Tim Shoppa wrote:
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
For that matter you can also get a T-37-6 or T-50-6 toroid and make your
own transformer. You'll have to use a trimmer cap to tune it since
toroids don't have tuning slugs but the end result won't be much bigger.
www.kitsandparts.com has toroids cheaper than anybody else and they have
all the winding data on their site.