On Oct 26, 3:36*pm, "QRP'er" wrote:
But it is amazing because the two FI have very different impedances :
42IF122 = 15K/0.3K
42IF123 = 25K/4K
Which are rather distant from one another...

KD1JV is using them as readily available components that can be made
to resonate on 40M.

That section is both a narrowband filter and a matching network; the
topology around two coupled resonant LC's guarantees the
narrowbandedness more than any individual component values. And
mismatches can largely be accomodated by tweaking the cores. I'm sure
there's some difference in core position depending on what part you
use but as long as it peaks up in the 40M band you're good to go.

If you don't have those Mouser cores, don't worry, you can tear apart
any FM radio from the past chunk of a century and find 10.7MHz
transformers in it.

Tim.