Ft240-31 toroid RF choke/1:1 balun build help.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7LPI, Apr 17, 2021.

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  1. KJ7LPI

    KJ7LPI Ham Member QRZ Page

    Hello, I am in the process of building a 5 band fan dipole ( 80-40-20-15 and 10 meter) and am wrapping the ft240-31 ferrite toroid with 16 gauge silver coated copper insulated wire. I am wondering how many wraps and what directions? I am finding multiple ways of doing it and the one I tried which says 12 wraps one way and 12 wraps the other way with 2 different wires didn’t fit on the toroid. So now I have 13 wraps of 2 wires and that’s all that will fit. Am I on the correct track or is there a calculator that I can use to figure out what I need to do? Thanks all in advance. I’m just learning this stuff.

    Attached Files:

  2. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    This website has a number of FT240-31 core choke designs to chose from. For 80-10 meters the 12 turns of RG-58 looks to be the best compromise. To see how much the ~30" of ~100 ohm line comprising the choke alters the 10 meter SWR I ran an EZNEC simulation. It lowers the 10 meter band resonant frequency and that can be compensated for by a bit of dipole trimming, 3 inches or so. 24 turns is too many and will compromise the choke common-mode impedance on the upper end of the HF band as shown by the 17 turn design.
    Last edited: Apr 17, 2021
  3. K9UR

    K9UR Subscriber QRZ Page

    Last edited: Apr 17, 2021
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  4. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    That is an interesting choke design. Rather than built as a transmission line as RF chokes tend to be it utilizes two coupled inductors as is used in things such as EMI CM chokes.
    SM0GLD likes this.
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    It is not a suitable design for RF. It will result in increased leakage inductance.
    The leakage inductance is equivalent to putting series inductors on both wires and that will de tune the antenna.
    KM1NDY likes this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    More misinformation on YT. That is not the way to build a 1:1 current mode balun (aka Coax CommonMode Choke). This way works:


    You can also use two parallel, insulated wires wrapped together bifalar style. Look at K9YC's 2018 Cookbook for details.
    Last edited: Apr 17, 2021
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  7. KJ7LPI

    KJ7LPI Ham Member QRZ Page

    Thanks for the info! Looks like I need to use 2 cores for a 80-10 meter choke? I’m wanting to use wire wound because my feed point will be wire with each dipole feed point coming off that wire every 5.5 inches
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    You can split coax center and shield to two parallel conductors which are then wound through the core or you can wind (small diameter) coax through the core; either one adds a short section of transmission line to the total feed line length. Both methods are mostly equivalent except that the characteristic impedance of a short section of coax is different than the parallel wire transmission line, so the final overall antenna system SWR can be a bit different.

    One reason to stack two cores is to get sufficient choking impedance at 80m. Problem is, that usually means that the choke isn't great above ~40m. A single 2.4inch type 31 core with 15 turns of coax would work well on 80m to 30m. If you put that in series with a second 2.4in core wound with 10turns, that series combination creates a choke that is usable from 80m to 10m.
    Last edited: Apr 17, 2021
    K9UR likes this.
  9. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    A single core choke will work and coax or wire can be used and wire, I would use twisted pair.

    SM0GLD, I agree. The choke shown in the video is not as good as the usual transmission line transformer. As you say it has additional leakage inductance and I see it has increased capacitance between the two wires further increasing the effective transmission line impedance. The ideal for this application is a 50 ohm transmission line and that is coax.
    Last edited: Apr 17, 2021
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  10. K0IZ

    K0IZ Ham Member QRZ Page

    Jerry Sevick's W2FMI (SK) book "Transmission Line Transformers" is a superior technical and practical text on the subject. Highly recommended.
    VK4FUQ likes this.

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