Antennas: The Key to Maximizing RF Coverage

By Jim Geier

May 24, 2002

Setting up an enterprise WLAN is not a plug and play process -- you need to carefully consider range and coverage of signals, which you can manipulate though effective use of antennas.

RF coverage throughout a facility is of prime concern for companies deploying wireless LANs, and the antenna plays a vital role. If you ignore the antenna, then the access point may not attain maximum effective range. This can be a costly mistake. An effective antenna solution increases the range and corresponding coverage of a wireless LAN, which decreases costs because of fewer access points.

Antenna basics

An antenna couples RF energy to the air medium. A transmitter within a radio NIC or access point 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 either 2.4 GHz (802.11b) or 5 GHz (802.11a). 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 NIC or access point. For most applications, the antenna power specification won't be of too much concern to you because of the relatively low power that wireless LANs 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, with decibels (dB) 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 wireless LANs

Some Radio NICs and access points have integrated antennas that you can't change. For example, laptops such as Apple iBook integrate the antenna within the cover or body of the device, which is not visible or changeable by the user. Some radio NICs and access points 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 access points 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 (i.e., throughout the facility), 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.

Omni-directional antennas, having gains ranging up to 6 dB, apply to most applications inside buildings. Omnis provide the widest coverage, making it possible to form somewhat circular overlapping cells from multiple access points located throughout the building. Most access points ship standard omnis having relatively low gain. Consider using higher gain ones to increase range, which enables wider spacing of access points. This can reduce the number of access points and lower costs.

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.

High gain antennas work best for covering large, narrow areas, or supporting point-to-point links between buildings. In some cases, a directional antenna will reduce the number of access points needed within a facility. For example, a long loading dock of a distribution center many require three access points having omnis, but the use of a high gain directional antenna would likely only require a single access point.

When looking for antennas, start with the wireless LAN vendor supplying your radio NICs and access points, such as Proxim, Cisco, and Symbol. Be sure, however, that the components you purchase enable the use of different antennas. For a wider selection, consider 3rd party companies specializing in antennas, such as Cushcraft and Til-Tek.

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 wireless LANs. 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 most radio NICs and access points, you'll be well under the FCC limitations using the transmit power options available to the end user. For example, a radio set at 100 milliwatts transmit power with a 3 dB antenna will only have 200 milliwatts (0.2 watts) EIRP. A 6 dB antenna doubles the EIRP to 400 milliwatts, a 9 dB antenna doubles it again to 800 milliwatts, and so on.

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. The reduction, however, is only 1 dB for every 3 dB of additional antenna gain beyond the 6 dBi mentioned above. This means that as antenna gain goes up, you decrease the transmitter power by less. Thus, the FCC allows EIRP greater than 4 watts for antennas having gains higher than 6 dBi. Of course these higher gain antennas would mostly apply to point-to-point solutions having longer-range requirements, which is not common for most indoor applications.

In summary, be sure to consider antenna selection as part of the wireless LAN deployment. Provide flexibility by purchasing access points accepting interchangeable antennas. This will enable the use of higher gain omnis and perhaps directional antennas to enhance the range and minimize the costs of the wireless LAN.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.



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