Notice that the primary to secondary (total) turns ratio is
almost 1:1, that both shunt capacitors in the band-pass filter are
of the same value and that the path resistance on either side of
the secondary, through the FST3126M to the low-pass filters is
around 20-ohms. So what you are looking at (to a crude
approximation) is a design impedance of 50-ohm all the way
through.


OK, that's about what I thought, but I was estimating closer to 15
ohms for ac resistance between the secondary winding and ground. So, I
was having a hard time believing it could be 50 ohms in and out. It
probably makes little difference though in the filter values or
passband response.


As for the toroid, for optimum Q you might want to look for a
mix more suitable for the frequency range you quoted (maybe 15?)
and increase the diameter to take the extra wire as you will need
to scale the impedances for use at 50kHz. The FT series, I think,
need less turns for the same inductance and so should be easier to
wind.


I was looking at the CT series, specifically a CT-50-57, which is a
little bigger, but probably can contain all the wire easily.

Finally, a simple low-pass filter would probably be all you
need here as the transformer primary impedance will be a limit the
low end. Filter response is dependant on source as well as load
impedance and most antennas that you would use at those
frequencies will be quite reactive and difficult to design a fixed
input network for. You may be aware that a lot of HF receivers use
a separate hi-Z antenna terminal for LF/MF.


I'm aware of the separate antenna inputs some LF radios have. The
lowfer group has quite a bit of information on antennas, and it seems
that making a 50 ohm antenna or something approaching 50 ohms from a
large single or multiturn loop isn't that difficult.

Making a 15 or 20 ohm antenna is even easier, perhaps I should think
about 15 ohm input impedance and a 15 ohm output impedance since it's
actually easier to handle the antenna step up.

Having said that, if you do want to use a simple low-pass
configuration, remember that a symmetrical 'Pi' filter with source
and load impedances of 1-ohm and a cut-off of 1-rad/sec would use
two 1-Farad capacitors and a 2-Henry inductor. To scale to new
impedances and frequency, divide C (and multiply L) by the design
impedance and then divide both C and L by the new cut-off
frequency in rads/sec.


Most likely it's probably best to keep it at 50 ohms and not to worry
about the slight mismatch in that frequency range.

Thanks to you and to all who commented, it was just he sort of
information I needed to put me back on track.

Regards,

T