Leaving one wire open and grounding the other you typically achieve 12 dB
F/B that's finally not bad; the primary windings of transformers at RX side
need always to be alternatively terminated when not used (or connected to
different receivers).
If you use too high initial permeability core (i.e like some cores used at
50-60Hz) you might deal with a core material that has already high losses at
a few MHz.
Increasing the number of turns won't cure core losses and will result in a
transformer that's unable to reach 7 Mhz, a pity for an antenna that can be
instead designed to work decently on 3 octaves.
Amidon best materials for your purposes are 73 75 and 77 althought material
73 is not available in a toroidal form.
73,
Mauri I4JMY
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