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Getting the signal
Hint: Do not leave antenna selection until the last step of a wireless design
By Gary Legg, Contributing Editor -- Design News, December 16, 2002
All too often, the antenna for a wireless design is an afterthought. In a typical scenario, says Dax Craig, vice president of business development for antenna manufacturer Centurion (Lincoln, NE), a design is complete and the designers finally want to attach an antenna—any antenna. That's a costly mistake, says Craig, because even though antennas may seem simple, they're not.
And it isn't just the choice of an antenna that's complicated. Designing an antenna into a system is even more complicated, because antennas interact with components around them, and those interactions alter antenna performance. "Antennas are fussy little devices," says Craig.
Putting an antenna inside a device, which is now the norm for mobile phones and PDAs, adds still more complexity. Most cases are grounded, notes Patrik Byhmer, director of marketing and sales for gigaAnt (Lund, Sweden), either because the case is made of metal, such as magnesium, or is covered with paint that has metal or carbon in it. The result, Byhmer says, is that getting wireless signals in and out of the case can be tricky. Mechanical constraints come into play, too, Byhmer adds. For instance, he says, the speaker in a PDA is normally the same length as a quarter-wavelength antenna for the 2.4-GHz frequency band used for 802.11b (Wi-Fi) wireless LANs and Bluetooth devices. That means that it works as a parasitic and absorbs energy from the antenna.
Complexities abound. Because engineers who aren't antenna experts are often unaware of these kinds of complexities, antenna companies like to work with application designers as collaborators. Engineers from gigaAnt collaborated with Compaq on the iPAQ PDA, for example, and with Sony Ericsson on the Bluetooth-enabled T68 mobile phone with a custom-developed antenna package that includes two antennas—a 2.4-GHz antenna for Bluetooth, and a 3-band antenna that works at the different frequencies used in different mobile-phone standards.
In choosing an antenna, says Centurion's Craig, the first question to ask is whether the antenna is going to be internal or external to the device. It's easier to get good performance from an external antenna, but it has less esthetic appeal, especially in consumer products, and can break off, resulting in costly product returns. In the latest mobile phones and PDAs, almost all antennas are internal.
For many products, a key consideration in antenna selection is cost. Cell phone makers, for example, usually won't accept antennas that cost more than about $1.50, and they prefer much cheaper. An important question to ask, says Centurion's Craig, is, "Are you looking for value, or are you looking for superb, excellent performance?" The way you answer that question, he says, will determine the path you go down.
Retractable and nonretractable external antennas provide a good example for the value-or-performance design decision. A retractable antenna, Craig notes, is a better performer, because it has more surface area, but it is far more expensive than a fixed antenna. A fixed antenna can perform fairly well, he says, but rarely will rival a retractable antenna's performance, although it's much cheaper and makes a greater visual impact
Choices, choices. You might have to make decisions such as this for all kinds of antennas, however. Antenna choices include devices known as whips, stubbies, patches, chips, and PIFA, which stands for planar inverted-F antenna, because it looks like a capital F lying on its side. All have different shapes, sizes, and performance characteristics. Each has advantages and disadvantages, and some types suit certain applications far better than others.
If your antenna doesn't have to be internal to a wireless device, for example, then high isolation isn't usually a major requirement, and a quarter-wavelength stub antenna can be a good option. A "stubby" is fairly small and, because it mounts externally, has a fairly clear path for transmission and reception without interference from nearby electronic components. It does, however, require a ground plane, such as a chassis or a metal plate. In this instance, a metal product case is an advantage, whereas it creates complications for an internal antenna.
If your design can tolerate a slightly larger antenna than a stubby, using a ground-independent half-wave dipole gets around the ground-plane requirement and leaves you free to choose a product case of nonconducting plastic. The antennas length is nominally half a wavelength of a signal at the frequency of interest.
Other antenna choices include chip antennas, which are very small, but not the best performers. Patch antennas are noted for their low profiles, while a PIFA model provides good electrical performance but at a cost of being much thicker than a patch antenna.
Antenna selection almost always involves evaluation of technical parameters. Radiation patterns, for example, can be directional or omnidirectional, and they can be aligned near the earth—good for cell phones—or can cover the sky—for reception from GPS satellites. High isolation is important if an antenna has to be near other electronic components. High efficiency is necessary for long battery life in portable equipment. Antenna manufacturers can help you evaluate these parameters and others. It's not a job for the uninitiated.
To make the antenna-selection process even more difficult, some new types of antennas, based on unconventional antenna technologies, are just now starting to become available. Startup companies e-tenna, Ethertronics, and SkyCross, for example, use either new technology or technology previously used only in military applications to create antennas of extremely small size and weight, very high efficiency, and very high isolation. Antenna isolation is so high, in fact, according to these companies, that the antennas can often go on a circuit board among other electronic components and still not suffer from component interaction.
Consider these benefits of the new antennas, too. The 2.4-GHz ClearLink antenna from e-tenna is about the size of a cuff link and weighs only 0.12 g. SkyCross says its antennas are so efficient that, when one is installed in a cell phone, it can decrease the phone's battery-power consumption by 40% compared to the same phone with a conventional antenna.
New antenna technologies even promise to radically alter wireless product design. The technology used by e-tenna, for example, produces antennas that are essentially a foil-like material that inherently incorporates some shielding from electromagnetic interference (EMI). In a joint project undertaken by e-tenna and Ericsson, the Swedish mobile-phone giant, the two companies put a radio transceiver inside one of e-tenna's antennas, measuring only 10 × 14 × 2.5 mm. The antenna actually shields the radio from stray electromagnetic interference.
Go to www.designnews.com/info and enter the numbers below:
Antennas from Centurian;
www.centurion.com:
Enter 545
Antennas from e-tenna;
www.etenna.com:
Enter 546
Antennas from ethertronics;
www.ethertronics.com :
Enter 547
Antennas from gigaAnt;
www.gigaant.com:
Enter 548
Antennas from Skycross;
www.skycross.com:
Enter 549
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