The impedance for a half wave antenna is high at both ends. Around 3 kΩ.
For a 50Ω coax feed line a 50x (2,5kΩ) to 70x (3,5kΩ) impedance transformer is needed.

A winding ratio of 4 (for example) transforms the voltage a factor 4 up and the current a factor 4 down (equal power). This means the impedance transformation is the square of the turn / winding ratio. So in this example a impedance transformation of 16, giving a 800Ω output impedance.

Primary and secondary windings are bifilar wound (and loosely twisted).

As the 30m band is CW-only, I will skip this band for the moment. I'm not (yet) a CW guy.

Leaving 17m an 12m to combine in one antenna.

Considerations

How to make the antenna resonant for both bands. The frequencies relate to each other as 3 to 4.

On the Internet there's lot of discussion about this End Fed.
One thread uses a full wave for 12m a coil and after that a wire of less than a meter to make 17m and 30m resonant. But how dus this work in theory?

So I dicide to start with one band: 17m.

These antennas can be compared on 20 and 40m. On 20m both antennas are half wave length. On 40m the Fan is half wave dipole, where the End Fed is extended and effectively a quarter wave, so about -3dB less gain.

Both antennas are about 10m above the ground at there feed point. Sloping down to about 5m above ground. Radiation direction is approximately North - South.

Base

End-Fed antenna for 20m, 10 meter in length.
FT-140-43 ferrite toroid impedance transformer (UnUn).

Extension

To make it a multi band (10/20/40/80m) antenna when little space is available: