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A 100 uH to 1000 uH choke with a could coupling (low differential mode impedance respectively leakage inductance). I previously used Wuerth SL2 for a similar 13.56 MHz transformation. At 40 MHz, you have a considerable effect of leakage inductance, it would be possible better to use a mini-circuits RF transformer.
As another point, the transformer output is loaded with 100 ohm, so the input impedance is about 25 ohm.
To achieve an voltage increment at all, the driver impedance must be below 25 ohm.