Ferromagnetic Inductors

Like most hams, I’m kind of a pack rat. I’ve got boxes of resistors, capacitors, and even vacuum tubes that I’ll probably never use. But you never know….

I’m kind of like that with e-mail, too. When I receive an interesting e-mail, I save it to a folder or to a file on disk where it will sit until I junk the computer. You never know, though. Every once in a while I’ll come across one that’s actually useful.

This afternoon, I was searching through my voluminous saved e-mail looking for something I sent to a business associate, when I happened upon an e-mail from a local ham. The message included a URL to a web page that described an interesting QRP antenna, the RockLoop. The RockLoop is a small, square loop antenna measuring 33-in. on each side, tuned with a 100 pF variable cap and a 4:1 toroidal transformer. (Heres’ another URL describing the RockLoop.)

It looked easy enough, and I thought I might actually try to put one together, except that I didn’t have any of the T-50-2 cores on which to wind the transformer. So, I called up my local radio store to see if they had any. Of course, they didn’t have any T-50-2s, but they did have a bunch of T-50-6s. That got me to wondering aboutthe difference between the two cores. So, I did a Google search.

One of the first URLs I came up with is a great tutorial on ferromagnetic core inductors. It talks about the different types of core materials and how to choose the right core for your application. There’s also a chapter on winding tips. This is a very cool site.

Next, I found the Amidon website. This website has a product selector online that describes the properties for all the various. ferromagnetic cores they sell.

Finally, it struck me that I sell a book that talks about all this stuff, namely Ferromagnetic Core Design and Application Handbook by Doug DeMaw W1FB. This book talks about all the different applications of ferromagnetic cores, rods, bars, and beads, as well as give technical data on the various materials and commercially-available products. It’s cool because it not only delves into the theory, but also gives you some circuits to build, if you’re so inclined.

Using these three resources, I was able to determine that the T-50-6 would indeed work for the RockLoop antenna. Its frequency range of 10 MHz – 50 MHz, while the T-50-2 has a frequency range of 2-30 MHz. Since the RockLoop is designed to operate on the 30m, 20m, and 15m bands, the T-50-6 should work OK, although I’m a little concerned that I may be pushing it on 30m. So, while it may work just fine, I think I’ll ask them to actually order some T-50-2s. They look to be a lot more versatile, anyway.

Update 4/26/04: Since writing this, I’ve been searching for sources for these cores. I’ve come up with three:

The first two sell Amidon cores, but the selection is limited. For example, in one article, the author used an FT-240-67 core for a balun, but neither of the first two sources seem to carry them. CWS Bytemark does carry them, though, and you can order right from their website. They also carry all the wire and tape you need to build your baluns.

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