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folded dipole

Folded Dipole Antenna

Another common place one can see dipoles is as antennas for the FM band – these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. The main advantage of this arrangement is an improved bandwidth over a standard half-wave dipole.

Folded dipole design

folded dipole

The Length of folded dipole
* La = 145 / freq ( MHz )

Length of coil

* for 75 ohm cable : Lb = La x 0.8
* for 50 ohm cable: Lb = La x 0.66

Using a 3 – 12 V lightbulb we can make a symmetrical antenna. Hold the lightbulb and touch the folded dipole, move from end point to middle. If the lightbulb goes off in the middle everythimg is ok.

12 Comments

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  • dan

    Hello all,
    the initial explanation was given by somebody who speaks english as a 2nd/3rd language, hence the coil vs stub.
    otherwise the explanations are good and right on.

    at the end there is a lot of trial-and-error, but thats normal.
    typical optimization problem.

    gl

  • Phaneendra

    Dear Sir
    we want 90.8 MHz fm folded dipole specifications. most urgent.

  • smseeja

    Thank you …Dan. i liked.

  • Dan

    please feel free to visit link below, more of enter information and it will design the antenna of your choice, As i mentioned, the article is correct, but a bit crude, and did not fully explain, why you need a hairpin impedance matching network. and a good class in ac circuit theory in conductor capacitor and antenna theory. Most people who read this sight may not have this behind them, and just want to build an antenna.

    http://www.k7mem.com/Electronic_Notebook/antennas/folded_dipole.html

  • smseeja

    Thanks. I got it. good explanation.

  • Dan

    This folded di-pole does not have a coil. it has a matching stub to correct for impedance, the article calls this a coil which is incorrect.

    The Length of folded dipole
    * La = 145 / freq ( MHz )

    La is the distance in empirical feet, which is not specified.
    La or length in feet = 145 / desired frequency in Mega-hertz or Mega-cycles per second. in this case, your radio frequency

    electricity travels faster in air, then it does through copper, so each coax cable has a velocity factor. so we need to calculate a matching stub with a correction factor of the type of cable used.

    * for 75 ohm cable : Lb = La x 0.8
    * for 50 ohm cable: Lb = La x 0.66

    in general rg6 is 75ohm tv coax cable and its velocity factor is 0.8 rg58 is 50ohm cable used for cb and ham use, its velocity factor is 0.66

    take the last calculation you had La and multiply it by the velocity of the coax you are using, that calculation is also in feet. and is left in a U shape, not in a coil.

  • smseeja

    i need explanation about antenna coil and la and lb calculation.

  • estefan2020

    This is a crude explanation of how to calculate a folded dipole with a hairpin match. A much better explination with calculators at http://www.k7mem.com/Electronic_Notebook/antennas/folded_dipole.html

  • Mahesh A Birari

    Such a good and nice explanation.

  • dna

    Please elaborate on the procedure to be followed to test the folded dipole using lightbulb. Also let us know the theory behind it.

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