Antennas 101

Antennas 101

The antenna has two basic functions. On the receiver side, it converts electromagnetic energy into voltage, which gets processed and converted to digital data. On the transmitter side, the antenna transforms voltage into electromagnetic energy that radiates into the air. Complex modulation schemes carry the digital data streams over radio signals.

Just how well your antenna pulls off this bit of magic determines whether your users get connected and, if so, how fast the system performs. If your antennas don't radiate properly for your facility, your WLAN interfaces will drop their data rates to adjust to the antennas' lower signal level. And your performance goes down the tubes.

The right antenna maximizes your coverage and makes sure your WLAN transmissions stay inside the building instead of sending signals out to the parking lot.So how do you design your antenna system for optimal performance? RF engineers use the hypothetical isotropic antenna as a model for their antenna design. This conceptual model radiates the available radio energy from a point in space, forming a globe-shaped antenna pattern. You can use this model as a baseline, 0dBi (decibels relative to isotropic), for comparing the effective signal amplification, or gain, of various antenna designs. An antenna with a gain of 2dBi, for example, is two decibels above an isotropic antenna.

Antenna patterns are represented as 3-D or in a pair of two-dimensional plots. The most common WLAN antenna design is the omnidirectional dipole configuration, which is used in WLAN APs and wireless home routers. When the omni-dipole is oriented perpendicular to the ground, it radiates a signal in a 360-degree pattern that resembles a donut. The antenna itself "sticks out" of a small hole at the center. The diagram on page 79 shows two- and three-dimensional plots of a 2.2dBi omnidirectional antenna.

The leftmost plot assumes the antenna is perpendicular to the ground. Unlike the globe-shaped isotropic antenna, the dipole pattern is elongated. This concentrates radio waves along the X/Y axis, producing antenna gain and, in most cases, more coverage area. The rightmost plot shows the companion 2-D coverage patterns. The left side represents the coverage area if you're looking directly down on the antenna, and the right side shows the view from the side.

Because electromagnetic energy is focused, the antenna produces about 2dBi of gain (along the X/Y axis). Each 3dBi of gain effectively doubles the signal level. So an AP outfitted with an omnidirectional antenna could radiate its signal through a large room full of office cubicles with less of its signals seeping to the upper or lower floors of the building.Antennas also can be designed to focus their energy in narrow and highly directional beams, such as a large parabolic dish antenna that links cell towers 25 miles apart. These antennas can provide gain of 25dBi or more.

Making a simple change to your antenna design can pump up performance: If you orient your omni antenna horizontally rather than vertically, for instance, its signal will propagate between floors rather than across a floor. That works for a tall, narrow building or a multistory home.You can optimize your wireless coverage by connecting external antennas to most enterprise APs (within the limits imposed by the FCC). But in portions of the 5-GHz band, the FCC prohibits external antennas, so you're stuck with the antenna on your AP.

Leading AP vendors such as Cisco Systems, Proxim and Symbol Technologies offer several antenna options for their products, whereas other vendors, including most of the new WLAN switch companies, use fixed omnis. Airespace provides external antenna connections as well as an integrated dual-patch, directional antenna design. This approach gives Airespace's AP greater range than its competitors. In our tests at the Network Computing Real-World Labs, the product provided about 15 percent greater range than other products we've tested (not including those with smart antennas).

Notebook computers are equipped with PC Card WLAN NICs or integrated adapters, usually in the form of a mini-PCI WLAN NIC inside the system.

If you have a PC Card NIC, you're stuck with a simple omni antenna. Because the NIC is oriented on its side, the antenna signal pattern usually radiates up and down rather than side to side, so it limits your range (unless, of course, you're communicating with an AP on the floor above or below). The Asante FriendlyNET AL1511 adapter with pop-up Xwing antennas has the best range of 802.11b WLAN NICs, according to our test results. It's not the prettiest or most physically robust system we've tested, but its vertically oriented antennas make a significant difference. It helps if you need coverage in fringe areas of a large facility. Some 2.4-GHz NICs also offer a jack for an external antenna--a capability proven useful not only to enterprise WLAN users, but to war drivers as well.An embedded NIC that's installed in a notebook PC typically comes with a preinstalled antenna in the system enclosure. In most cases, these antennas are dual-band-capable, meaning they have separate antenna elements for supporting 2.4-GHz and 5-GHz transmissions. Ideally, the best location is around the perimeter of the display, but that requires cable connections, which attenuate the radio signal (a typical notebook antenna cable can result in 3dBi or more of loss). So many manufacturers instead install the antenna near the mini-PCI card, which is typically below the keyboard. Trouble is, this can make the performance even worse than that of a PC Card NIC antenna.The good news is that wireless antennas are getting smarter and more efficient in how they transmit and receive radio signals. Simple smart-antenna design, known as switched antenna diversity, has been used for several years for enhancing wireless systems. Switched antenna diversity consists of two antennas and an internal switch, and the radio receiver chooses the antenna with the best signal. This technology is widely available on WLAN APs and NICs, but unless the antennas are far enough apart--typically three wavelengths--it doesn't buy you much.

