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why does 1/4 wavelength have to have a ground plane and 1/2 doesn't?

Discussion in 'CB Antennas' started by LUFER, Feb 10, 2020 at 1:12 PM.

  1. LUFER

    LUFER New Member

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    Hello everyone, does anyone explain?

    why does 1/4 wavelength have to have a ground plane and 1/2 doesn't?

    I know that an antenna that has half a wavelength does not need the ground plane (example a dipole antenna) and when using a wavelength of 1/4 it is usually necessary to have the ground plane (planar antennas [PCB]).
    My problem: I can't understand why we need to use a ground plane at 1/4 wavelength and 1/2 wavelength not needed.

    Please could also provide source of relevant information, examples, books or scientific article, something like that.



    Regarding the subject
     

  2. sp5it

    sp5it Master of puppets

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    Marconi, Redbeard U812 and VoX 997 like this.
  3. Marconi

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

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    gamegetter, Tallman, Tokin and 2 others like this.
  4. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Well, while you're looking thru the sties the above mentioned, I'm just going to add a simpler effort to look at the way you are thinking antennas work...

    1/2 Wave antenna?

    Divide it by 2 - you get 2 - 1/4 Wave antennas Right?

    Where are you feeding the antenna? Where are you attaching your COAX - also known as FEEDLINE

    You say why does 1/4 wave need a ground plane - but 1/2 wave doesn't...(well, yes it does, but it's not what you think)

    Re-read at the above carefully. You can look at this problem in different ways, change your point of view. How? Start with - It's in how you're feeding the antenna - in a 1/2 wave, it's being 2 - 1/4 wave antennas, you feed it at one end.

    Now take a look at your other antenna - your 1/4 wave antenna. It is still the same size - BUT YOU FEED IT AT ONE END, IN THE MIDDLE - You connect one part of your coax wire to it - use the CENTER for that - connected the SHIELD to the other 1/4 wave feed it there THAT THE OTHER HALF STARTS FROM (The Middle) AND GOES IN THE OPPOSITE DIRECTION. Only you don't make the other half, the ground image does. Put the 1/4 wave with the ground image of another 1/4 wave - you make a 1/2 wave LENGTH TOTAL overall antenna correct? Only you feed it at a 1/4 wave point (the middle) - but you make a 1/2 wave antenna you just let the ground image form from the COUNTERPOISE you need to make that 1/4 wave work.

    upload_2020-2-11_8-1-18.png

    If you feed a 1/4 wave antenna - you sill need ANOTHER part of it - that missing 1/4 wave is in your ground image...your Counterpoise...or what you use your vehicle - or any other metal you've attached that 1/4 wave antenna to - used as a means to COUNTERPOISE (Offset a lack of) otherwise.
     
    #4 Handy Andy, Feb 11, 2020 at 1:07 PM
    Last edited: Feb 11, 2020 at 5:59 PM
  5. HomerBB

    HomerBB Sr. Member

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    Hi Lufer.
    To keep it simple I will make a short statement and simple illustration (to understand the technical aspects of your questions answer requires study).

    Statement:
    All antennas of every type require a counterpoise although it is not immediately recognized as such.

    Illustration:
    To leap upward you must squat and use your legs to thrust yourself upward against the floor. Simple. Now suspend yourself from above, take the same squatting posture with your legs and thrust them downward. You go nowhere because you have nothing to thrust your legs against.

    RF is the same. It needs something against which to "push" or "thrust" itself against in order to project into space. Without this opposing something it will go nowhere just like your leap does not happen without the floor.

    For any and all antennas there is an opposing current equally providing the floor against which to "push" or send your signal on it's way. Whether you provide it by a groundplane or not doesn't mean it isn't necessary or that it doesn't find one by some other means.

    Knowing all antennas need one introduces the real question - Where is the counterpoise for this antenna?

    Study, and continue to ask questions.
     
    #5 HomerBB, Feb 11, 2020 at 2:17 PM
    Last edited: Feb 11, 2020 at 5:23 PM
  6. Marconi

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

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    Lufer, I see the distinction above in bold type, that you bring to the CB Antenna Section. Maybe a visit to the Ham Antenna Section will get you the proper information for your project.
     
  7. gamegetter

    gamegetter Well-Known Member

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    greetings lufer, i don't get on much any more, hope you enjoy the hobby as antenna making has been the most enjoyable aspect of the hobby for me.
     
  8. Marconi

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

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    Hello gamegetter. It seems a long time since I heard form you, to long.
     
  9. gamegetter

    gamegetter Well-Known Member

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    greetings marconi, i hope all is well with you. are you still broadcasting live and in color? :)
     
    Marconi likes this.
  10. Marconi

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

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    Oh yes.
     
  11. gamegetter

    gamegetter Well-Known Member

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    good good, keeps the air waves interesting for sure.
     
  12. M0GVZ

    M0GVZ Sr. Member

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    A half wave DOES need a ground plane, so does a 5/8 wave or any other combination of wavelength for an end fed antenna.

    A dipole isn't end fed, it's fed in the centre therefore the "other leg" acts as a ground.
     
    Marconi likes this.

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