What is the directionality of a Wi-Fi antenna?

　　What is the directionality of a Wi-Fi antenna?

　　The radio frequency signal power output by a radio transmitter is transmitted to the antenna via a feeder (cable), which radiates outward in the form of electromagnetic waves. Once the electromagnetic wave reaches the receiving address, it is followed by the antenna (which only receives a very small portion of the power), and then transmitted via the feeder to the radio receiver. Therefore, the antenna is the primary radio device for transmitting and receiving electromagnetic waves. Without antennas, there would be no radio communication.

　　There are many types of antennas for different frequencies, uses, scenarios, requirements, and other applications. Given the wide range of antenna types, it's necessary to classify them appropriately: by use, they can be categorized as communications antennas, television antennas, radar antennas, etc.; by operating frequency, they can be categorized as shortwave antennas, ultra-shortwave antennas, microwave antennas, etc.; by direction, they can be categorized as omnidirectional antennas, directional antennas, etc.; and by shape, they can be categorized as linear antennas, planar antennas, etc., and so on.

　　One of the basic functions of a transmitting antenna is to radiate the energy received by the feeder into the surrounding space; another is to radiate most of the energy in the desired direction. A half-wave symmetrical oscillator placed on a straight line has a flat, "doughnut"-shaped three-dimensional pattern. While robust, a three-dimensional azimuth pattern is challenging to manufacture.

　　The input impedance of a Wi-Fi antenna is the impedance at the interface between the receiving antenna circuit and the receiving circuit. Impedance matching between the Wi-Fi antenna and the transceiver circuit is used to minimize return loss caused by mismatches. Therefore, the input impedance of the Wi-Fi antenna must match the output impedance of the transceiver circuit. This prevents significant power loss between the Wi-Fi antenna and the transceiver. In typical Wi-Fi antenna designs, the input impedance cannot be significantly varied. A common design practice is to design the Wi-Fi antenna input impedance at 50 ohms, which matches the output impedance of the transceiver circuit, which can reach up to 50 ohms. However, in designs with additional transceiver circuitry, the output impedance may not be 50 ohms. In such cases, an additional impedance matching network is designed between the transceiver circuitry and the Wi-Fi antenna input to convert the Wi-Fi antenna input impedance to the transceiver output impedance.