An omnidirectional Wi-Fi® antenna is a vertical antenna that typically radiates its signal equally well in most directions. The signal strength decreases with distance and is often weaker directly above and below the antenna; a longer vertical frequently gives better performance. An omnidirectional antenna can also receive signals from all directions. Many wireless routers and Wi-Fi® access points use an omnidirectional Wi-Fi® antenna to reach all of the rooms in a home or office.
Several different designs exist for omnidirectional antennas. Some designs have more gain — signal amplification and reception sensitivity—than others. One of those with the least gain is commonly used on most consumer wireless access points and routers. Often known as the "rubber duck," it is named for the flexible rubber sheath that makes the Wi-Fi® antenna more durable. It is basically a piece of coax cable with a simple metal decoupler separating the shielded and unshielded portions of the cable. Compared to a perfect isotropic antenna, this design has a typical gain of 2-2.2 decibels (dBi).
An omnidirectional Wi-Fi® antenna with better gain is a long piece of copper wire with several carefully crafted loops in it; a gain of 4-15 dBi can often be produced, depending on the length and number of loops used. The wire is frequently encased in some form of plastic tubing to preserve its critical shape. Vehicle-mounted whip antennas are variations on this design. An omnidirectional Wi-Fi® antenna with about 6-8 dBi gain can also be created from eight very carefully constructed segments of coax cable. If space is very limited, a lower-gain alternative is a quarter-wave "spider" antenna—a radio frequency (RF) connector with a metallic base, a bit of coax and four stiff wire "legs" to form a ground plane.
A directional Wi-Fi® antenna may also be used in a home or office environment. In contrast with an omnidirectional Wi-Fi® antenna, it focuses the transmitted signal in just one direction; it also receives signals mainly from that same direction. The gain of a directional antenna is often much higher than an omnidirectional one, by 15-24 dBi or more. The signal is usually concentrated with the help of a metallic cylinder, dish, mesh or other reflective surface; this makes it ideal for longer-distance point-to-point communications as well. Both types of Wi-Fi® antennas can be used together — an omnidirectional one to serve a small area, in combination with a directional one to connect to a distant network.
It is possible to build a Wi-Fi® antenna or purchase one ready-made. Many books and websites illustrate how to assemble different types of Wi-Fi® antennas. They can often be constructed using simple tools and inexpensive materials. Some creative people have even built directional antennas from tin cans and cooking woks. A large number of commercial vendors sell mass-produced Wi-Fi® antennas as well.
