Wireless Systems Design

Antennas: The Black Art Of Wireless

New designs boost efficiency as multiple antennas per product becomes common

I have always been amazed by how much difference a good antenna makes in a wireless application. It became pretty clear to me at an early age as a ham radio operator that a good antenna is the difference between making a connection or not. A poor antenna, or one that is incorrectly oriented, or not properly Z-matched to the equipment, will leave you disconnected—while a good antenna will make it seem like the transmitter and receiver are actually wired. Anyway, you get the point. The antenna is the transducer to the ether and just as important as a good connector in a wired communication. But now, having stated the obvious let me say that designing a new antenna is just as much art as it is a science. The theory is well known and the design/simulation software is better than ever, but antenna design seems to thrive on intuition, a bit of art and some believe actual magic, and lots of experimentation. Empirical design is a good term to use here. Unique configurations and combinations are leading to some interesting new things with those little pieces of metal we call antennas. Over a Billion Units Sold
The antenna business if flourishing because of the enormous growth of all things wireless. With cellular handsets selling at well over one billion units per year these days, that equates to well over a billion antennas. Remember that most cell phones today have more than one radio in them. Multiband cell phones with 800-900-MHz coverage also usually cover 1800-1900 MHz. That often means two antennas. Bluetooth is another example and it shows up in roughly half the cell phones today. Add another half billion separate antennas for that. Then realize that many cell phones also have GPS and/or Wi-Fi—add another couple hundred million units. And what about all those Wi-Fi antennas in laptops, access points, home routers, and soon-to-come WiMAX antennas? Now that Wi-Fi and WiMAX are using multiple-in multiple-out (MIMO) that means multiple antennas per product. Maybe the antenna business is not as good as the razor blade, panty hose or battery business, but it is certainly high volume. The key to making the antenna work effectively is to make it as efficient as possible and provide isolation between antennas. That is hard to do in a handset that is very small and you are loading the antenna as you hold it to your head. Yet manufacturers are making some great progress in these areas. I spoke with two companies recently that have some interesting antenna developments you should be aware of. Ethertronics
Ethertronics (www.ethertronics.com) just launched their new Savvi line of ceramic antennas. These antennas use a patented design called Isolated Magnetic Dipole (IMD). Its unique shape gives it not only an omnidirectional radiation pattern but also provides excellent isolation from surrounding items. This makes it immune from frequency shifts caused by loading of close by objects. And that isolation characteristic makes it possible to mount the antenna close to another active antenna. One of their first Savvi products is a dual-band, dual-feed GPS-Bluetooth antenna. It solves the problem of adding Bluetooth and GPS to a cell phone. The antenna is housed in a 14- by 4- by 1.3-mm package and it can be co-located with the main cell-phone antenna. The isolation spec is greater than -20 dB. Efficiency average is in the 64% to 75 % range. A Bluetooth-only antenna is also available. Ethertronics also plans additional antennas in this line for Wi-Fi (2.4 to 2.5 GHz and 4.9 to 5.8 GHz), WiMAX (2.5 to 2.7 GHz and 3.3 to 3.8 GHz) as well as a PCS (1.93 to 1.99 GHz) plus WiMAX. SkyCross
Another unique design uses the Isolated Mode Antenna Technology (iMAT) of SkyCross (www.skycross.com). The iMAT addresses the MIMO antenna problem. MIMO uses two or more transceivers and antennas to create several parallel and independent streams of data on the same channel. This multiplies the data rate in the same channel size and also provides some relief from the multipath problem that plagues microwave devices. And it improves overall link reliability. Technologies like 802.11n Wi-Fi, WiMAX, HSPA 3G, and LTE 4G all use MIMO. The key to making it work and deriving the benefits is to keep the antennas as separated from one another as possible. That is OK in a basestation, home router, gateway, or a laptop. But it is tricky, if not impossible, in a handset. It is just too small. So antennas get squeezed together and couple to one another thereby lowering efficiency. The SkyCross iMAT design uses a single antenna in place of two antennas. By providing multiple feed points with high isolation between them, a single antenna can give the effect of two. With two feed points properly spaced and matched, the two signals can be isolated from one another sufficiently to prevent significant coupling. And because of the two feed points, the radiation pattern for each independent signal is different. The result is what SkyCross calls pattern diversity. It's a cool concept that lets you do a x2 MIMO with a single antenna. SkyCross has iMAT antennas for WiMAX and Wi-Fi now with other designs on the way. SiBeam
Another innovative antenna design is the phased array on-a-chip developed by SiBeam. It is part of the chipset for the 60-GHz band WirelessHD standard. WirelessHD defines a way to transmit high speed video for HDTV in the consumer space. Although I do not have details at this time, the antenna does automatic beam steering to initiate and maintain a link between source and destination. That process avoids the need to set up precise line-of-sight (LOS) antenna alignment usually required at these millimeter wave frequencies. I hope to have more on WirelessHD and the 60 GHz mm wave movement for a future issue. One Last Word
A recent article in the Wall Street Journal on antennas caught my eye. It talks about the problems of car antennas. The older whip or stick antennas are slowly going away. Less than 50% of all new vehicle antennas are of this type and that is only 25% on cars. Whip antennas are very easily broken and power-antenna mechanisms are even more prone to problems. Yet the whip is still the best antenna for AM and FM radio reception. More and more manufacturers are embedding the antenna into a window although these designs always seem to be less effective. Manufacturers are continuing to seek an antenna that is not only effective but unobtrusive and damage proof. The small GPS and satellite radio antennas on newer vehicles are a good example. At those high frequencies, small antennas are possible but for AM and FM stations you need more length to get a decent signal. Anyway, there is a great opportunity for anyone wanting to invent the next great vehicle antenna. High volume is assured.

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