Diversity antennas help to combat problems with multipath, a phenomenon where signals bounce off solid objects and multiple instances of a signal arrive at the AP at different times. In the worst case, the signals cancel each other out, and the signal is then so low that it can't maintain a connection.

Extending the range of WLANs requires even smarter antenna design. Advancements in signal processing let radios intelligently control and decipher complex radio signals. For example, phased-array antennas, such as Wi-Fi switch manufacturer Vivato's, pack multiple discrete antennas into a single AP to "steer" the radio beam between the AP and a standard 802.11 client. Each antenna is designed with a slightly different directional beam pattern, and the AP chooses the best one for each client, depending on where it's located. This improves range, particularly outdoors, and it operates with standard, off-the-shelf WLAN client adapters. But phased-array antennas do little to combat the ill effects of multipath, and these systems tend to be large and expensive.

Another smart-antenna design is the adaptive array, which Motia and other vendors are developing. Rather than choose a specific antenna and beam pattern, an adaptive array simultaneously transmits and receives signals on multiple antenna elements (usually three or four). It uses signal processing to combine the signals of each antenna. This "antenna appliqu" is designed to bolt onto existing Wi-Fi systems.

Although this approach is promising, taking advantage of these and other advanced smart antenna designs requires fundamental changes to WLAN standards. Already under way within the IEEE 802.11n working group is a next-generation WLAN standard for throughput of at least 100 Mbps. Today's 11a and 11g standards specify a maximum data rate of 54 Mbps, with actual throughput of 25 to 30 Mbps.It's widely anticipated that the next-generation 802.11n standard will rely on a MIMO (multiple input, multiple output) radio design. A version of MIMO is available in new chipsets and reference systems from Airgo (see "Airgo's Smart Antenna"). Rather than being thwarted by multipath, these systems rely on multipath signals to deliver more than 10dBi of system gain.

MIMO is expected to be just one ingredient in the emerging 802.11n standard. Supporting 100-Mbps throughput over reasonable distances may also require significant changes to the 802.11 MAC design, introducing potential issues with backward compatibility. Stay tuned for 802.11n. It will be fast and, thanks to some ingenious antenna design, very smart.

Dave Molta is a senior technology editor at Network Computing. He is also assistant dean for technology at the School of Information Studies at Syracuse University and director of the Center for Emerging Network Technologies. Write to him at [email protected]. Talk about a smart antenna. We got some promising results when we tested a prototype system from Airgo Networks at NETWORK COMPUTING's Real-World Labs at Syracuse University. The prototype included an AP (access point) and a NIC, each equipped with three dual-band antennas. We ran TCP throughput tests using NetIQ's Chariot, with Airgo's AP and an Airgo NIC installed on a Dell Latitude notebook. With all devices in the same room, we measured TCP throughput of slightly greater than 40 Mbps--the fastest we've seen for a single-channel WLAN system, and faster than proprietary turbo designs that bond multiple radio channels to improve performance.

The Airgo system, while operating in the 5-GHz band, maintained a connection at a distance significantly greater than any 802.11a system we've tested. The range improvement wasn't dramatic--especially when cinderblock walls in one portion of our test facility separated the devices--but it was notable.

You need Airgo's WLAN chips to use the new antenna. The company has developed reference designs for NICs and APs, and intends to compete with Atheros Communications and other chip/system vendors. Its equipment also works with legacy wireless equipment. We discovered that, with a current-generation Netgear 802.11a NIC, the Airgo AP's range exceeded that of Airespace's APs. And Airgo's AP covered nearly twice the area of a Cisco 1200 AP equipped with an 11a interface. Airgo just completed a fourth round of venture funding, so the company has the financial resources to make a serious run at the WLAN market.Etenna, internal antennas for notebook computers, www.etenna.comTrevor Marshall's Byte article on antennas, www.trevormarshall.com/byte_articles/byte1.htm

Greg Rehm's homebrew antenna, www.turnpoint.net/wireless/has.html#intro

Summary of antenna patterns, www.rfcafe.com/references/electrical/antenna_patterns.htm

Smart-antenna apps in cellular communcations, www.cdg.org/technology/cdma_technology/smart_antennas/smart.asp

WLAN antenna purchasing, www.wlanantennas.comPost a comment or question on this story.