impedance of a 1/2 wave dipole antenna

Discussion in 'Wireless & RF Design' started by donut, Jun 22, 2012.

  1. donut

    Thread Starter Member

    May 23, 2012
    The theoretical impedance of a half wave dipole antenna is 73 + j 42.5.

    My question are:

    1) Is there a pragmatic explanation how it is 73 + j 42.5?

    2) Can I radiate rf from a half wave antenna dipole antenna that has an impedance of 73 + j 42.5? Or do i have to remove the j42.5?

    3) How would I remove the inductive reactance to make the antenna only real?
  2. davebee

    Well-Known Member

    Oct 22, 2008
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    1. Probably. Maybe someone will chime in with that explanation.
    2. Yes. It is not manditory to have a totally resistive load in order to radiate. But there are some issues including reflected power that will preclude maximum power from the source being radiated which is presumably what is desired. So you do not absolutely have to remove the j component of the antenna characteristic impedance but it may be helpful. And for maximum power transfer the 73 Ohm resistive component of the load impedance must be matched to (equal to) the source impecance. All of that is what antenna matching devices do. There are a plethora of configurations such as pi, T, L etc methods. Which is 'best' depends on the source and load impedances.
    3. It could be as simple as a capacitor. Remember capacitance is a -j so the proper value of C will cancel the +j.

    To what extent you go depends on several other factors which you do not provide info on.
    Last edited: Jul 11, 2012
    donut likes this.
  4. Papabravo


    Feb 24, 2006
    That impedance is not constant with frequency. For any dipole you construct there will be a resonant frequency where the actual impedance of the antenna AND the transmission line will approach several tens of dB return loss and radiate like crazy.

    Also don't forget that your antenna and feedline are in parallel with free space.

    An antenna tuner is exactly the device you need to adjust a real antenna to a transmitter output. For receiving you don't need to be nearly so precise.
  5. donut

    Thread Starter Member

    May 23, 2012
    so when they say that the theoretical impedance of λ/2 wave antenna is 73 + j42.5, what frequency is the antenna tuned to with this so called theoretical impedance?

    Yes i understand your point. But knowing that the antenna impedance changes due to frequency what does that actually mean to me. How do you find the impedance of a dipole for a specific frequency?

    Are you talking about the 377Ω intrinsic impedance? Please elaborate a bit further so that I can understand what you are trying to tell me. What does it mean to me to know that the antenna and feedline are in parellel with free space?

    So the antenna tuner is a Balun? matching the transmitter side to 50 ohms but what is the tuner circuit antenna side impedance value adjusted to?
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Probably - like any physical load the antenna has a complex impedance. The resistive part accounts for heat losses in the antenna, in the radiated energy and losses in the medium surrounding the antenna. Since there will be an electromagnetic field near the antenna parts then there will be energy stored in the field(s). This stored energy gives rise to the imaginary part of the effective antenna impedance.
  7. KL7AJ

    Senior Member

    Nov 4, 2008
    The +45j is what happens if you ignore the velocity factor of the antenna's about 95%. Most practical applications actually cut the antenna to true resonance, which, by definition is 0j. The ARRL Handbook and Antenna Book goes into great detail on this.

    Hope this helps!

  8. KL7AJ

    Senior Member

    Nov 4, 2008
    As a rule of thumb.....for very large length/diameter radio of wire, the formula for a resonant dipole (in feet) is 468/F(mhz). If you ignore the velocity factor, the dipole is 492/F(mhz)

    Now you know!