Antenna Types
A variety of antenna types are available. Your choice will be driven by the needs of your particular application. Because antennae are working with a fixed amount of power generated by the system, antennae reshape their energy pattern in a specific direction. For example, a busy coffee shop requires a signal in all directions (but not directly above or below the AP); point-to-point applications require highly directional antennae that focus all of the energy in one particular path.
To put it another way, you can think of antennae as sources of light. When faced with the choice of using a flashlight or a fluorescent bulb, you must understand if the application involves lighting up a hiking trail at night or a department store aisle.
Isotropic Antennae
The first type to consider is an isotropic antenna. By definition, isotropic antennae do not exist. They are a purely theoretical construct, designed to be used as a frame of reference for measuring other antennae. A hypothetical isotropic antenna (Figure A.7), radiates energy in all directions equally. Imagine a point in space, radiating energy in all directions—like a beach ball. Note that there are no physical parts (like a conductor) in an isotropic antenna. It simply distributes and receives energy in 360 degrees.
Figure A.7. Isotropic antenna.
Manufacturers will often rate the gain of their antennae in terms of dBi, the number of decibels when compared to an isotropic antenna. For example, the standard “rubber ducky” antenna built into a typical AP has a gain of 2.2 dBi. It should be logical that the gain for all antenna types exceeds the gain of an isotropic antenna, because energy from an isotropic antenna is essentially wasted in the area directly above and directly below the antenna (for most applications). Typically, we are not trying to provide coverage to the area directly below the AP (the ground) or the area directly above the AP (the sky). Of course, this changes if we are trying to cover multiple floors of an office building.
Omnidirectional Antennae
Omnidirectional antennae (Figure A.8) cover an area 360 degrees around the antenna in the horizontal plane, but limit energy directly above and directly below. This configuration creates a shape that resembles a doughnut. Most client cards and APs ship, by default, with an omnidirectional antenna. 5 dBi magnetic mount antennae have proven very popular for war driving applications because the antenna base can be attached securely to the roof of a moving vehicle.
Figure A.8. Omni-directional antenna.
Patch, Panel, and Sector Antennae
Patch, panel, and sector antennae all ship with a specific horizontal and vertical beam spread. For example, if you wanted to limit the range and directivity of the energy pattern, antennae could be purchased with a 95-degree horizontal and 7-degree vertical beam spread, or with a 60-degree horizontal and 60-degree vertical beam spread, etc. This is particularly useful for custom deployments that require a narrow coverage area (such as a specific classroom). Patch, panel, and sector antennae are also very popular because of their narrow profile. They are typically only a few inches thick and can be mounted on a wall or ceiling with very little protrusion. This is especially important for indoor deployments that have aesthetic requirements. Figure A.9 illustrates a patch antenna.
Figure A.9. Patch antenna.
Note that some sector antennae (Figure A.10) have the ability to manually adjust their range. Some manufacturers sell variable range sector antennae if your application requires a modifiable beam spread.
Figure A.10. Sector antenna.
Parabolic Grid Antennae
Parabolic grids (Figure A.11) are typically used for long distance point-to-point applications. They have a very high gain and highly focused beam spread. For example, you could purchase a 24 dBi antenna with an 8-degree beam. These are generally used in outdoor rooftop applications due to their larger size and less attractive appearance.
Figure A.11. Parabolic grid antenna.
Yagi Antennae
Yagis (Figure A.12) are another form of highly directional and high-gain antennae. Rather than being shaped in a grid fashion, Yagi antennae are narrow cylindrical objects, typically 1 to 2 feet long and a few inches in diameter.
Figure A.12. Yagi antennae.
Pringles® Can Antennae
Pringles® can antennae (Figure A.13) are an experimental form of the Yagi antenna. They are not FCC compliant and should not be used in a production environment. However, their creation can be a valuable educational experience. See Appendix D for detailed plans on building one.
Figure A.13. Pringles® can antennae.
Vivato
One of the most exciting areas of Wi-Fi discoveries has been in the area of phased array antennae (Figure A.14). Imagine having a directional antenna with a narrow beam width (say 9 degrees). This beam would have a long range, but narrow coverage area. What if you could tune this narrow beam to follow a client everywhere she went? That's the idea behind Vivato's antennae. In essence, they are an electronically steerable, directional antenna. This is very exciting because we can now cover an entire office suite with one antenna instead of placing APs every 150 feet. Vivato can provide several miles of Wi-Fi coverage from a single antenna!
Figure A.14. Vivato phased array antenna hanging on the wall.
