The correct answer is: C. air core
A high frequency transformer is a type of transformer that is designed to operate at high frequencies. High frequency transformers are used in a variety of applications, including radio transmitters, receivers, and amplifiers.
The core of a transformer is the material that is used to conduct the magnetic flux. The core material must have a high permeability, which is a measure of how easily a magnetic field can be created in the material. The most common materials used for transformer cores are iron and steel. However, these materials are not suitable for high frequency applications because they have a high hysteresis loss. Hysteresis loss is the energy that is lost in a magnetic material when the magnetic field is changed.
Air is a good conductor of magnetic flux, but it has a very low hysteresis loss. This makes air a good choice for the core of a high frequency transformer. Air cores are also lighter and less expensive than iron or steel cores.
The main disadvantage of air cores is that they have a lower permeability than iron or steel cores. This means that they require more turns of wire to produce the same amount of magnetic flux. However, the higher number of turns of wire does not offset the advantages of air cores in high frequency applications.
In conclusion, the core used in a high frequency transformer is usually air. Air cores have a low hysteresis loss and are lighter and less expensive than iron or steel cores. However, they have a lower permeability than iron or steel cores, which means that they require more turns of wire to produce the same amount of magnetic flux.