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. It's important to realize that some manufacturers offer "sector panel" antenna systems without making it possible to activate a full 360 degrees of coverage. This is simply the proverbial conflict between marketing, engineering, and finance. If you're considering using a sector array be sure you ascertain how many separate radio transmitter/receivers units will be required to simultaneously operate some number of sector panels. It may be that a single radio uses an intelligent array, or it may be that multiple radios each use a single sector. The old saying, "Caveat Emptor" ("Let the Buyer Beware") applies. Connect802 is happy to discuss the options, features, benefits, and cautions for using sector array antennas in any particular design. Give us a call at (925) 552-0802, email, or use a feedback form for more information.
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.

Going Deeper..

Diffraction is the bending in the direction of travel of part of a wavefront resulting from the presence of a radio-opaque object in the transmission path. This is a characteristic of electromagnetic radiation that can not be explained solely on the basis of a particle view of signal propagation. Diffraction is a phenomena unique to the wave-like nature of the electromagnetic field. We leave it to research in quantum mechanics to suggest reasons why diffraction is consistent with both the particle and wave view of electromagnetism and, from the perspective of 802.11 wireless networks, think of diffraction only from the wavefront perspective.

Diffraction effects are most noticeable when objects are close to 1 wavelength in size (roughly 12 cm for an 802.11 2.4 GHz transmitter). By the time an object gets to roughly 60 wavelengths in size (7.5 meters) diffraction becomes negligible. This means that typical objects in any building’s interior space will introduce diffraction. A tall cubicle wall, high shelving unit, or free-standing kiosk in an auditorium will all fall under the 7.5 meter height guideline. Horizontally, wall segments (not separated by doors) will typically be narrower than 7.5 meters and will act like horizontal barriers, also introducing diffraction at their edges. In essence, waves propagate around electrically small (close to 1-wavelength) objects without any crisp shadows. The umbra region has sufficient energy to often go unnoticed by the casual user of 802.11 equipment.

The bending effect observed when a wave is diffracted around an object is the result of interference between different parts of the electromagnetic field. This concept is very mysterious and a concise explanation of the field behavior can only be expressed through some very complicated equations. The effect can be understood, however, by considering the basic characteristics of field propagation as expressed in the Huygens-Fresnel principle. Understanding Huygens’ wave theory for electromagnetic field propagation provides the basis for not only formulating a better mental picture of what’s actually going on in the air in an 802.11 environment but for understanding signal obstruction in accordance with an aspect of field diffraction known as the Fresnel Zone. A Fresnel Zone is a 3-dimensional oval volume defined along the axes formed by the straight line between a transmitting antenna and a receiving antenna.

A directional antenna offers some particular total field energy in some particular direction. That field energy, whatever its value, is either unopposed or opposed by Fresnel Zone interaction when an obstacle gets too close to the straight line-of-sight between the antenna and the receiver. The location of the Fresnel zone is unrelated to the directionality of the antenna. It’s only relative to the direct line between transmitter and receiver. It has been determined that the diffraction characteristics of the first Fresnel zone are the most critical to received signal strength. Calculations have been made concerning the impact of obstructions in the first Fresnel zone and it’s been found that the first zone must be at least 60% clear of obstructions.

The radius of the first Fresnel zone, which must be 60% unobstructed, may be found using the Connect802 On-Line Antenna System Designer, a free calculator for RF antenna installation design.

To Infinity... and Beyond!

Maximum power output levels aren't the only things about 802.11 that are regulated by the Federal Communications Commission (FCC) in the United States. Part 15 of the FCC regulations requires that unlicensed operators accept interference from outside sources. This can significantly impact business operations, particularly when the "interference" is from a free HotSpot access provider or private individual who's taking business away from your pay-for-service HotSpot.

At some airports the Continental Airlines Presidents Club is offering members free Wi-Fi service in contrast to pay-for-service providers using services such as T-Mobile. At Boston Logan airport, Massport has a contract with T-Mobile in which T-Mobile is specified to be the designated Wi-Fi provider. They've asked Continental Airlines to shut down their free system to avoid a contractual conflict. Continental's position is that only the FCC can require the shutdown of an unlicensed transmitter and, in accordance with Part 15, the airport must simply tolerate their signal without consideration for the Massport contract. They stated, "We believe Federal Communications Commission regulations give us the right to provide our own seamless services, which is free, and do not require you or us to pay fees to Massport's vendor to have a wireless Internet access in our Presidents Clubs. At issue is whether or not landlords and local authorities have the right to restrict users of the unlicensed spectrum. Previously (June 2004) the FCC issues clarification that unlicensed spectrum rules may not be controlled by local authorities, municipalities, or landlords. The final rulings have not yet been delivered by the FCC but, unless they've changed their position in the past year, it looks like the free service will be allowed and attempts to shut it down will fail.

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 Suite Spot Predictive Site Survey 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.

Looking at the issue from a slightly different perspective, the FCC's stance is good news for you if you're a tenant in a commercial or residential building and the terms of your lease, your homeowner's association, and so forth, prohibit you from using wireless networks for whatever reason. The FCC's stance suggests that you have strong grounds to insist that you have the legal right to operate an 802.11 network as long as you comply with FCC regulations and that any private contract that limits that right won't stand up to legal scrutiny.

Some landlords try to get around the FCC's rulings by prohibiting not the wireless network itself, but the antennas. The FCC has, however, clearly ruled in the renter or lessor's favor on this issue as well. FCC CFR number 47, section 1.4000 "prohibits most restrictions that: (1) unreasonably delay or prevent installation, maintenance or use; (2) unreasonably increase the cost of installation, maintenance or use; or (3) preclude reception of an acceptable quality signal." This rule was originally intended to apply only to receiving antennas, such as those used for digital satellite television, but was extended in October, 2000, to apply to antennas that transmit and receive fixed wireless signals.

Although the tenant's rights to operate a wireless network and to erect antennas are substantial, they are not unlimited. You can read more about this issue, including relevant cases, and the specific requirements for an antenna that is covered by the ruling, in the FCC's OTARD (Over The Air Reception Devices) information sheet, which is available at: http://www.fcc.gov/mb/facts/otard.html