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Troubleshooting antenna issues on WLANs

The problems that plague WLAN antennas

When troubleshooting a wireless network problem, it’s important to have a thorough understanding of the technology and your options for fixing the problem. In this Daily Drill Down, I’ll show you how to troubleshoot signal problems on wireless networks, focusing on potential problems with antennas.

A few assumptions
As Sherlock Holmes said: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” This holds true when troubleshooting network problems. For the purposes of this Daily Drill Down, I’m assuming that you’ve taken troubleshooting steps at the access point and physical network and that the problem has been narrowed down to the antenna or connecting hardware.

Get the pieces right to start with
Understanding the components that make up a wireless network is critical to network support and troubleshooting. One of the primary components of a wireless network is the antenna. The type of antenna you choose directly affects its performance as part of the network, as well as the application for which it's suitable.

Wireless networking commonly uses two types of applications. The first is a site-to-site application in which two physically separate sites or buildings are connected to each other using a wireless bridge. The second type is client-based. A wireless access point is deployed to directly support laptops or other wireless clients for network connectivity. Each of these two types of applications has antennas that are better suited to it than the other.

Four basic types of antennas are commonly used in 802.11 wireless networking applications: parabolic grid, yagi, dipole, and vertical.

Parabolic grid
Perhaps the most powerful antenna for site-to-site applications is the parabolic grid antenna. A parabolic grid antenna can take many forms, ranging from something that looks like a satellite TV dish to one that has the same shape but is made of a wire grid instead of having a solid central core. This type of antenna is a unidirectional antenna, meaning that is transmits in one specific direction: the direction at which the antenna is pointed. Figure A depicts a parabolic grid antenna.

Figure A
A parabolic grid or dish antenna and its radiation pattern


Yagi
A yagi antenna is slightly less powerful than a parabolic grid, and it’s suitable for site-to-site applications at lesser distances that a parabolic grid. Like the parabolic, the yagi is also a unidirectional unit. A yagi antenna consists of a series of metal spokes radiating from a central core. The whole thing is covered by a tubular plastic housing called a radome, so you seldom see the actual antenna elements. Figure B depicts a yagi antenna with a cutout showing what the internal elements look like.

Figure B
A yagi antenna and its radiation pattern


Dipole
A dipole is a bidirectional antenna, and its radiation pattern extends in two directions outward, as shown in Figure C. It generally consists of a base with two antenna spokes going in opposite directions. You’d generally use a dipole antenna to support client connections rather than site-to-site applications.

Figure C
A dipole antenna and its radiation pattern


Vertical
A vertical antenna is exactly what it sounds like: an antenna that sticks in the air. A vertical antenna’s radiation pattern extends in all directions from the unit, losing power as the distance increases, as shown in Figure D. Like the dipole, you’d primarily use a vertical antenna for client support. Most wireless base stations come with a small vertical antenna. A vertical antenna is omnidirectional, meaning that the signal radiates in all directions.

Figure D
A vertical antenna and its radiation pattern


Antenna specifications
Understanding the different antenna types is only the beginning. Each antenna type has a number of specifications that directly affect how well it works. These specifications are antenna gain, beam width, loss, and radiation pattern.

Antenna gain
This is a measurement of how well the antenna focuses a signal. This is typically measured in dBi (decibels relative to isotropic radiator—a theoretically “perfect” antenna) and is based on decibels, which is a logarithmic measure of relative power. The dBi is computed by comparing the output of the antenna to a theoretical isotropic radiator (antenna) with a dBi of 0: the higher the dBi measurement, the higher the power level of the antenna.

Beam width
The beam width is the area radiating outward from the antenna where the signal within a specific angular distance is above the “half power” of the peak intensity of the antenna. The beam width is also loosely used to determine the antenna type. A parabolic grid antenna is a unidirectional antenna with a very low beam width, which means that it needs to be very carefully aimed at its partner in order to be effective. A vertical, omnidirectional antenna has a very high horizontal beam width, which is why it’s suitable for roaming client connections; however, its vertical beam width will be lower. In general, there’s an inverse correlation between beam width and antenna gain, which means that the required accuracy for aligning antenna goes up as the gain increases because the beam width decreases.

Loss
Loss is an important factor when deploying a wireless network, especially at higher power levels. Loss occurs as a result of the signal traveling between the wireless base unit and the antenna. Since these units are always connected by a cable, there will always be loss. You can minimize loss by using the appropriate type of cable in the minimum length required to make the connection.

Radiation pattern
In Figures A through D, I showed you a sample radiation pattern for each type of antenna. Every antenna has a unique radiation pattern determined by its construction. This radiation pattern is a three-dimensional radiation field of the antenna’s output. Some antenna manufacturers supply sample radiation pattern specifications for their equipment.

