Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


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September 1, 2005

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
 
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
 
To Infinity... and Beyond!
News from the wireless marketplace...
 

Essential Wi-Fi

This month, we continue our discussion of antenna propagation patterns. In July, we covered the concept of antenna beamwidth. Then, we discussed different types of omnidirectional antennae and when they might be used. In August, we moved on to directional antennas, covering the patch/panel and yagi types. If you missed those articles, we recommend that you read them addition to this one, for maximum completeness.

In this issue, we'll continue our coverage of directional antennas. The major types of directional antennae are: patch/panel, yagi, sector, and dish/grid. This month, we'll cover sector antennas.

Single sector antenna adjusted with downtilt
 

Sector antennas are usually long and rectangular. They are typically mounted with their central axis perpendicular to the ground, similar to an omnidirectional dipole, but their coverage area is different than a dipole's. Sector antennas put out flat, wide coverage patterns that can be roughly visualized as slices of a pie, with the point of the pie slice at the antenna.

Sector antennas are usually used in cases where you need a very specific arc of coverage. Therefore, they are designed so that, as their gain increases, their vertical beamwidth shrinks while their horizontal beamwidth stays the same. In other words, as the sector antenna's gain increases, the "pie slice" gets thinner and longer, but it is always the same width. A sector antenna with a 90 degree horizontal beamwdith will always have a quarter-circle's worth of horizontal coverage, no matter if it's designed with 6 dBi or 20 dBi of gain. In this way, sector antennas behave very much like an omnidirectional antenna--in fact, sector antennas can be thought of as "slices" of an omnidirectional "pie". Sector antennas typically have horizontal beamwidths of between 120 and 90 degrees.

Another defining characteristic of sector antennas is that they have very small "back lobes" and "side lobes" (the amount of RF energy that is propagated behind and to the sides of the antenna). Remember that the beamwidth of an antenna is usually the point where the signal is half as strong as it is directly in front of the antenna. But the beamwidth doesn't really tell you much about what happens outside of that point--for that, you need azimuth and elevation graphs. You can buy both a sector antenna and a panel antenna with a 90 degree beamwidth, but the sector antenna will typically have much less signal propagation outside of that beamwidth compared to the panel.

Sector antennas' small side and back lobes makes them more efficient at covering an area, since less energy is wasted by being transmitted outside of the intended coverage area, and also means that you can place sector antennas closer together without interference than you could any other type of antenna. It's common to place several sector antennas on top of a mast, pointing in opposite directions, to cover a full 360 degree arc--for example, you might use four 90 degree sectors or three 120 degree sectors.

You might wonder why not just use an omni antenna in this case, especially considering that sector antennas are one of the most expensive antenna types. One reason is that the sector antennas can each be tilted slightly down towards the ground, allowing the antenna to be mounted higher in the air than it could if it were a single omni. Another reason is that each antenna in the array can act independently of the others, allowing more than one station to use the array at a time, whereas a single omni antenna could only serve one station at a time.

Sector antennas should only be used in very specialized cases. Remember that sector antennas are similar to "slices" of an omnidirectional "pie". But unlike omnidirectional antennas, which have gains as low as 2 dBi, sector antennas typically range from 12 to 18 dBi of gain. This means that their coverage is always very flat! Therefore, they're not appropriate for cases where users will wander out of the sector antenna's relatively narrow coverage arc. Sector antennas are quite expensive, especially since you typically have to buy more than one. In many cases, a high-gain omnidirectional antenna may suffice at lower cost.

In general, consider using sector antennas in situations where you might also use a high-gain omnidirectional antenna. If a high-gain omni couldn't do the job--e.g. because the users are all at different heights and would be outside the thin arc of coverage--sectors probably aren't appropriate either. As an example of a case where a sector might be appropriate, consider if you planned to put an antenna on top of a central building and wanted wireless coverage with several other buildings of similar height in a 270 degree arc. You might use three 90-degree sector antennas to create an aggregate 270 degree arc of coverage. This might provide better coverage than a single omnidirectional antenna, since the omni would waste the 90 degrees of coverage where there was no receiver. On the other hand, it might cost twice as much as a comparable omni, so you would have to justify that cost with increased performance.

