Martin,
Here my answers to your last 3 posts.
Re splitting the winding:
Yes, as an autotransformer with a tap after 3 turns.
Below an example of a (not recommended) splitted winding, meaning the first 3 common turns. Also not recommended is the Reisert winding method, which is intended, for practical reasons, for winding toroidal cores with coaxial cables.
Photo below copied from the beginning of this thread.
Re measuring transformer efficiency:
Yes, these points have also been discussed on this reflector, but don’t worry about your question.
Useful and also recommended by Owen Duffy: A transmission measurement (s21) with a single impedance transformer (terminated with the nominal resistive load, or for perfectionists: the nominal load minus 50 ohms to account for the meter input impedance), usually performed with a VNA (photos of my measurement setup below).
Back to back measurements with 2 impedance transformers are often made for practical reasons (e.g. because of the capabilities of the measuring device or the math skills of the tester) and are not good enough for certain purists, hi.
BTW, The transformer efficiency of my very first 1:64 EFHW impedance couplers with a FT-82-43 toroid and 3 primary turns was something like this:
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65% at 3.6MHz
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70% at 7.1MHz
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72% at 10.1MHz
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74% on 14.2 and 18.1 MHz
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75% on 21.2, 24.9 and 28.5 MHz
The measurement of the transmission properties when the ferrite core heats up due to use and/or extreme ambient temperatures is perhaps a bit too academic for our needs. There are scientific studies to investigate the temperature-dependent behavior of the complex permeability of ferrite toroidal cores (example for NiZn ferrites below), busy readers could read chapters 1 and 4 at least …
So just complicate things as necessary - and use the time to climb mountains and make QSOs, hi.