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