Toroids do not require a ferrite or iron powder core at all. The necessary
characteristics of a toroid are simply that it is in the shape we call a
toroid. The self-shielding characteristics of toroids are not dependent
upon the core.
For low frequencies, the advantage of the core is that it reduces turns
required - just as a ferrite or powder core reduces the number of turns of
a conventional coil.
As a lark, I prepared a portable field day station a few years ago. It was
powered by a small steam generator powering a qrp station. Since weirdness
was important, I assembled a balun using a bagel. It was the bagel balun.
One of the requirements of this balun is to have a hard bagel.
After winding it, I thought it proper to test it. Unfortunately, I must
report that it did not work well. SWR was acceptable at about 25 mhz only.
Losses were high, but I did not record them. Unfortunately, I was running
less than 100 mw so was unable to take advantage of any heating effect on
the bagel.
Perhaps someone can improve on this design. I have yet to figure out how
to take advantage of the losses in such a device to heat the morning bagel,
since it is inconvenient to have to wind the wire around the bagel to heat
it and then remove the wire prior to consumption. Some clamp arrangement
would be preferred.
Colin K7FM
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