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Antenna impedance

Discussion in 'Electronic Basics' started by riccardo manfrin, Nov 3, 2013.

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  1. With reference to this schematic:
    http://www.pyroelectro.com/projects/pyro_rf_transmitter_27mhz/img/schematic.png

    I'd like to know if there is a general rule of thumb for determining the antenna impedance Z(w) and therefore the matching inductance L3.

    Also, I'd like to know what is the meaning behind the choice of the "12 inches of AWG 28" (cause 12inches do not seem to me like any fraction of the wavelength lambda).

    Thanks in advance.
    RM
     
  2. amdx

    amdx Guest

    Might be better to ask on,

    rec.radio.amateur.antenna

    It is a fraction of the wavelength, a very small one. (1/36)
    Many remote control toys use 27Mhz and a similar short antenna.
    I would think the designer picked the L and C for the 12 in antenna.

    Mikek
     
  3. Fred Abse

    Fred Abse Guest

    A vertical 12 inch piece of #28AWG wire, over ground, at 27MHz, will look
    something like 0.3 ohms radiation resistance, in series with about 2.5pF.
    Required series inductance will be 13.92uH.

    Radiotron Designer's Handbook, Chapter 22, Section iii gives:

    -jZo cot(2*pi*L/lambda) for the reactive component, where:

    Zo=138 log(lambda/d) - 104 ohms, where d = wire diameter.

    I do it on a spreadsheet I adapted from an HP41 calculator library program.
    Just a convenient piece of wire? No other reason I can see. #28 wire won't
    self-support very well, anyway.

    A quick and dirty simulation using the above antenna model shows resonance
    at about 28.37MHz, with your published component values. Probably not much
    radiation from the "antenna" at 27MHz.
     
  4. Fred Abse

    Fred Abse Guest

    With a properly-designed PA stage, which the quoted example isn't...

    Out of a hat, according to my calculations, and simulation. Preferred
    value components, without any adjustment are a dead giveaway. Resonance is
    over a meg away. 2N2222 isn't a very good choice, either.
     
  5. "License free" transmitters often have quite strong limitations, not just
    power input, but antenna length too. 12inches seems short for 27MHz, but
    that may be a factor. I'm not even sure if the laws deal with input power
    anymore, or just radiated power. The latter is harder to measure for the
    hobbyist, so things may be kept very simple to avoid too strong a signal.

    I can't emember what the antenna length is for the 160 to 190KHz band, but
    it's extremely short for the frequency.

    Michael
     
  6. Tom Biasi

    Tom Biasi Guest

    Some designs use the inductor series circuit as a loading coil for the
    antenna. calculations don't always hold up in real life.
    No idea if this applies here, just sayin'.
     
  7. Yes, but I can't remember if loading coils are allowed by the rules for
    unlicensed operation or not. I seem to recall something about "well we'll
    just use a giant loading coil" but then "no, that's not allowed".

    Michael
     
  8. Tom Biasi

    Tom Biasi Guest

    Back many years ago when I first got my FCC First Class License I think
    the rule came under part 95, unlicensed transmitters. Then 11 meters was
    taken from the hams and CB was born. The unlicensed band was limited to
    100mW and a 5 foot antenna. I don't recall any restrictions on loading
    coils but that was back when the Pope was an Alter Boy.
     

  9. Never heard about "forbidden loading coils"
    Is that another world domination conspiracy or what?


    w.
     
  10. Il giorno lunedì 4 novembre 2013 04:38:18 UTC+1, Tom Biasi ha scritto:
    I've been reading about the loading coil, but I just can't figure out the purpose? I thought that the inductor was there to match the antenna load. Has the "loading coil" this purpose? I mean, is it just another name for impedance matching? Or does it serve different purposes?
     
  11. Phil Allison

    Phil Allison Guest

    "riccardo manfrin" = Goggle Groups fuckwit
    ** No fooling ??

    ** How surprising...........


    ** It might be there match your stupid ass.


    ** **** knows that this retard thinks "match " actually means

    Least of all him.

    ** Not exactly, actually


    ** Hmmm, is there a tiny flicker of insight ..........

    It might just dawn on this fucking idiot that "match" has
    more than its literal meaning.

    Like in the old joke:

    " Got a match ? "

    " Yeah - your face and my ass. "




    .... Phil
     
  12. On my way to download that.
     
  13. It is interesting, but I'm looking for a (possibly simple) mathematical formulation that

    - applies to a generic dipole of arbitrary length (not lambda /2 or lambda/4, and in general way smaller than the wavelength),
    - does not take in integrals nor starts from Maxwell's equations
    - possibly takes in all conventional approximations/simplifications for far field, isotropic, linear, homogeneous, non-dispersive medium,
    - binds the antenna complex impedance to the length and lambda

    The closest to this was Fred's answer but the source is not exactly an "handbook".

    I'm now starting from scratch to understand the current and voltages distribution across the dipole to understand how the ratio between them is defined and therefore the impedance.

    R
     
  14. Phil Allison

    Phil Allison Guest

    "riccardo manfrin"
    ** You are so far off the basic issue.

    An antenna radiates RF energy in proportion to the VOLTAGE on its driven
    elements.

    A series inductor operating at resonance with some capacitance *magnifies*
    the voltage appearing at the output of the driving amplifier.

    Soooo - it boosts the energy radiation if the antenna is capacitive.

    Which all short antennas are !!!!!!!!!!!

    **** equal value impedance matching nonsense.



    .... Phil
     
  15. I always thougt it is the moving electron which radiates.
    When you charge a metal object to a million volts,
    will it radiate?

    w.
     
  16. Phil Allison

    Phil Allison Guest

    "Helmut Wabnig"

    ** **** off retard.
     
  17. I bet your antenna radiates into the ground.

    w.
     
  18. Fred Abse

    Fred Abse Guest

    Depends how big your hands are...

    My hardback copy is only about 2.5 inches thick ;-)
     
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