SO WHAT?  It is just an antenna.

One of the most overlooked aspects of a wireless local area network (WLAN) are antennas. For most organizations, which install their own WLAN’s in a small office or home, the antenna is not a major factor since most cards and access points have built in antennas.  However, for large installations such as warehouses, manufacturing plants, hospitals, airports and similar areas, the antenna becomes a critical part of the entire system. The correct selection and usage of antennas may mean the difference between a cost effective installation with robust reliable performance, and a network with areas of weak coverage, unreliable communications and poor performance. The correct selection and usage of antennas may also mean the avoidance of potential expenses due to too many access points and regular visits by technicians searching to fix a network problem.

 Critical Choices 

Many times WAV is called to a customer site to troubleshoot a network with problems. Often we discover that in an attempt to save costs an organization will install their access points without a technical site survey source, without regard for access point location, and/or without regard to antenna choice.

Due to the nature of WLAN radios, antennas are critical components of large WLAN installations. The majority of radios transmit a very small signal, typically 30mw. There are a few that transmit higher such as the Cisco 350/1200 series, however, most are in the 30mw range.

Chuck's Corner Info Bite
Twice the power output does not translate to twice the distance covered. Covering twice the area may mean 3 times more power and a higher gain antenna.

Antennas generally fall into the following categories: Omnidirectional and Directional. Although there are many different antennas, most are just variations of these two basic types.

Omnidirectional

Omnidirectional antennas (omnis) radiate a pattern in all directions, IE: 360 degrees. Omnis are good for large open areas where there is little in the way of obstructions. Warehouses with low racking and high ceilings, and manufacturing areas are examples. Omnis need to be mounted out in the clear. Many times an omni is found mounted on a ceiling extending down from the support beams.  

Omnidirectional antennas can vary in shape.  Depending on the gain, most are just black or white sticks in varying lengths. Others look somewhat like smoke detectors or simplified small, flattened hockey pucks. 

Some omnidirectional antennas have customizable patterns. The radiation pattern can be modified during installation to provide more coverage in some areas and a less in others. Even the "uptilt" and "downtilt" of the signal can be adjusted. This allows the pattern to be customized to the installation. This insures that signal is not wasted in directions where it is not needed and/or wanted. These kinds of antennas are used outdoors to cover large open areas such as theme parks or outdoor malls.

The gain of the antenna affects the coverage pattern. A low gain omni will have a relatively small coverage area, but it will be very broad vertically. This is why low gain omnis are used for high bay warehouses where the antenna is mounted in high ceilings, generally 35 feet or higher. This broad coverage also wraps around racking better. These kinds of antennas work well when the antennas need to be mounted high and the user population is at ground level.

High gain omnis radiate a signal further in a more narrow form. These antennas are deployed for outdoor use where users are not near, but more of a distance away. A good example of this is point to multipoint bridging. The center point would use a higher gain omni with the outer areas using directional antennas pointing towards the center point. (Directional antennas are discussed below.) It would not be wise to use a high gain omnidirectional antenna in a high bay warehouse. The signal would be radiating outwards, without enough downward signal to the users at ground level.

Directional

There are varieties of directional antennas. Although these are all directional antennas, a large difference exists among each. The difference is the coverage patterns.

Yagi antennas are the most well known. The Yagi looks a lot like an older television antenna. A long boom with horizontal sticks (elements) along its length. The higher the frequency, the smaller the elements. A Yagi for 2.4Ghz has elements less than 3 inches long. In fact, the most common Yagi antenna for 2.4Ghz looks like a long cylinder. The cylinder is just a weatherproof cover.  Yagi antennas work by focusing more signal in one direction like a mirror behind a light bulb. The higher the gain of the antenna, the narrower the radiated signal will be. One use for Yagi Antennas is within large warehouses with high racking and long aisles. An Omni may not fit between the top of the racks and the ceiling, therefore, a series of Yagis become the antenna of choice. They are used to fire a signal down the aisles. Yagi’s can also be used outdoors as bridge links between two locations over a long distance. In many cases a Yagi may cover up to 3 or more miles.

Sector Antennas are somewhat similar to Yagis, however, they present a much wider coverage. Yagi’s tend to be less than 35 degrees in coverage where as a Sectored Antenna typically is between 60 and 120 degrees in coverage. Sectored Antennas are used mainly outdoors where the antenna may be at the edge or corner of the coverage area.

Patch or Panel Antennas are flat, square, or round and used where a low profile is needed. Many times this is due to esthetics or reducing the risk of an antenna being damaged. Patch and Panel Antennas are much like Sectored Antennas. One type of Patch is a hemispherical. This antenna type has a 180 degree coverage. It’s well recognized for coverage in retail stores, parking lots, and convention halls. Another style of Patch is the bi-directional. It’s a small antenna that fires a signal in two directions, 180 degrees from each other. These are know for coverage in long hallways such as hospital corridors.

The Parabolic is the "big gun" of the directional antennas. The Parabolic is used exclusively for outdoor, long distance point to point bridging. Typically demanding coverage of 10 to 35 miles.

 Antenna Gain

One item that needs to be discussed in more detail is Gain.  Gain is defined as the compressing of the vertical component of the antenna pattern, in effect causing the radiation pattern of the antenna to reach out further toward a base station or cell site (http://www.antenna.com/faqs_theory.html). Antenna Gain is an antenna’s ability to gather in signal and radiate signal. Usually the higher the Gain, the larger the antenna will be.  For an omnidirectional, that means height. For Yagi Antennas, length. Gain is expressed in dBi-a unit of antenna gain. The dBi measure is referenced to a theoretical, dimensionless point source with a completely spherical radiation pattern (http://www.antenna.com/faqs_theory.html). This is the only way to compare relative performance when looking at similar antennas. Most manufacturers rate their antennas in dBi.  A few still use dB. When comparing gain, the units must be the same for both.

Chuck's Corner Info Bite
You can convert dB to dBi by adding 2.2. For example: A 3dB gain omni is comparable to a 5.2dBi gain omni.

You don’t get something for nothing. In order to earn more gain or radiate more signal in one direction, you must take it from somewhere else or refocus it. This can be easily compared to taking a single light bulb in the middle of a large room and placing a reflector behind it. You’re redirecting all the light energy in one direction at the expense of the rest of the room.

 How do I know what antennas to use in a given situation?

How do I know what antennas to use in a given situation? This is where a well trained Systems Engineer and a Site Survey come in. A proper Site Survey takes into account the topography of the area to be covered, the limitations of antennas location, and the dynamics of the structure.  All this information is used to determine how many access points and what kind of antennas are needed to provide robust, reliable coverage. The proper use of antennas often means LESS access points are needed.  The results are a more cost effective and efficient network.

WAV is a distributor of the complete line of MAXRAD Wireless LAN antennas. WAV currently uses MAXRAD antennas for Site Surveys conducted so that we have the unique ability to match the correct antenna to the coverage area. Contact your  WAV Representative and ask how MAXRAD and WAV can work with you in deploying successful WLAN implementations.