RF Choke with toroid?

Discussion in 'Homebrew and Kit Projects' started by YY5ARR, Aug 28, 2010.

Thread Status:
Not open for further replies.
  1. YY5ARR

    YY5ARR Ham Member

    Hi Experts,
    I know it is possible to build up a "home made" RF choke with a ferrite rod. Is it the same if a use toroid? any special instructions for the winding?

    I know the issue is: let the DC goes trough and Block the RF. But is there any diferent behave when using toroid?

    mil gracias!
  2. KI6J

    KI6J Ham Member

    Since the experts are sleeping now, I will answer in the meantime.

    Use the toroid. No special instructions, just make sure you have enough turns to get the "choking" resistance sufficient at the frequencies of operation. Toroids became more popular than rods because the toroid wound inductors have less tendency to radiate and interact with other components. Use a binocular (multi aperture) core or two beads side by side if that is all you have; fewer wire turns are needed.

  3. VK2TIL

    VK2TIL Ham Member

    The principle is the same whether you use air, ferrite rod or ferrite toroid; as you say, block RF, pass DC.

    There's a bit more to RF chokes than it seems at first glance; the cause is stray capacitance.

    This combines with the inductance to form a resonant circuit; indeed, there can be more than one such resonance.

    I have borrowed this plot from K8ZOA's website (the complete article is at http://www.cliftonlaboratories.com/self-resonant_frequency_of_inductors.htm and it's very much worth reading);


    Note how the impedance of this choke varies with frequency.

    So yes; you can use windings on toroids as RF chokes. In fact it's widely done; the book EMRFD, for instance, contains many examples and you will find lots of others.

    Ideally you would test your choke to ensure that it's providing the impedance you want at the frequency you wish to block.

    Here's a randomly-chosen data sheet to show how reputable manufacturers specify the self-resonant frequency (SRF) of their products;


    The general rule is to have the SRF well-above the frequency at which the choke is to be most effective.
    Last edited: Aug 28, 2010
  4. W9GB

    W9GB Ham Member

    Building an RF Choke -- DIY Style


    A Grid Dip Oscillator (GDO) is a useful tool

    RF Coil Inductance

    H = Coil height (inches)
    R = Coil radius (inches)
    P = Coil pitch - distance between turns (inches)

    This is what your DIY home-made choke balun should look like:

    Balun Designs - 1:1 Balun 1.5 - 54MHz 5kw - #1115

    The usage of Teflon coax permits a small O.D. and kW usage!

    Last edited: Aug 28, 2010
  5. YY5ARR

    YY5ARR Ham Member

    Hi Guys, thanks for the prompt answer and the several source of info you have provided to me.

    Important thing is to be sure that the choke is not resonant with the frreq i am pretending to bloc, otherwise will do the oposite.

    I Have been moving with some project and it is time now to buy / build up a grip dip meter!


  6. VK2TIL

    VK2TIL Ham Member

    Important thing is to be sure that the choke is not resonant with the frreq i am pretending to bloc, otherwise will do the oposite.

    Not exactly; the principle is to have the choke SRF above the frequency you wish to block.

    Let's again borrow K8ZOA's diagram (thanks Jack);


    This choke has a SRF of about 1.8 MHz; at that frequency its impedance is very high, about 200 k-ohms.

    Many designs use this "resonance" feature to advantage; if we were only concerned with blocking 1.8 MHz and weren't concerned about other frequencies, this choke would be perfect.

    If we were only concerned with blocking frequencies of, say, 150 kHz to 1.8 MHz, this choke would also be good; at 150 kHz it has an impedance of about 1 000 ohms and this increases to the maximum of about 200 000 ohms at 1.8 MHz.

    If we wanted to block frequencies around, say, 100 Hz (power supply ripple), this choke would be useless.

    There is something that this particular chart doesn't mention, although it's well-covered in the article itself; above 1.8 MHz, this inductor becomes capacitive!

    It's a bit difficult to grasp the concept that an inductor becomes a capacitor but read Jack's article carefully.

    So that's why a choke should be used at frequencies below its SRF.

    You will find a GDO useful if you are playing-around with coils and tuned circuits.

    A GDO may not give perfectly-accurate results when investigating the SRF of toroids; this is because you require fairly long leads to allow the GDO coil to resonate the toroid.

    An alternative is a link-coil connected to the GDO but, in either case, the already fairly-small stray C of the toroid winding is altered by the leads or by the link coil.

    But the results should be OK for most work; there will be stray C when the choke is connected into the circuit and any added stray C would give a lower SRF so, if you stay below the SRF you measure, you will be safe.

    A GDO is a very good way to learn about resonant circuits; they can be built or bought; they seem not too expensive these days on ebay and elsewhere.

    This is a great site for learning about GDOs;

  7. AB3CV

    AB3CV Premium Subscriber

    google k9yc
  8. YY5ARR

    YY5ARR Ham Member

    Hi Kerry, Thanks for the clarification! I will make a homemade GDO as a starting.

    thanks again,

  9. AI3V

    AI3V Ham Member

    That's OK, because above the capacitor's SRF it becomes inductive (at least for a little frequency):eek:



    P.S. Be carefull about running DC thru a inductor, the current will cause more or less of a change in the magnetic permiability of any iron powder or ferrite core.

  10. KI6J

    KI6J Ham Member

    "Be carefully about running DC through an inductor, the current will cause more or less of a change in the magnetic permeability of any iron powder or ferrite core."

    Oooh. I hadn't thought about this before. To the test bench...
Thread Status:
Not open for further replies.

Share This Page

ad: ARR