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The antenna feed is the combination of all of the components of the antenna that are used for the receiving and transmitting purposes of radio frequency waves. The antenna feed can be regarded as that portion of the antenna ranging from the first amplifier to the front end transmitter in the case of the receiving antenna. In a transmitting antenna, the antenna feed can be regarded as the portion after the last power amplifier. The antenna feed includes the functions of transforming the radio waves into electrical signals and transmitting them to the receiver components. In general, it is considered to be part of the antenna that is being used for the conversion purposes of the radio waves into the electric signals and vice versa.
Antenna design should be made considering the maximum power transfer possibilities and the efficiency. For that purpose, the antenna feed impedance must be matched with the load resistance. The antenna feed impedance is the combination of the resistance, capacitance, and the inductance. To ensure the maximum power transfer conditions both impedances — load resistance and feed impedance — should be matched. The matching could be done by considering the frequency requirements and the design parameters of an antenna such as gain, directivity and radiation efficiency.
The feed impedance includes two resistive elements, which are the loss resistance and the radiation resistance. The loss resistance is the resistance that is offered by the actual components of the antenna and the feed impedance is the resistance offered at the input of the antenna to the signal. Thus, the loss and the feed impedance must work together in order to achieve a proper working antennae feed. The radiation resistance is the resistance offered by the antenna to the radiation power, or in other words, it presents the dissipated radiation power.
The efficiency, gain, and absolute gain are very important considerations in the design of the antenna feed. The total efficiency of the antenna describes the losses at the input terminal and through the antenna components. It accounts for the reflection, conduction, and the dielectric losses inside the antenna. The gain of an antenna can be regarded as the ratio of the radiation efficiency to the total input power. It is the product of the radiation efficiency and directivity. The absolute gain is the product of maximum efficiency and the directivity. The polarization of the antenna is also a very important consideration for the design and function of the antenna.