I hadn't expected this to work out so easily. I already had a selection of bifilar wound transformers wound on these cores so it seemed like a good place to start. Terminated in 12.5Ω I looked at the 50Ω connection with the nanoVNA:
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| 8t bifilar on a single core |
Another single core with 12turns bifilar was tested and it was too many turns. Good for 160m to 40m though.
Shifting to two cores side by side like a binocular core with 7 turns proved useful for all of the HF bands. Five turns could be a suitable starting point for 6m.
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| 7t bifilar on two cores binocular style |
To check the insertion loss I wired two of these transformers back to back. (50:12.5 + 12.5:50)
The loss was negligible.
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| Loss across two transformers back to back |
With 1W applied to this arrangement for 15 minutes the cores showed no sign of heating, perhaps 1degree above ambient.
The goal was to find a suitable transformer for matching 50Ω to input of a 10W amplifier. If a 4:1 transformer is the solution then this has been achieved. But a 9:1 transformer may be required so another round of testing is needed.
73's
Richard
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