Using the right antenna will be one of the most important aspects of your ability to connect to a Wi-Fi source. If you ignore the antenna, then you may not attain maximum effective range. This can be a costly mistake. An effective antenna solution increases the range and corresponding connectivity of your Wi-Fi setup.

Antenna Basics

An antenna couples RF energy to the air medium. A transmitter sends an RF signal to the antenna, which acts as a radiator and propagates the signal through the air. The antenna also operates in reverse by capturing RF signals from the air and making them available to the receiver.

The following are common antenna characteristics:

• Frequency. For wireless LANs, you need to use an antenna tuned for 2.4 GHz (802.11b/g/n). An antenna will only work efficiently if the frequency of the antenna and radio matches.
  
  
• Power. Antennas can handle a specific amount of power put out by the transmitter. In the case of 802.11, the antenna will generally be rated greater than 1 watt in order to handle the maximum peak transmit power of the radio. For most applications, the antenna power specification won't be of too much concern to you because of the relatively low power levels that Wi-Fi transmit.
  
  
• Radiation Pattern. The radiation pattern defines the radio wave propagation of the antenna. The most basic radiation pattern is isotropic, which means the antenna transmits radio waves in all directions equally. An isotropic radiation pattern resembles the shape of a beach ball, with an antenna at its center. Other radiation patterns are also possible, and we'll discuss those in the next section below.
  
  
• Gain. The gain of an antenna represents how well it increases effective signal power. Think of antennas as being more of a lens focusing the transmitted signal. The signal is needed to radiate more in the horizontal than in the vertical. Think of the signal pattern starting as a round balloon (isotropic). By pushing in on the top and bottom, the sides move outward. This would then represent the gain. 
  
  Decibels (dB) are used as the unit of measure. The number of dB is 10 times the logarithm of output power divided by input power. As an example, an input power of 30 milliwatts and output power of 60 milliwatts equates to 3 dB of gain. A convenient fact to remember is that 3 dB of gain means the doubling of power. For instance, a transmitter outputting 100 milliwatts to an antenna having 3 dB gain produces 200 milliwatts effective power.
  
  Most antenna manufacturers specify gain as dBi, which is the gain relative to an isotropic source. In other words, dBi is how much the antenna increases the transmitter's power compared to using a fictitious, isotropic antenna. dBi represents the true gain that the antenna provides to the transmitter output.

Antennas for Wi-Fi

Some Wi-Fi radios have integrated antennas that you can't change. For example, laptops integrate the antenna within the cover or body of the device, which is not visible or changeable by the user. Some Wi-Fi radios also use permanently mounted antennas. With these types of products, you have no choice but to use the antenna the vendor supplies.

Other wireless LAN devices have antennas that are interchangeable. In fact, it's a good idea to purchase Wi-Fi devices with removable antennas. These allow more flexibility by enabling the selection of an antenna having characteristics better suited for your specific application.

The more common antenna types for wireless LANs have omni-directional and directional radiation patterns. Omni-directional antennas propagate RF signals in all directions equally on a horizontal plane, but limit range on the vertical plane. This radiation pattern resembles that of a very large doughnut with the antenna at the center of the hole. Omnis provide the widest coverage.

A directional antenna (often called a yagi) transmits and receives RF energy more in one direction than others. This radiation pattern is similar to the light that a flashlight or spotlight produces. Most antenna manufacturers provide illustrations indicating the radiation pattern. The higher gain antennas will have a narrower beam width, which limits coverage on the sides of the antennas. Directional antennas have gains much higher than omni-directional antennas, such as 12 dBi and higher.

FCC Rules Dictate Antenna Use

In the United States, the Federal Communications Commission (FCC) regulates the use of antennas through FCC Part 15.247, which defines power limitations for Wi-Fi devices. The key to applying these rules is to understand EIRP (equivalent isotropically radiated power), which represents the total effective transmit power of the radio, including gains that the antenna provides (and losses from the antenna cable). When using omni-directional antennas less than 6 dB, the FCC rules require EIRP to be 1 watt or less.

With higher gain directive antennas, the FCC relaxes EIRP limitations. When using antennas having a gain of at least 6 dBi, the FCC allows operation up to 4 watts EIRP, which is 1 watt plus 6 dB of gain. The reason higher EIRPs are acceptable is that the higher gain antennas are more directive, which reduces the possibility of RF interference with other systems.

For antennas with gain greater than 6 dBi, the FCC requires you to reduce the transmitter output power if the transmitter is already at the maximum of 1 watt. This means that as antenna gain goes up, you must decrease the transmitter power. These higher gain antennas would mostly apply to point-to-point solutions having very long range requirements, which is not common for most Wi-Fi hotspot access applications.