I have built a simple impedance matching autotransformer for LF/MF use. There is nothing special about this design, except perhaps the addition of a DPDT switch which allows the transformer to be used as a Lo-Z (50 ohm or less) or Hi-Z (50 Ohm or more) transformer. That is useful to me as my antenna has a higher than 50 Ohm impedance at 136 kHz, and lower than 50 Ohm at 500 kHz.
The circuit diagram should be obvious by the picture below ;-)
