Smart Antennas Provide Lifeline For Wireless Design

For many wireless products, the antenna is often the toughest part of the design. It’s a mechanical thing that takes up too much space, but has to be present for the radio to work. As a result, most designers say it’s more fun to work on the electronics and the software.

If you have ever worked on a wireless project, you know the real value of a good antenna. It is often the single item that makes the difference between whether the radios work or fail. Antennas are “where the rubber hits the road,” so to speak. They are the interface to the ether; the more efficient the interface is, the better the performance of the system.

With today's wireless standards, designers have very little choice over details that could make the difference between a good radio and a mediocre one. You are essentially stuck with the receiver sensitivity of the chips you are using, the signal to noise ratio (SNR) of the environment, and the maximum power as dictated by the regulations. The only variable left to play with is the antenna. And, to your benefit, the varieties are infinite.

Antenna design, being a real black art, is not something even many wireless engineers like to tackle. Luckily you can buy commercial antennas for virtually any application. But most of these antennas are the run of the mill standard types for common applications. If you want real, special performance such as higher data rates at longer ranges more reliably, you are going to have to use a special antenna.

But designers are in luck. Smart antennas have finally arrived in a practical and affordable form, making them a viable option in many new designs. One company, Airgain, has a new antenna that is bound to fill the needs of improved performance from established standard wireless products.

The Smart Antenna

If you want to improve the range of a wireless device you can do two basic things: increase the transmit power and improve receiver sensitivity. Increasing antenna height can help, but sometimes you just have no control over that. One thing you CAN do is use directional gain antennas on both ends of the link. The transmit power is effectively increased by the gain of the antenna (due to its directionality) and the receiver gets extra sensitivity from the gain. And the directionality reduces interference from other sources. With gain antennas, link reliability (as well as the data rate) greatly improves because with the extra signal strength the speed does not back off as it does in some standards if the signal strength is not sufficient to maintain a reasonable bit error rate (BER).

The downside of this arrangement is that the antennas have to be accurately aligned because of their highly directional characteristics. Such an exercise is often not just a pain in the neck, but an unworkable arrangement in some systems. A Wi-Fi hot spot is a good example. Most access points (APs) can get a signal from any direction or any distance. An omnidirectional AP antenna is the usual solution, but the trade-off is limited range.

Now, with smart antennas, you can have your cake and eat it too. A smart antenna is an antenna that uses directional antennas, but has the ability to steer the beam in any direction. Some kind of antenna array that can produce multiple beams is hooked up to some fast switches so that the beam can be quickly pointed in the optimum direction to maximize the signal strength. By using beam-switched gain antennas on both ends of the link, you can optimize performance to the conditions.

That is what Airgain has done. The company’s patented beam switching antenna can greatly improve the performance of some wireless applications. Their MaxBeam 75, shown in Figure 1, is designed for use in Wi-Fi APs, routers, and gateways used in enterprise and home networks. Airgain CEO Pertti Visuri explained the antenna this way: If you look closely, you will see four unusually-shaped monopole antennas at each corner of a ground plane. In the center is an X-shaped passive reflector for the antennas that provides the gain and directionality. All of the antenna elements are fed by transmission lines and matching circuits that tie together back to the single 50-ohm input/output connector. PIN diodes connected to each antenna switch each antenna off or on. By turning on two adjacent antennas at a time, it is possible to create eight beams over a 360° range. Turning on a third beam in some positions creates a total of 10 beams. The final result is an antenna that produces as much as a 300% increase in performance over that of a conventional dipole. Typical gain is 7 to 7.5 dBi.

The MaxBeam 75 is only 90 mm square, so it can fit inside most AP housings. And it is fully compatible with any 802.11b/g AP. The beam switching software runs on the host processor in the AP and the control signals are sent to the PIN diodes over a 6-wire cable. The software makes the decisions about what antennas to turn off and on to zero in on the signal for the best transmission and reception.

Airgain makes a number of other antennas that use the same beam switching techniques. Their MaxBeam65N incorporates MIMO, so it is compatible with the forthcoming 801.11n standard. A version called MaxBeam80N also uses MIMO. It covers the 5.7-5.9 GHz range, in addition to the 2.4 -2.49 GHz band. The H Smart antenna is designed for CardBus wireless products. And their MaxBeam 30 is a collinear array that can replace dipoles in some Wi-Fi and ISM band products.