https://www.dxzone.com/dx33342/efhw-autotransformer-comparison.html
EFHW autotransformer comparison - Antennas/End-Fed/End Fed Half Wave Antenna | The DXZone
Two different ways to create autotransformer for end fed half wave wire antennas, by using ferrite or air core.
end fedhalf wavethe dxzoneefhwautotransformer