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QRP 1:1 balun

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8QVO, Jul 19, 2012.

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  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    I have a few Amidon T80-2 toroids and I am wanting to wind a 1:1 balun for my QRP radios. I would like to use insulated wire, #28 or so computer ribbon wire (2 strands). Will this work or do I need enamel coated magnet wire?

    Also, are there any good designs for a 3-ratio switched balun? I need the 1:1 but it would be nice to have a 2:1 and 4:1 as well, more flexible in impedance matches.

    Steve, KC8QVO
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Can you say a bit more about the application: what's the frequency range? Is this for use at the output of a tuner? You said QRP, but that can mean different things to different people, so what power level? What sorts of antennas will you be using with it?

    But in anticipation of your answers, anything wound on a T80-2 core is likely to be unsatisfactory - the wire choice is not particularly critical, but it's essential to get the core material right.

    Steve G3TXQ
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    HF - 80-6 meters, mostly 40-17 meters. Antennas will be doublets (balanced feed line feeding a dipole-like, non-resonant radiator/counterpoise). Balun will mount directly to my KX3 radio with built in tuner, or a few inches away. Other applications may be with an LDG Z11-pro tuner remote mounted, again the balun won't be more than a few inches from the tuner.

    Power levels up to 15 watts CW/SSB.
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    For that application the choice of wire for the bifilar winding is not critical, although something with a thicker conductor than your #28 would be preferable (less lossy).

    Don't use a T80-2 core - that wont be satisfactory. I would use 15 bifilar turns on a FT114-43 core to form a choke (1:1 current balun). Make two of them: use one on its own when you need a 1:1 transformation; wire the second as a 4:1 voltage balun and connect it between the 1:1 and the tuner when you need a 4:1 impedance transformation.

    Hope that helps,
    Steve G3TXQ
  5. WA3UCR

    WA3UCR Ham Member QRZ Page

    It sounds like the Elecraft BL2 may be what you are looking for. Switchable between 1:1 and 4:1 and good for up to 250 Watts. I have never used one myself, but have heard good reports from other Elecraft users.

    Enjoy that KX3 ! I've been very tempted to pick up the phone and grab the Mastercard....
  6. KZ4Y

    KZ4Y Ham Member QRZ Page


    You can make/create a multi-balun/un-un for the ratios you want. The book by Dr. Jerry Sevick, "Understanding, Building, and Using Baluns and Ununs" has several designs for ratios from 1:1 to 9:1 using two small cores. As mentioned the core choice is critical. Most of W2FMI (J.Sevick) baluns/Ununs were built on 'K' cores. I've made several over the past couple of years and all work great. There are low and high power models and all are easy to build. My latest balun was a multi-core for 1:1, 1:2 and 1:4 ratios. I used #14 coated wire on three cores (one for 1:4 unun, one 1:2 unun and the last was 1:1 balun as the output to the antenna). While I used 2.4 inch cores for high power capability, you can could use the 1.5 inch or less and #18 for your qrp power levels. The design was simple: 1:4 was 10 bifilar windings; 1:2 was 6 trifilar windings with output tapped 5 turns from input side, and the 1:1 was 10 bifilar windings. I used three connectors - one to each core to select the desired ratio and one output from the 1:1. As mentioned, two cores were designed as unbalanced to unbalanced while the third core was unbalanced to balanced (going to the antenna). The 1:4 design you can find using Google (dx zone). If you want the designs send me a message.

    K8TDN likes this.
  7. NA7U

    NA7U Ham Member QRZ Page

  8. K8JD

    K8JD Ham Member QRZ Page

    Why don't you try the new MFJ balanced line tuner, you connect the balanced feedline (started with old TV twinlead feeder) right to the tuner input. The circuitry in the tuner is a little like the old Johnson Matchbox with a true balanced input fron the antenna and 50 ohm unblanced output to your rig..forget external baluns !Mine works great, I ordered the 160-10M model but there is a slightly cheaper version for 80-6M.

  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    I'm curious which MFJ model that is? I know they do a balanced tuner with a balun at the input followed by a balanced T-match, but didn't know they did a link-coupled model like the Matchbox.

    Steve G3TXQ
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