Antennas are necessary for transmitting and receiving the radio-frequency energy that forms the basis of Wi-Fi communications. The underlying rule is you need a better antenna to improve coverage. Understanding fundamentals is essential for selecting a proper antenna for your application.
In general, antennas radiate radio waves when fed with the right kind of electrical power. Conversely, an antenna can also covert radio waves received by it into electrical power. There are different forms of antennas, some created intentionally, such as those on your wireless router, and others created naturally, such as the wires on your earbuds, which act as antennas. Antennas are usually directional, meaning they are better in transmitting and receiving radio waves in some directions than in others. However, there are omnidirectional antennas that work nearly equally in all directions.
Wi-Fi antennas are mostly dipole types, or more specifically, half-wave dipoles. They consist of two halves, each equal in length to a quarter of the wavelength they are to transmit or receive. A separate conductor from the feedline feeds each half separately. For example, for a frequency of 2.45GHz, a half-wave dipole antenna would be 61.22mm from one end to the other, while each half measuring 30.61mm. However, other parameters also affect the length of the dipole and the resulting antenna may differ considerably from theoretical calculations.
Examining a Wi-Fi antenna from a 2.4GHz wireless router reveals a hinged base connected to a plastic cover. The hinge allows antenna rotations irrespective of the mounting position of the router. Within the plastic cover, you can see the entire dipole antenna. One-half of the dipole is made of a metal cylinder through which the feeder wire passes. The other half is the wire itself that protrudes to the other side of the cylinder. With the metal cylinder and the wire insulated from each other, they form a dipole of approximately one-quarter wavelength long. Such antennas have a gain of about 2dBi and their radiation pattern is circular.
The antenna connects to the Wi-Fi radio transceiver via a wire feedline – a coaxial cable. This has an insulated inner copper conductor covered with an outer braided shield made of copper wires. A clear plastic cover encases the entire feedline. Wi-Fi devices use these feedlines, also known as coax and designated RG-178, specifically for their small size and relatively low RF losses.
Antennas are usually better in transmitting and receiving radio waves in certain directions. Their ERP or Effective Radiated Power is greater in those directions. Although the total radiated power remains the same, antenna gain refers to the increase in strength in several directions than in others. Therefore, simple horizontal dipoles show gain in two directions – parallel to the radiators on both the front and backsides.
Depending on the country that is using the Wi-Fi signals, there are five different bands of transmission – 2.4GHz, 3.6GHz, 4.9GHz, 5GHz and 5.9GHz, with correspondingly matched antenna lengths. Although the general principles apply to all bands, the most widely used transmission for Wi-Fi signals is the 2.4GHz band. Usually, this extends from 2.4GHz to 2.5GHz.