• Register
  • Login
  • this message might be for you!

    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.

    ask a question

    12 Responses to "Folded Dipole Antenna"

    1. can u explain, how to measure La.What unit u used.which one La and Lb.TQ

    2. JACQUES Francois Goualbert Patrice says: on February 23, 2011 at 6:02 pm

      I want to have a perfect calculation about this antennas

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

    4. Such a good and nice explanation.

    5. 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

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

    7. 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.

    8. Thanks. I got it. good explanation.

    9. 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.


    10. Thank you …Dan. i liked.

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

    12. 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.


    You need to log in if you need to post comments on ElectroSchematics.com or register if you do not have an account.