another 9:1 UNUN question!

Hi all,

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 chech 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. L15 is easy to get too).
Regarding wires for the turns. Some use enamelled copper wire, some 3-colour for like a power socket.

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

Any help appreciated!

QRPGuys use a T106-2 Red for the 9 t 1 unun part of the tuner
Kit for QRP . 20AWG magnet wire for the windings.
Get a kit from EACHI they have all the right bits to make an UNUN box and all.
vk5cz …

Thanks for the pointer but I have all the bits to make one. No need for a kit but the question is about the toroidal type vs loss

I also like to future proof for more power.


You could do a lot worse than looking at the Amidon application notes and data sheets.

I have found the many articles available on absolutely fantastic. He is a very clever rf engineer who backs up his articles with calculations and measurements. None better.

Good luck

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I’ve done a fair bit of experimenting with small multi-tap UNUNs using FT143, FT243, and stacked FT143 cores… and have recently posted a vid on YT regarding UNUN power loss if you’re interested. Never, ever again will I wind a UNUN for a single Z ratio. 73 OM

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Hi Chuck,

Welcome to the group. I gather you had excessive losses. How much? Do you think it is related to the core material? Some materials are awful for transformers but OK for transmission baluns. What was the impedance of the winding? If too low losses would have been high.


Extra question. 9:1 turns ratio? (81:1 impedance ratio).


That’s not a great idea. Broadband impedance transformers need to be wound on ferrite cores, not powdered iron. This reference makes it quite clear: Unun 9:1 vs Unun 9:1 ferrite

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i used to be one for using Ununs on me portable antennas but have come away from there making more res antennas and antennas that don’t need it even at home.

One of the lastest I have made an inverted L being two traps 40m and 80m is 25m total length and is even res on 40m and upper part of the 160m band .But has at mo 40m of ground radials soon to be 115m. Works very well with tuner on 80m but trap needs shortening to bring its res freq higher. 40m is nicely res.

Play with ideas and designs on the net and so forth I started out with a simple G5RV when first started the hobby only brought antenna i have used to move on to a tad more complicated antennas.

Its a learning process but have moved on since the G5RV of many moons ago.

Karl 2E0FEH Kernow

so I should better go with this trifilar design?

Regarding other pionts: The question is not if its better not to use an UNUN, I need an UNUN to match a random length so Im trying to get the ‘best’ loss.


We need to clarify a couple of things as I am now unsure what you are trying to achieve…

I thought you were talking about a transformer even though you called it an UNUN.

The 81:1 impedance transformers (9:1 turns ratio) are designed to match resonant half waves, not any old length of wire. They are not really UNUNs.

A UNUN which is a transmission line device usually using 2 wire transmission lines. They most commonly have impedance ratios of 4:1. If you use three windings, preferably on three cores, you can get a 9:1 impedance match but it will be a BALUN.

Neither will match a random length of wire. A random length by definition has a random input impedance.

To match such an unknowm impedance an adjustable L network would be the first approach. Any standard ATU will match a small range of wire lengths.

Maybe you meant a half wave antenna and not a random length.



I agree. A random wire will present a different impedance on each band. I would be thinking of something similar to the MFJ16010, which is an L network, a variable capacitor and a toroidal coil with taps. You can connect it to the transceiver either way around depending on whether your random wire presents a high or a low impedance.

@VK3AFW and @G8ADD
My understanding (and what I actually use for years) is that with a 9:1 UNUN like the links I have, you can get a non-resonant length down to a manageable 3 SWR. When I measure my random wire antenna I get around 3 SWR over most bands. And then the KX3 tuner makes it 1.1 usually.
All this include 10 meters of coax before the KX3.
I know there are other “tuning” typologies that are better but my setup and location of the ‘random length’ element require the setup Im aiming for.

My question is specific to the UNUN and the core material. Especially regarding the power loss.

Was this a bit clearer?
Thanks a lot

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Hi, yes much clearer thanks.

Peter VK6YSF has published his tests using an L15 core trifilar 5 turn winding (1:9) vs a 10 turn trifilar winding on a 200-2 core (1:9) . The first transformer had good performance (low loss and consistent ratio) on a 450 ohm resistive load. The other core gave poor results. VK1OD confirms this in comments on a similar design and points out the flux leakage is high for transformers wound on type 2 cores and so the results (loss and ratio constancy) will be disappointing.

The 61 material may give better results than the 2 material but given that the L15 is so good why bother? I’ll let you do your own research on the 61 material.

So bearing in mind my approach would be a remote ATU I suggest you use your L15 core for a 1:9 transformer with low loss coax back to the ATU. How well it will work with unknown reactances I don’t know. For the lower frequencies a 1:1 or 9:1 ratio might be more effective. Remote switching?


hehe this will be a crazy setup for my little space :slight_smile:

Thanks for the overall response!