Reg, G4FGQ wrote:
"If the input impedance is LESS THAN 50 ohms then an unun makes it
worse"

A transformer can step down as well as step up. Early crystal sets
stepped down antenna impedance to the low impedance of the rectifier
using a continuously variable autoformer (a sliding contact on a coil).
Position was adjusted for best results.

Best regards, Richard Harrison, KB5WZI