Radio-frequency transformers:

In the radio receivers and transmitters, transformers are categorized generally by the method of construction used. Some of them have primary and secondary windings, similar to utility and audio units. Others uses transmission line sections. These are the 2 most common types of the transformer found at the radio frequencies.

Coil types

In the wound radio-frequency (RF) transformer, powdered iron cores are used up to quite high frequencies. The toroidal cores are common, as they are self shielding (all of magnetic flux is confined within core material). The number of turns depends on frequency, and on permeability of core. In the high power applications, air core coils are sometimes used, because air, however it has a low permeability, has extremely low hysteresis loss. The disadvantage of Air core coils is that some magnetic flux extends outside of the coil. This affects the transformer's performance when it should be placed in the cramped space, such as in the transmitter final amplifier compartment.

A main advantage of the coil type transformers, especially when they are wound on the toroidal cores, is that they can work over a wide band of frequencies, like from 3.5 MHz to 30 MHz. These are called as broadband transformers.