Application note from Mini-Circuits about transformers, this time the RF transformers. Link here (PDF)

RF transformers are widely used in electronic circuits for

* Impedance matching to achieve maximum power transfer and to suppress undesired signal reflection.

* Voltage, current step-up or step-down.

* DC isolation between circuits while affording efficient AC transmission.

* Interfacing between balanced and unbalanced circuits; example: balanced amplifiers.