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Antennas

Discussion in 'Electronic Basics' started by thejim, Jan 23, 2006.

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  1. thejim

    thejim Guest

    I have concuded somewhere regarding Electricaly short antenna.
    I just want to mention it to see if my conclusions are correct.
    If the antenna is shorter than 0.25 of the wavelength then there will
    be a phase angle as a result of disturbances and that is why there is
    impendance rather just resistance
    Am i saying it correctly?
     
  2. I don't understand what you say. The outgoing wave bounces off the
    end of the antenna and returns back toward the transmission line, with
    some energy lost to radiation. Right at the end of the antenna, the
    current of the outgoing and reflected wave must sum to zero, since
    there is no other currents involved except the two waves. So the
    returning wave is current inverted.

    The thing that is hard to wrap your brain around is that you are
    dealing with traveling waves, not lumped components (like capacitors
    and inductors). So to understand how these two waves act at any point
    on the antenna or transmission line, you have to deal with the time it
    takes for any part of the wave to get from point A to point B.

    If the antenna were electrically resonant (not too long or too short),
    the outgoing wave would take a quarter of a cycle time to get to the
    tip of the antenna and a quarter of a cycle time for its reflection to
    get back to the feed point. But there is also the inversion at the
    reflection, which adds another 180 degrees or one half cycle to the
    total effect, so the returning wave meets the outgoing next cycle, in
    phase.

    If the antenna is short, the reflected wave gets back too soon, and
    the next cycle sees the last one at an earlier phase in its cycle.
    For instance, if the outgoing cycle is right at zero volts and going
    positive, it meets the reflection at a negative voltage heading for
    zero volts. This is a lot like how it would be if the outgoing wave
    were driving a lumped capacitor. The capacitor voltage would always
    be lagging the applied wave. Short antenna = capacitive reactance at
    the feed point.

    If the antenna were long, the reflection would have the (travel) time
    to get ahead of the drive wave, so it would have a voltage that leads
    the drive in phase, just as an inductive load would have a voltage
    than leads its current.
     
  3. John,

    Can you please explain how the length of a flattened loop, like a TV
    antenna, should compare with an ordinary dipole antenna.

    R
     
  4. Phil Allison

    Phil Allison Guest


    ** They are virtually the same length.

    Do a Google search on " folded dipole ".

    Beware of the trap of assuming that any Yagi antenna with the main element
    folded is a 300 ohm antenna.




    ......... Phil
     
  5. What is that supposed to mean? Were you trying to say they are the same
    "electrical" length? Either way, you're wrong. Try again later.

    A full wave loop (like the guy asked about) has a feed point impedance
    of around 100 ohms when it's shaped like a circle compared to the ~75
    ohm impedance of a dipole. When you squash it down till it's nearly the
    length of a 1/2 wave dipole the feed point impedance shoots up to around
    300 ohms while the gain of the antenna drops with the shrinking loop
    area. To further answer the OPs question, a full wave loop has a
    theoretical gain of 3dB vs. a dipole. I'm sure it's going to be
    somewhat less than that when you mash it to a narrow oval, but it will
    still have more gain than a 1/2 wave dipole. It will also be much
    "quieter" in terms of the amount of noise it receives.
    Why don't you do us all a favor and stop trying to answer antenna
    questions.
    I'm guessing this is in reference to some other occasion where you put
    your foot in your mouth.
     
  6. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"

    ** Freemont is *hell bent* on proving, over and again, just what a
    criminal, psychotic pile of sub human shit he really is.


    "Roger Dewhurst"

    ** It is a perfectly clear statement.

    But Fremont's grossly autistic brain has mangled it - as usual.



    ** No, I am not wrong.

    A regular di-pole and its folded counterpart have essentially the same
    length.


    ** No he did NOT - you illiterate, brain fucked ASSHOLE !!!



    ** Fremont, why not do the whole planet a favour.

    Go shoot yourself in the head.


    ** Every this Fremont **** posts is a wild guess.

    Then what would you expect from a pig ignorant, septic tank ham radio
    puke.




    ........ Phil
     
  7. Your reference to a full wave loop implies, to me, that the circumference of
    the loop approximates to the wavelength. Is this correct?

    R
     
  8. "Phool Allison"
    Please don't top post.
    Only in the physical sense of the space it takes to erect it, and even
    that is a twisted viewpoint since it's still a mashed up loop. That
    means its length is really twice that of a dipole.
    You really need help phool. A folded dipole IS A FULL WAVE LOOP
    squashed down to regular dipole size (more or less). I see you snipped
    away about 90% of my post. I'll take that to mean you verified it as
    factual information after you frantically rifled your way thru the
    google archives.
    I don't see how that will answer the OPs question, but then you aren't
    really interested in doing that are you? You just wish to disseminate a
    bunch of BS mixed in with a little bit of decent information. A true
    savant when it comes to audio and an absolute knuckle dragging simian
    when it comes to everything else.
    Yet another imbecile jealous of a ham ticket, what an absolutely pitiful
    sight. I'm sure you could probably get one now phool, since they made
    it so easy. You don't even have to learn morse code, but you WOULD have
    to learn something about antennas.
     
  9. Yes, but as phool said, if you mash into the shape of a folded dipole
    then stand back and squint a little, it will appear to be the size of a
    regular dipole which is 1/2 wavelength long.
     
  10. If the loop is flattened enough that the half look more like pieces of
    transmission line than they do like a loop antenna, then the
    electrical length of the folded dipole is about the same as a simple
    dipole. I think, that the propagation speed may be just a bit lower
    for the folded dipole, so it may look act a bit longer than a dipole
    of the same physical length, but I an not too sure of that.
     
  11. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"

    ** Freemont is *hell bent* on proving, over and again, just what a
    criminal, psychotic pile of sub human shit he really is.


    ** It is a perfectly clear statement.

    But Fremont's tortured, autistic brain has completely mangled it.


    ** No he did NOT - you illiterate, brain fucked ASSHOLE !!!

    Fremont, do the whole planet a favour.

    Go shoot yourself in the head.




    ........ Phil
     
  12. ....nothing technical

    I guess I win again. Thanks for playing.
     
  13. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"


    ** Fremont is *hell bent* on proving, over and again, just what a
    criminal, psychotic pile of sub human shit he really is.

    Fremont, do the whole planet a favour.

    Go shoot yourself in the head.




    ........ Phil
     
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