Having unwound some of the cores I recently measured and finding some were not bifilar wound at all I started again.
I took another, different small core, and wound a few turns on it. The exact number is unimportant at this point. It's the red curve below. I then wound a bifilar winding on another identical core with the same number of turns, that's the yellow line when the second winding is open circuit. What I noticed was the bifilar wound toroid appeared to be "stretched". When terminated in 200 ohms the blue curve eventuated.
Where either the red or yellow curve crossed the R axis, that frequency was a reasonable approximation of the frequency where the 200 ohm load was transformed to 50 ohms.
I picked up three cores, the same as each other but different from previous tests, and wound 9,10 and 11 bifilar turns on each one.
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| 9 turns |
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| 10 turns |
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| 11 turns |
Again, this relationship of the frequency where the R axis is crossed when the load is open is close to the best frequency for a 200:50 ohm transformation.
I believe I'm onto something now. Let's repeat this with 3 larger, EMI suppression cores taken from a VGA cable. Not the huge ones, but about 15mm long.
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| 7 turns |
With 7 tuns I had a transformer suitable for 2 MHz (marker 1) to over 10 MHz (marker 2). I will repeat this for 6 turns and if suitable use that transformer in the test boards for general purpose transistors as RF amplifiers I am working on.
Conclusion:
You can quickly determine the frequency a toroid can be used to make a 200:50 ohm transmission line transformer with a bifilar winding.
- If the toroid has a single winding, or adding some turns if it has no windings, adjust the turns until the R axis is crossed at the lower end of the frequency range you are interested in. Then replace the winding with a bifilar wound transmission line.
- If the toroid already has a bifilar winding you can treat it like a core with a single winding by leaving the hot end open circuit.
73's
Richard VK6TT
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