Define impedance matching requirements
The antenna impedance is not always 50 ohms, so impedance matching technology is required. Some antennas, such as chip antennas, have low or high impedance during manufacturing. For other antennas, such as printed antennas, it may be difficult to design antennas that fully meet the 50 ohm target impedance; The wiring may be very wide, or the antenna may take up space on the circuit board. The ultimate result is the need to reduce the antenna size, resulting in impedance mismatch.
In addition, the antenna and its matching network may be connected to a short feeder line, so the input impedance of the feeder line may not be equal to its characteristic impedance. On the contrary, the input impedance needs to match the output impedance of the transmitter in order to minimize the return loss at the antenna input and feeder input as much as possible (S11). Although devices with integrated RF transceivers typically use or can be configured with 50 ohm on die termination, this does not mean that impedance matching is perfect.