Sector antennas are also commonly used on top of cell towers. Several sectors are combined to create a 360 degree zone of coverage around the tower. The antennas are mechanically tilted down towards the ground so that their coverage doesn't blast off over the horizon. Although the antennas' vertical coverage arc is relatively small, it adds up to a wide coverage area by the time the signal gets down to the ground.

   
Sector panel with adjustable tilt bracket
 
   
Three 120-degree sectors combined into a single antenna array. Downtilt is adjusted separately for each sector
 
 
The azimuth graph (left) shows the top view of a single sector. The elevation graph (center) is the side view. When multiple sector panels are combined to create an antenna array a 360-degree coverage pattern is attained, as shown in the azimuth (top) view to the right.

 Technology and Engineering

In the last two months, we covered the basic RF behaviors of attenuation, absorption, reflection, and refraction. This month, we'll conclude with a discussion of diffraction. Diffraction is common in water waves, but you may not have noticed it as such, and you've probably never noticed it at all in visible light!

Diffraction occurs when a wave moves past the edge of an object. The effect of diffraction is that the wave is spread in the direction of the object. For example, when you talk into a cardboard tube, the sound coming out of the end of the tube doesn't propagate just in the direction that the tube is pointed, even though the sound waves in the tube obviously must be propagating in that direction. Instead, the sound coming out of the tube propagates in all directions. This effect is caused by diffraction at the edge of the end of the tube. Another example of diffraction occurs when a wave hits the entrance to an atoll that is protected by a barrier. Even though there is just a small entrance into the atoll, the wave spreads out and propagates throughout the atoll.

In the RF environment, diffraction usually occurs when a wave moves past the corner of a rectangular object such as the corner of a hallway or a skyscraper (in a point to point outdoor link). In this case, the wave will be bent in the direction of the object. In the case of the hallway, this have the desirable effect of helping the coverage to extend around the corner. In the case of the building, diffraction may make it more difficult to align the antennas at the ends of the link. Diffraction can also occur when an RF signal enters a room through the door. In this case, the signal will expand to fill the room, much like the wave entering the atoll. If a diffracting object is directly in line between the transmitter and the receiver, diffraction may actually improve coverage by allowing the signal to "wrap" around the object and continue on in the direction that it was going. Diffraction can be helpful or harmful, depending on circumstances. It becomes a problem when it "bends" signal away from its intended recipient, which is most noticeable in long-distance point-to-point links.

Diffraction becomes more pronounced when the dimensions of the diffracting object are small relative to the wavelength of the wave. Objects that are large relative to the wavelength simply absorb, reflect, and/or refract the signal. Therefore, an object that has gently curved edges will have less pronounced diffraction effects than an object that has sharp, well-defined edges. The wavelength of an 802.11b signal at 2.4 GHz is about 4 to 5 inches, so you probably don't have to worry about diffraction from any obstruction that is larger in diameter than this. For 5.8 GHz transmitters, the wavelength is about 2 to 2.5 inches.

If you're a business entrepreneur in the HotSpot space it would be wise to keep current on the status of "free versus paid" service conflicts. A HotSpot business model that's dependent on subscription fees will fail if users have the option of a free service in the same geographic or indoor area. If you're providing a Wi-Fi coverage area, and your property boundaries, are such that you don't have to worry about nearby potential free competitors; good. If someone could set up a free service and cover your area; Oops. At Connect802 the Connect EZ Predictive RF CAD Design can be used to create an RF predictive coverage map showing how your Wi-Fi equipment will provide RF coverage, and how your neighbor's equipment may wash an RF signal across your property. This is one proactive approach to the problem.