The price of wireless networking hardware has fallen dramatically in a very short time. Wireless adapters now come standard with many computers, and off-the-shelf access points are commonplace, making it easy for just about anyone to set up an off-the-shelf wireless network. However, the prices of antennas and related components have also fallen sharply as high demand and extreme competition have driven the industry to an increasingly high-volume, low-margin business.
These lower prices have enabled do-it-yourselfers to experiment and find out just how little it takes to build a working network. There is something almost magical about radio networking. Tales of war driving (and even war walking) aside, just imagine that today in many cities around the world, dozens of invisible networks exist on any given street corner. As you sit at a cafe eating your lunch, you might be completely unaware of the dozens of people simultaneously using the environment around you to communicate with people around the world. I believe that it is largely this mysterious, intangible aspect of unseen global communications that draws people to embark on their own antenna projects. The deeply rewarding feeling of making something useful out of virtually nothing is worth much more than saving a few dollars on an off-the-shelf network component.
When comparing antenna designs, there are a number of important factors to keep in mind. The first antenna property that people usually refer to is gain. The gain of an antenna is a measurement of how well it radiates in the direction you intend it to, measured in decibels. This measurement is actually the antenna's performance as compared to an imaginary invention called an isotropic radiator (this is the i in dBi).
Imagine an infinitely small light suspended in the vacuum of space. It radiates light equally in all directions, and by definition has no gain in any direction. Now, take this light and place it in the head of a flashlight. Without increasing the brightness of the bulb, you can turn the head of the flashlight to focus its beam in a particular direction. This is gain. By directing the energy in a particular direction, you both make the light cover a smaller area and appear to be brighter in the area it does cover. The higher the gain, the tighter and brighter the beam appears to be. Also note that antenna gain is reciprocal, meaning that it works for both transmission and reception. Adding an antenna to either end of a radio link will help performance for both ends of the link.
Another important property to keep in mind when designing or purchasing an antenna is that it must be tuned to the frequency for which you are using it. An antenna that is well matched to the radio it is connected to is said to have a low standing wave ratio (SWR). The SWR of an antenna is measured using an SWR meter or reflectometer. It is a measurement of how much energy actually leaves the antenna versus how much energy is reflected back at the radio from the antenna itself. At (legal) 802.11-power levels, a badly mismatched antenna with a high SWR simply results in poor performance. At higher power levels, a mismatched SWR can actually damage your radio or amplifier. As you'll see in the antenna designs in this chapter, the antenna is tuned by manipulating a number of factors, including the size of various active components, and their relative distance away from reflective components.
One property of antennas that is frequently overlooked by beginners is their front-to-back (F/B) ratio. This is a measurement of how much energy radiates in the expected direction (at the center of the strongest beam) versus the average amount of energy radiated in the opposite direction. A high F/B ratio means that most of the energy goes in the direction that the antenna is pointed. A low F/B ratio means that more energy is lost in the reverse direction, potentially causing unwanted interference with nearby devices. This is particularly important if you are using two or more antennas adjacent to each other, pointed in different directions. A higher F/B means that it is less likely that adjacent antennas will interfere with each other.
Finally, one last important property of antennas to keep in mind is their polarization. Briefly, this refers to the orientation of the electrical and magnetic parts of the radio wave as they leave the antenna. Polarization is discussed in greater detail in "Take Advantage of Antenna Polarization" [Hack #100]. There is also a comparison of the various general types of antennas and their typical uses in Appendix B.
The hacks in this chapter describe a number of inexpensive, highly effective antenna designs that you might find useful for your own wireless networking project.
Bluetooth, Mobile Phones, and GPS
Network Discovery and Monitoring
Wireless Network Design
Appendix A. Wireless Standards
Appendix B. Wireless Hardware Guide