Bill,
If you are using the autoformer (single winding with a tap) design
from Tom's web site, I'm not surprised to hear about the limited high
frequency response. Autoformers are not particularly broadband (see
Fig. 3-12 in W2FMI's Transmission Line Transformers book). And they
can be sensitive to winding technique, but more so on a toroid than a
binocular core.
I'd recommend a transformer with separate primary/secondary
windings. 5 turns / 7 turns on the same BN202-73 core will give you
an accurate 1:2 impedance ratio and will easily cover from BCB to 20M.
73, Gary
K9AY
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