1/4 wave resonance calculation

Discussion in 'The Projects Forum' started by clone477, Jul 28, 2009.

  1. clone477

    Thread Starter Member

    Oct 3, 2008
    Hello everyone,
    I'm playing around with some tesla circuits, wanted to know the calculation for 1/4 wave resonance. Ive search around but can't seem to find that simple equation. Also does it matter what shape or form the lenght of wire is in?? example would be solenoid or flat spiral?? Thanks Guys
  2. k7elp60

    Distinguished Member

    Nov 4, 2008
    If you are talking about a length of wire then, the 1/4 wave length of an antenna in meters=150,000,000/frequency in cycles per second.
  3. clone477

    Thread Starter Member

    Oct 3, 2008
    Thats exactlty what I wanted. So that will give me the length of wire needed for a given frequency, so it will be 1/4 wave. Could you please explain, does that mean that with that formula, that anything that is at it's 1/4 wave in wire length will automatically be in resonance at that same frequency?? Thanks guys, Fern
  4. yourownfree

    Active Member

    Jul 16, 2008
    one problem though. When you wrap the wire into a coil form, you now have what is called inductance. Inductance together with capacitance will make your unit resonant. That is what you want. So if you know the frequency of your unit or what you want to output then after you wrap all the turns of wire to get the voltage you desire then you will need to measure the coil to find the inductance. When you know that then you calculate the capacitor you will need. I am assuming your coil will be in the mH range. In which case an easy way to calculate is take 25330/ frequency in mHz squared. use that s your constant value. now take that constant value and divide by the inductance in mH and you will get the capacitance you need in pF. Or you could divide the capacitance and hget the inductance you need. A simple inductance meter are simple and on the internet that you can use your dvm as a readout. hope this helps. 234/frequency in mHz will give you the length in feet for a straight length of wire 1/4 wave. wire is not perfect so the wave travels slower in wire than in free air. Also when wrapped in a coil the length of wire calculated will not be 1/4 wave resonant in a coil. The wire diameter is also a big role in your coil inductance. so when the coil and capacitor are resonant at your frequency, maximum power will radiate from the coil end (the sparks)

    Senior Member

    May 26, 2009
    For a 1/2 wave it is 468/f in MHz for a 1/4 wave it's 234/ f in MHz