Question about ferrite mix selection for a homebrew 4:1 Guanella unun

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9YOJ, Oct 26, 2021.

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  1. KB9YOJ

    KB9YOJ Premium Subscriber QRZ Page

    Hello All-

    So I am in the process of figuring out how to make a 4:1 UNUN, to transform 200:50 ohm feed point impedances for various antennas. I was thinking I would like to experiment with loops and OCF dipoles.

    I've also been studying Jerry Sevick's Transmission Line Transformers book to try to emulate his designs, but I have some questions about ferrites:
    • Many of his designs use ferrites that list the permeability of the core material, but not the core type. When it does, it seems like they are difficult to find. Q: Can I pick and choose from readily available ferrites? Some of the designs ask for high permeability. Is higher better, or should I stick to the book?
    • His designs sometimes use teflon wire and teflon tubing. Is this to get the proper wire spacing away from the core in order to replicate the desired results? Could I substitute teflon tape on the core in order to accomplish the same thing?
    I've made many chokes and ununs from scanning the internet, but I feel like I need to delve into here so I can learn a thing or two. Here is what I was thinking after some study. Make a 4:1 Guanella UNUN using a type 31 core for the 1:1, and a type 61 (or 52?)core for the 4:1 like you see here:

    [​IMG]



    Values listed for the cores:


    Mix # Material Initial Permeability RFI/EMI Common Mode Suppression Range Tuned Circuits – Coil
    Wide Band Transformer

    31 (1) MnZn 1500 1-300 MHz – 1:1 only, <300 MHz
    61 (3) NiZn 125 200-1000 MHz <100 MHz 1-300 MHz
    52 (6) NiZn 250 200-1000 MHz < 20 MHz 1-60 MHz

    I noticed the type 52 core has the same permeability (250) as what is listed in the design text, but I know that the type 31 is better at 1:1 common mode. Maybe I should use that?

    I am not an engineer, just a normal guy. So I don't know what any of this stuff really means beyond matching line to antenna. Any thoughts?

    Matt
     
  2. W1VT

    W1VT Ham Member QRZ Page

    Typically you want the same permeability as the original design unless you are trying to change the frequency range of the unun. A higher permeability will shift the optimum frequencies lower.
    But, when it comes to raising the frequency range, the length of the wire also comes into play.

    The Teflon tubing is for the spacing between the wires, not the wires and the core.
     
    Last edited: Oct 26, 2021
    KB9YOJ likes this.
  3. KB9YOJ

    KB9YOJ Premium Subscriber QRZ Page

    Thanks for the practical advice. With that, I can use type 52 cores which are readily available and copy the design. I don't have any reason to shift the optimization around, but it answers my questions.

    Matt
     
  4. W9IQ

    W9IQ Ham Member QRZ Page

    I presume you are talking about a Guanella balun that consists of two separated cores, each wound with parallel transmission lines that are then series and parallel connected. In this case, the common mode choking effect of each core is essential to its performance. For HF, this means the cores should be type 31 material. Generally the FT240-31 core size is used.

    Be a discerning reader when you have a Sevick book in your hand. He got a lot of things wrong including misidentifying his autotransformer unun designs as TLT baluns.

    - Glenn W9IQ
     
    WA7ARK and KH6AQ like this.

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