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I need to build another 9:1 unun and I start searching for my toroids... (I have build 2-3 something like 4 years ago)... In parallel I checked on Google about the correct toroid etc so I found 2-3 pages with designs. My focus is on power loss as I run QRP on batteries.

9:1 voltage unun using a L15 ferrite core

9:1 Voltage Unun - FT240-61

9:1 voltage unun using a T-200-2 powdered iron toroid core

What do you think regarding power losses and materials? Which toroid should I use? (I already have the *-61 and the *-2).

Regarding wires for the turns, some use enameled copper wire, some 3-colour for like a power socket.

The unun is not for portable. So a size 200-240 is okay for some future proofing.

If I can get ok losses down to 80m, I'll be very happy, I mainly use 15 to 40m.
It's for a “random wire” end fed. Probably around 58ft max. Or maybe 53ft (space restricted)

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  • $\begingroup$ Welcome to this site! The [balun] tag also covers unun, so I added that. $\endgroup$
    – Mike Waters
    Commented Feb 20, 2020 at 21:55
  • $\begingroup$ This is for an end-fed antenna, right? Is it the same antenna for all those bands? How long is it? $\endgroup$
    – Mike Waters
    Commented Feb 21, 2020 at 2:19
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    $\begingroup$ Its for a “random wire” end fed. Probably around 58ft max. Or maybe 53ft (space restricted) $\endgroup$
    – OHTO
    Commented Feb 21, 2020 at 7:43
  • $\begingroup$ You might find this similar question and the answers particularly useful. On some bands, a 9:1 transformation ratio (50 ohms to 450 ohms) might not be optimal, as the feedpoint impedance might be much higher or much lower than 450 ohms. $\endgroup$
    – Mike Waters
    Commented Feb 21, 2020 at 13:36
  • $\begingroup$ I copied your comment with your antenna info into the answer. Note that comments are not searchable, and such details should be in the answer itself. $\endgroup$
    – Mike Waters
    Commented Feb 21, 2020 at 13:56

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9:1 transformer (Ruthroff transformer) is often used with antennas of high impedance (450 ohm or higher). So, the transformer is best wound with a transmission line (twisted or parallel wire) with a characteristic impedance about 150 ohm. However, that would require too thick insulator and very thin copper core, which in turn requires big cores and too few turns due to the size of the hole(s). So, a compromise is to use a transmission line of about 100 to 120 ohm lightly twisted trifilar wires. That can be accomplished copper wire jacketed by PTFE, FEP, or polyethylene, and about 24 AWG is good enough for 200W or less. I would avoid more common PVC jacket because the dielectric loss of PVC is 10x higher than PTFE, FEP, or PE. PTFE and FEP are most desirable because they are heat resistant. PE is good enough for QRP if you are careful in soldering.

If you have some odd ends of Cat5 or better ethernet cable in your junk box, break the sheath and untwist the twisted pairs. You can pick 3 and make a trifilar and use that to wind your 9:1 Ruthroff transformer. Those individual wires in the Cat5 cable are high quality oxygen-free copper of 24 AWG with polyethylene jacket.

Regarding the core, the best shape is binocular, but most factory-made binocular cores have small holes. The next best is to construct a binocular core using two tubular-shaped cores (like rigatoni pasta). Fair-rite 43 or Laird 28 material is good for your frequency range. Fair-rite 61 core is also good but you'll need to wind a lot more turns. I would NOT use carbonyl iron dust (like Micrometals 2 core) for this transformer.

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