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A typical antenna has symmetrical output of 300 ohm impedance, while coaxial cable is unsymmetrical load of 75 ohm impedance. This is why an impedance transformer must be used. The antenna impedance transformer is passive RF device which both transforms the impedance and adjusts the output to standard 75 ohm lines. Usually the HF transformer has ferrite core. Sometimes such transformers are optimized for work in a particular band, e.g. C0001 - for channels 1-12, while impedance transformers working on channels 21-69 are commonly made in PCB form.