tuning an antenna, making it resonant, and impedance matching. That assumes that you want a resonant antenna to start with, which isn't necessarily what you may want or need.
    Sound like it's really complicated and contradictory? Yep, certainly can be.
    - 'Doc
     
  4. RustDemon

    RustDemon Active Member

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    LOL thanks, its all relative I guess. I will pick one of the longer measuments and go from there. and it will be an inverted v formation which lengthens it out some more. no worries. this is half the fun of starting up in a new hobby, asking all those questions that makes oneself feel ignorant once answered.

    fortunately as I read the answers as well as the various sources online and the books from the library my knowledge pool is getting deeper bit by bit.
     
  5. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    You are asking questions, learning, then building. Nothing gets better than that. Especially when you build the antenna and make a rare contact with it.

    Everyone starts somewhere and many knowledgeable members on this forum.

    1005/freq full wave length
    468/freq half wavelength
    234/freq 1/4 wl

    Straight from the antenna handbook.

    Why is it not 936/freq for a full wavelength? I once got an answer to that question. It is 1005/freq in free space. Is this true? Darn if I know and I am not going into free space to find out.

    The formula's get you in the ball park as DOC said, then you just snip and trim snip and trim.

    Oh and wire antennas work better when they are built in the middle of winter with snow coming down and freezing rain.(y)

    Doc says his Pink antenna wires resonant better than other colors. I dunno about that though.
     
  6. Beetle

    Beetle Sr. Member

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    I'd change what Wavrider said just a bit: you're asking questions, then building, and THEN is when the learning starts: when you actually use the antenna you've built yourself, on the actual air, and hear somebody actually replying to your call! At that point, everything you've done comes together; the door opens and your learning process begins.
     
  7. RustDemon

    RustDemon Active Member

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    its all a learning process beetle, just learning that I should start with a Di-pole instead of a long wire is learning. I don't have to wait until its up and running to learn something. That seems to me to be a very narrow view of the definition of learning. granted I'll learn more after completion but I'll learn though the whole process.


    anyhow as it stands I have the approximate lengths needed. I'm going to build the stand offs for the roofline in the next couple days. I need to pick up some more heat shrink tubing so I can waterproof all my connections since I am almost out.

    thanks for your help guys.
     
  8. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Liquid electrician tape.

    It applies like paint and seals like electrical tape once dry, great stuff for weather proofing connections.
     
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Yep, I have used it before too and it works great.
     
  10. Shockwave

    Shockwave Sr. Member

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    The reason the full wavelength deviates from the calculations for the 1/4 and 1/2 wave is because the full wave is intended to be used as a closed loop. The reactance at the ends of an antenna element have a different effect when there are no ends and the loop is closed. This is why the lengths are slightly different then what you would expect. This is not due to velocity factor. That is an independent calculation that should be added to the previous formulas to determine the correct length for the wire and insulation type being used.
     
  11. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Thanks shockwave, I never heard that explanation before.
     
  12. W5LZ

    W5LZ Crotchety Old Bastard

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    And to keep it straighter...
    Those free space numbers were derived from calculations a very long time ago. The numbers used to find those 'ball-park' lengths are from experimentation, resonating an antenna then measuring it's length. The differences apply to all the wave lenghts normally used, 1/4 wave, 1/2 wave and full wave. EG: 234/246, 468/492, 984/1005. Do some 'playing' with those relationships and I think you'll see they tend to reflect some velocity factors you my be familiar with.
    None of the 'magic' numbers for finding the lengths of antennas is ever going to 'exact', there are just too many variables.
    - 'Doc
     
  13. RustDemon

    RustDemon Active Member

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    what type/brand of antenna tuner do you guys recommend for this dipole to be?
     
  14. RustDemon

    RustDemon Active Member

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    any how here is where I am at, for my inverted v the calculator says 14.83 feet per leg.

    so I cut wire to 15 feet 1 inch
    [​IMG]

    then stripped 2 inches on the outer of a piece of coax and one inch on the inner core. and stripped one inch on both wires or legs rather.
    [​IMG]

    soldered them together heat shrank them then re-enforced the resultant t shape with lots of electrical tape for support so there wont be any strain on the solder
    [​IMG]

    and thats as far as I have gotten today.

    need to pick up a pl259 for the end of the coaxial then find a tuner somewhere.
     

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