Ignacio,
If in a previous post I recommended that you omit the designations 64:1 and 49:1 in your diagram, I was afraid that you might not fully understand the function of the circuit used. That actually seems to be the case now.
No problem, just consider the primary winding as a coupling loop for the tuned parallel resonant circuit, formed by the secondary winding and the variable capacitor. What is required here is the well-known Thomson oscillation equation, ok.
BTW, to achieve maximum efficiency with a multi-band coupler, both the degree of coupling (number of primary turns) and the parallel resonant circuit (capacitance AND inductance) would have to be variable, because both are frequency-dependent.
Note: Due to the load of a connected EFHW antenna, the parallel resonant circuit is of course slightly detuned, but can easily be brought into resonance with the variable capacitor.
BTW, Doug DeMaw W1FB did a pretty good job of illustrating his explanation of the subject, didn’t he?
QST April 1985, Resonance and Tuning Methods