Guest Post: A guide to wireless antenna basics

Editor's Note: Emmett Dulaney writes about certification topics for Certification Magazine and CertMag.com. You can find more of his guides to certification topics at CertMag.com

Wireless networking related topics appear in a number of certification exams, most notably CompTIA's Network+ (both the current N10-005 exam and the upcoming N10-006). For that reason, my focus this month is on wireless antenna and what you need to know about that topic for passing a certification exam. Much of this information is excerpted from the CompTIA Network+ Exam Cram.

A wireless antenna is an integral part of overall wireless communication. Antennas come in many different shapes and sizes, with each one designed for a specific purpose. Selecting the right antenna for a particular network implementation is a critical consideration, and one that could ultimately decide how successful a wireless network will be. In addition, using the right antenna can save you money on networking costs, because you will need fewer antennas and access points.

Many small home network adapters and access points come with a nonupgradable antenna, but higher-grade wireless devices require you to choose an antenna. Determining which antenna to select takes careful planning and requires an understanding of what range and speed you need for a network. The antenna is designed to help wireless networks do the following:

• Work around obstacles
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• Minimize the effects of interference
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• Increase signal strength
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• Focus the transmission, which can increase signal speed

The following sections of this article explore some of the characteristics of wireless antennas.

Antenna Ratings

When a wireless signal is low and is affected by heavy interference, it might be possible to upgrade the antenna to create a more solid wireless connection. To determine an antenna's strength, refer to its gain value. But how do you determine the gain value?

Suppose that a huge wireless tower is emanating circular waves in all directions. If you could see these waves, you would see them forming a sphere around the tower. The signals around the antenna flow equally in all directions, including up and down. An antenna that does this has a 0dBi gain value and is called an isotropic antenna. The isotropic antenna rating provides a base point for measuring actual antenna strength.

NOTE: The dB in dBi stands for decibels and the i stands for the hypothetical isotropic antenna.

An antenna's gain value represents the difference between the 0dBi isotropic and the antenna's power. For example, a wireless antenna advertised as 15dBi is 15 times stronger than the hypothetical isotropic antenna. The higher the decibel figure, the higher the gain.

When looking at wireless antennas, remember that a higher gain value means stronger send and receive signals. In terms of performance, the rule of thumb is that every 3dB of gain added doubles an antenna's effective power output.

Antenna Coverage

When selecting an antenna for a particular wireless implementation, you need to determine the type of coverage the antenna uses. In a typical configuration, a wireless antenna can be either omnidirectional or unidirectional. Which one you choose depends on the wireless environment.

An omnidirectional antenna is designed to provide a 360-degree dispersed wave pattern. This type of antenna is used when coverage in all directions from the antenna is required. Omnidirectional antennas are advantageous when a broad-based signal is required. For example, if you provide an even signal in all directions, clients can access the antenna and its associated access point from various locations. Because of the dispersed nature of omnidirectional antennas, the signal is weaker overall and therefore accommodates shorter signal distances. Omnidirectional antennas are great in an environment that has a clear line of sight between the senders and receivers. The power is evenly spread to all points, making omnidirectional antennas well suited for home and small office applications.

NOTE: Omnidirectional antennas provide wide coverage but weaker signal strength in any one direction than a directional antenna.

Unidirectional antennas are designed to focus the signal in a particular direction. This focused signal enables greater distances and a stronger signal between two points. The greater distances enabled by unidirectional antennas give you a viable alternative for connecting locations, such as two offices, in a point-to-point configuration.

Unidirectional antennas are also used when you need to tunnel or thread a signal through a series of obstacles. This concentrates the signal power in a specific direction and enables you to use less power for a greater distance than an omnidirectional antenna. The following table compares omnidirectional and directional wireless antennas.

Comparing Omnidirectional and Unidirectional Antennas

NOTE: In the wireless world, polarization refers to the direction in which the antenna radiates wavelengths. This direction can either be vertical, horizontal, or circular. Today, vertical antennas are perhaps the most common. As far as the configuration is concerned, the sending and receiving antennas should be set to the same polarization. 

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Wireless Signal Quality

Because wireless signals travel through the atmosphere, they are subjected to all sorts of environmental and external factors. This includes storms and the number of walls, ceilings, and so on that the signal must pass through. Just how weakened the signal becomes depends on the building material used and the level of RF interference. All these elements decrease the power of the wireless signal.

NOTE: Wireless signals degrade depending on the construction material used. Signals passing through concrete and steel are particularly weakened.

If you are troubleshooting a wireless connection that has a particularly weak signal, you can do a few things to help increase the signal's power:

Antenna: Perhaps the first and most obvious thing to do is to make sure that the antenna on the AP is positioned for best reception. It often takes a little trial and error to get the placement right. Today's wireless access cards commonly ship with diagnostic software that displays signal strength and makes it easy to find the correct position.

Device placement: One factor that can degrade wireless signals is RF interference. Because of this, you need to try to keep wireless devices away from appliances that output RF noise. This includes microwaves, electrical devices and certain cordless devices using the same frequency, such as phones.

Network location: Although there may be limited choice, as much as possible try to reduce the number of obstructions that the signal must pass through. Every obstacle strips a little more power from the signal. The type of material a signal must pass through also can have a significant impact on signal integrity.

Boost the signal: If all else fails, you can purchase devices, such as wireless repeaters, that can amplify the wireless signal. The device takes the signal and amplifies it to make it stronger. This also increases the distance that the client system can be placed from the AP.

To successfully manage wireless signals, you need to know which wireless standard you use. The standards used today specify range distances, RF ranges, and speeds. It may be that the wireless standard cannot do what you need it to.

Summing it Up

 To prepare for the wireless networking portion of a certification exam - particularly CompTIA's Network+ - it helps to know the basics on antennas. The three main topic areas to know are antenna ratings, coverage, and wireless signal quality.