I would like to construct a 9:1 unun that connects my QRP transceiver to a random-wire antenna. The transceiver's antenna connection is an unbalanced BNC jack and the random-wire antenna is, well, a random wire. ;)

I have several toroidal ferrite cores laying around from disassembled power-supplies and such, but the many websites I've visited that show the construction of the 9:1 unun have not specified what type/mixture/diameter of toroidal core I need.

I'll be operating mostly on 20 and 40 meters HF.

If I were to order a toroid just for this purpose, which would be ideal for this application?

From the future -- Here is an answer that gives a lot of good information: https://ham.stackexchange.com/a/6145/1738

  • $\begingroup$ Do you mean "balun"? $\endgroup$
    – Andrej
    Mar 10 '16 at 14:53
  • $\begingroup$ Welcome and thanks for asking a great first question! Here on Stack Exchange, we don't put signatures on posts — the user card automatically provided at the bottom is your signature, and you can put whatever you like in your name and profile. Also, it's better not to say “EDIT:” — just edit your post so it reads like it was always that way. I've made these changes for you. $\endgroup$
    – Kevin Reid AG6YO
    Mar 10 '16 at 16:31
  • $\begingroup$ @Andrej A BNC connector and a random wire antenna are both unbalanced connections, so this device is an unun. $\endgroup$
    – Kevin Reid AG6YO
    Mar 10 '16 at 16:38
  • $\begingroup$ @KevinReidAG6YO Thanks, I didn't know this. $\endgroup$
    – Andrej
    Mar 11 '16 at 12:07
  • $\begingroup$ How does one read a ferrite datasheet? $\endgroup$ Mar 15 '16 at 11:46

Most manufactures that sell ferrite materials will say something about their intended application. HF isn't especially demanding. You'll probably want to avoid anything intended for EMI suppression, since these materials tend to be lossy since the goal is specifically to convert EMI into heat.

You can also just search around the internet for people who have built similar things and do what they are doing.

Having narrowed down the choices, you'll want to read the datasheet and select a core which

  • has low loss and won't overheat,
  • won't saturate at your maximum transmit power,
  • meets any dimensional constraints you might have, and
  • provides sufficient inductance to not adversely affect the impedance seen by the transmitter.

KB9RLW Kevin Loughin has a good YouTube video on his 9:1 unun build.

He used it for his HF vertical fed like an end fed antenna. I built one of these with full success for an elderly ham who also uses a manual tuner and/or his rig tuner which will load so many HF bands 6-160 meters. Yah, hard to believe - how? There is a 3" coil with about 20+ turns inline on the vertical and with 2 tuners makes it happen – hi hi. Easily works on 15-80 so he was tickled happy.

As far as type of core I used - 31 mix 240 size x 2 similar to KB9RLW's. We used silicone insulated wire but try any good solid copper - heavier if running power. We used 14 and 16 gauge as he only runs 100watts. I believe this one will take 500 watts though with dual cores. It pays to have books on unun/baluns like the CQ publication with Jerry Cevick - or others.

Consider a hybrid design that has a 1:1 core wound above the 9:1 unun. Don’t be afraid to experiment some - you will be proud like us of the results. By the way, I wound a 4:1 balun 1st and it did not work for so many bands as this one did using a couple radials and copper rod - this is worth it.

  • $\begingroup$ Welcome to hamSE! Here on Stack Exchange, it is customary to use lower case, as ALL UPPER CASE is difficult to read. And we don't put signatures on posts — the user card automatically provided at the bottom is your signature, and you can put whatever you like in your name and profile. I've made a few of these changes for you. Also, kindly edit your post and place a link to that video here. :-) $\endgroup$
    – Mike Waters
    Nov 1 '18 at 20:17

Seek out retailers who cater to the Ham Radio consumer for information. For instance, Buxcomm sells toroids which are specifically indexed based upon your intended use. I am sure that there are other retailers who will provide the same help, and it may be wise to get multiple sources to seek a range of agreement.

Without your intended frequency range it is difficult to comment on which exact toroidal core material to use, but indications are that a "Type 43" with a mu of 850 is appropriate for 2 through 60 Mhz. Other manufacturers also use colored lacquer to indentify charactaristics. A 2 - 30 Mhz toroid may have a red lacquer color (type 2), at least from some manufacturers.

In any case, I would suspect that toroids which came from power supplies would not be appropriate as they deal in very low frequency AC. Thankfully, the Ham specific ones are relatively inexpensive.

  • $\begingroup$ Thanks for that "I'll be operating mostly on 20 and 40 meters HF." $\endgroup$ Mar 12 '16 at 0:29

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