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Electrically length

Discussion in 'Electronic Basics' started by thejim, May 28, 2006.

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  1. thejim

    thejim Guest

    What do we mean by saying "electrical length" of an antenna?
     

  2. Physical length based on the frequency.

    Roughly speaking, a quarter wave antenna is:
    VHF - 18"
    UHF - 6"
    CB - 108"

    Google is your friend.
     
  3. Phil Allison

    Phil Allison Guest


    ** Depends on the CONTEXT - idiot !!






    ........ Phil
     
  4. I'm tired of your crap. Killfiled.
     
  5. Phil Allison

    Phil Allison Guest

    "Abstract Dissonance"

    ** Thank god.

    Another ASD fucked, public menace bites the dust.





    ........ Phil
     
  6. Rich Webb

    Rich Webb Guest

    I suspect that most regular readers have killfiled Phil.
    See: Noise, signal to, ratio of.
     
  7. Ralph Mowery

    Ralph Mowery Guest

    In free space a very thin 1/2 wavelength antenna will have a lenght equal to
    492 devided by the frequency in MHz. In practical applications this length
    will be somewhat shorter depending on several factors such as the diameter
    of the conductor,if it is insulated, and what it is near. For frequencies
    up to about 30 MHz the electrical length will be around 468 devided by the
    frequency in MHz.

    The same applies to the feedline. If you see something like velocity factor
    of .66 or .8. This is the number you multiply the 492 by to get the
    electrical length of 1/2 wavelength of the feed line.
     
  8. It usually means the length as measured in units of wavelength at a
    particular frequency and at the propagation speed of light in that
    medium.

    For instance, a quarter wavelength long whip (one who's electrical
    length is 1/4 wave) is a vertical antenna that is a quarter of a
    wavelength long at one operating frequency, and at the slightly less
    than free space speed of light for RF energy traveling attached to
    that diameter conductor. This also has lots of uses when you are
    using specific length of transmission line to perform impedance
    matching functions, since all the calculations of what length is
    needed for a particular effect are calculated in electrical length or
    specific number of waves or fraction of waves that are stored in that
    length (based on the speed of propagation along the line, and the
    operating frequency), not the external physical length. But to cut
    the cable you have to convert electrical length to physical length.
    This is why every transmission line specification includes propagation
    speed relative to free space.
     
  9. Chris

    Chris Guest

    The other posts happened not to mention the most basic newbie fact:
    wavelength is inversely proportional to frequency. For electromagnetic
    radiation,

    l(wavelength in meters) = c(speed of light, about 3 * 10^8 meters/sec)
    / f (frequency)

    For air or other non-vacuum mediums, you have to tweak a bit. It's
    explained here:

    http://en.wikipedia.org/wiki/Wavelength

    Cheers
    Chris
     
  10. Pooh Bear

    Pooh Bear Guest

    492 what ? Feet, inches, yards, furlongs, metres, cm, mm ?

    Graham
     
  11. Ralph Mowery

    Ralph Mowery Guest

    Feet.
     
  12. tombiasi

    tombiasi Guest

    Phil,
    Why did you have to be rude to the guy for asking a basic question on a
    basic group?
    Tom
     
  13. Tim Williams

    Tim Williams Guest

    Because he's Phil, duh.

    Best solution would be to stick "killfile P.A." in the FAQ's to all the NG's
    he frequents... not to mention encourage voluminous abuse reports to his
    ISP.

    Tim
     
  14. Phil Allison

    Phil Allison Guest

    "tombiasi"
    ** Piss off - shithead.


    ** So you have no appreciation of the importance of CONTEXT either ?

    Read the OP's asinine question more carefully.

    HE **removed** a technical phrase from its context so that it no longer
    had a meaning.

    THEN the wanker asked this NG to explain it to him.

    Only a congenital, idiot does that.

    You must be one as well.




    ......... Phil
     
  15. Phil Allison

    Phil Allison Guest

    "Tim Williams"


    ** This trolling cretyn is human garbage.






    ......... Phil
     
  16. tombiasi

    tombiasi Guest

    Thank you Phil,
    I now have a better understanding of the situation.
    Tom
     
  17. It's cretin, you cretin.
     
  18. Phil Allison

    Phil Allison Guest

    "Rikard Bosnjakovic"

    ** LOL.

    Says some wanker who's name looks like a demented monkey was loose at the
    keyboard !






    ......... Phil
     
  19. Jamie

    Jamie Guest

    i'll give it a shot.
    in free space the wave is traveling in space
    of wavelength = 300/ ?? Mhz which basically means
    the distance the wave will travel in 1
    full cycle..
    now here is the real kicker, the energy being
    emitted from the radiator does no leave its end
    point in a perfect line.
    picture in you mind an omnidirectional antenna.
    " that would be a vertical with ground mass on it"
    the energy leaves the elements at an angle and if
    you were to map the actual point of where it completes
    a full cycle at this angle it would be shorter than the
    vertical element if you were to use a free space vertical
    element length. the larger diameter of elements will
    cause this effect to be increased thus making your electrical
    length shorter.
    then you have the velocity factor which is used to help reduce
    the wave length constant used in the math.
    normally the smaller mass elements has lower velocity and
    closer to free space size..
    if you were to look at a dipole and see the graph chart you
    will see how the electrical length is being effected.
    that is the closes i can explain it with out getting into a
    pile of mess .
    just think of Trig, that mite give you some insight.
     
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