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.

Impedance transformer (UnUn)

Ferrite Toroid FT-140-43. Should be enough for 100W PEP.

The impedance for a dipole 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.

As the frequencies (18MHz and 24MHz) are relative high, I will use the 2x14 windings UnUn.

See UnUn impedance transformer for End-Fed for more details.

Measurements

17m (within band)

17m broader frequency range

to be continued

Editors remark

File names: EndFedWarc99m[SWR|Z|Smith|Data]{optional range in kHz}
LCD2clip to make screenshot (F+6)
Alt+PrtSc and copy to Gimp export to png
FileZilla to ../files/inline-images/