You can use these specifications to determine how far the signal from a particular antenna can travel before becoming unusable. As a rule of thumb, a directional antenna has a conical pattern of coverage that radiates in the direction that the antenna is pointed, while an omnidirectional antenna’s area of coverage is shaped like a doughnut.

Troubleshooting some common problems
A good understanding of wireless networking antennas makes troubleshooting much easier. Exactly how you solve the problem depends on the type of connection you’re trying to make—site-to-site or local wireless connections.

Troubleshooting site-to-site connections
It’s late October in your beautiful upstate New York town, and you’ve just finished putting up your last wireless antenna. Now you’re going from site to site, aiming them in the right direction. You spend a few days getting everything just perfect and even get close to 100 percent efficiency on your link! Life couldn’t be better. Then, in April, your links start to degrade slowly, and by May, some are almost unusable. You check your antenna connecting hardware and everything looks good. What could have happened? Unfortunately, trees grow leaves, and leaves are a Wi-Fi killer because they contain water. You’d be better off with a brick wall in the way!

The best way to address this problem is to install your hardware at the time of year when conditions will be the most difficult, such as in the late spring when all of the leaves are out and the summer drought hasn’t yet begun. If this isn’t feasible, I recommend installing your antennas on towers on top of the buildings you’re connecting (keep in mind that trees also have a tendency to grow, so plan accordingly). If this isn’t possible either, you might consider a higher-power antenna, but keep in mind that the FCC limits the total transmission power to 1 watt, or about 36 dBm.

When problems arise suddenly with a site-to-site connection, it’s probable that something has happened to one of the antennas. You may need to physically check your antennas to make sure that none have been damaged, have fallen off the building, or have been bumped around and are now not aimed correctly. If you’re using a solid parabolic grid antenna and live in an area where high winds are common, you may want to consider replacing it with a mesh dish in order to prevent potential wind damage.

Other problems with site-to-site connections involve interference from various sources, including other 2.4-GHz installations. Most of today’s wireless networking systems use the 2.4-GHz spectrum. If your company and the company next door both decide to implement a wireless network between your various sites, you may notice degradation in performance because of interference caused by your neighbor. You may need to use different channels for your installation.

If that’s not possible, and you need a performance boost, you may have to migrate to 802.11a technology, which would require you to replace all of your equipment. Equipment using the 802.11a standard operates in the 5- to 6-GHz range, but it’s much more expensive than 802.11b equipment, and it’s not backward compatible with 802.11b. However, 802.11a gear can operate at speeds of up to 54 Mbps, or close to five times the 11 Mbps limit of 802.11b. If at all possible, I don’t recommend ripping out your 802.11b equipment and replacing it with 802.11a. A new standard, 802.11g, has recently been approved. It will allow transmission speeds of up to 54 Mbps in the 2.4-GHz range, and it’s compatible with 802.11b.

Troubleshooting local wireless network problems
Because of the “walking around” aspect to client wireless networking and because a wireless network is generally deployed inside buildings, you’re more likely to experience unusual problems with these types of connections than in your fixed point-to-point connections.

Some common wireless connectivity problems have to do with your distance from the antenna, as you’d expect. However, there are also certain places that can create problems with wireless connectivity. One such place is directly under a vertical antenna that is pointing upward. As mentioned earlier, an omnidirectional antenna has a doughnut-shaped area of coverage, which means that there’s a hole right in the middle. If you’re working in the area covered by the hole and you aren’t able to connect to the network, try moving your wireless device, moving your base antenna, or mounting the antenna upside-down on the ceiling instead. If you’re not using an omnidirectional antenna for your indoor client application, you should replace the directional antenna that you are using. If you’re using a directional antenna because of its increased range, just add a second access point to cover the same distance. In the long run, you’ll have a more efficient system in place, as well as better throughput from more areas.

Other problems have to do with the way that the wireless adapter fits in the PC. To achieve the most desirable coverage area, the antenna for a wireless adapter should be pointed up. Unfortunately, most wireless adapter antennas point horizontally, which greatly limits their range. The best way to correct for this is to attempt to point the side of your laptop with the adapter antenna toward the wireless access point antenna. This may solve your connection problem and potentially boost the strength of your signal at the same time.

My last point has to do with antenna placement. If you’ve positioned your base antenna or your client card antenna near a device generating a 2.4-GHz field, such as a cordless phone, you may experience major interference. Try moving the antenna or the source of the interference. Likewise, try not to place objects such as fish tanks or water coolers in the line of sight between antennas, since water will refract the signal. Finally, avoid placing antennas near large metal objects, microwaves, and other sources of electromagnetic (EM) interference.

That’s all there is to it
Troubleshooting antenna problems and boosting network performance are sciences in and of themselves. If you want to truly understand how the signal is generated and how it travels, there are numerous resources available on the Web. Knowing some potential trouble scenarios and how to deal with them can help you solve problems and increase performance at the same time.
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