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Dipole vs folded dipole

  1. Sep 3, 2004 #1

    What is the advantage to have a folded dipole? Well, the radiation pattern, the output power are the same? Except for the radiation resistance, but I don't understant, the resistance is 4 time the dipole so, why the output power is the same?


  2. jcsd
  3. Sep 3, 2004 #2
    Radiation resistance does NOT affect power if you match the antenna to the transmission line properly.
  4. Sep 7, 2004 #3
    Maybe I'm missing something. Just a simple calcule,

    Input let's say , V = 100 volts

    V = Z*I

    at resonnance, Zdipole = 73 ohm

    Idipole = 1.36 A

    For folded dipole, Zfolded = 300 ohm

    Ifolded = 0.33 A

    Output power,

    P = 1/2 *Z*I*I


    P = 1/2 * 73* 1.36 *1.36 = 67.51 W

    Folded dipole

    P = 1/2 * 300* 0.33*.033 = 16.34 W

    So, the output power is not hte same. Can you explain?

    Thanks for your help.

  5. Sep 8, 2004 #4
    I said properly matched. Matching a 300 ohm antenna to a source other than that impedance is not a proper match.

    You are assuming a voltage source is driving the antenna. This is not really the case. It can be pictured as an ideal voltage source with a resistor in series with it. The series resistor is the output impedance of the source. Maximum power transfer exists only when the source and load have the same impedance. Do the math. You seem to understand ohms law so I think you can handle it. So you want drive a 300 ohm load and radiate the same power as a 73 ohm load? Well, it requires a matching network of some sort at the load. An impedance transforming device. A 4 to 1 balun transformer would work for a 300 ohm antenna and a 75 ohm source.
  6. Sep 9, 2004 #5
    Why I cann't drive my antenna directly from the source? For example, if I have a dipole which it resonne at 1.5 GHz, let's say perfect match, so the frequency of my generator should by 1.5 GHz? The duration of my impluse should by 66 nano second (1/1.5 GHz) ? It doesn't matter the output of the wave of the generator (a square, rectangluar, sine wave form )? The important thing is the generator (voltage source) has the same frequency?

  7. Sep 9, 2004 #6
    Hold it. Let's just stick to sine waves for now. If you start throwing in square and whatnot you are driving the antenna with other frequencies besides the fundamental 1.5 GHz you have selected.

    I am pretty sure you have not done the math I asked you to do. Not only that, just because the antenna is cut to resonate at the frequency your generator is putting out does not mean it is properly matched to the transmission line and generator.

    Two basic things are required for maximum power transfer into an antenna. They have both been mentioned.

    1) The antenna must be resonant at the frequency of the generator. At resonance, the antenna 'appears' to the source to be a single resistor.

    2) At resonance, the 'resistance' (you have referred to it as radiation resistance and I believe that is correct terminology) that is presented to the generator as a load must be the same as the output impedance of the genearator and must be the same as the characteristic impedance of the transmission line.
  8. Sep 10, 2004 #7
    Thanks, this is clear. Does exist a software which I can design simple antenna system i.e. changing parameters of the generator, transmission line and see the output of the antenna system? It doesn't need to be in one software, severval softwares will be fine. Right now, I'am using NECWIN95 for antenna radiation plot. I need a software to analyse the output data of the generator ( if a give a square wave form, the output will be the resonance frequency of the generator, amplitudes,and other useful information for the design of the antenna), any suggestions?

    Thanks again,
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