Wireless Systems Design

A Bent Piece Of Metal

I was talking recently to Rick Segil, the VP of marketing of Ethertronics, an antenna company in San Diego, Calif. I got a laugh out of his comment that what his company does to be successful is sell bent pieces of metal. But I guess that is what any antenna manufacturer does, in general. Antennas are metal structures that are indeed usually made of some kind of bent metal. They are the transducers that convert the voltages and currents into electric and magnetic fields and vise versa. And every radio needs a good antenna. You can have the world's best transmitter, receiver, protocol, technology, etc., but without the antenna the signal goes nowhere. It is the critical link in any wireless system, despite the fact that many wireless engineers forget how vital it is.

As wireless devices have continued to move to higher and higher frequencies, antennas thankfully have gotten smaller. Now instead of the "whip" antenna that used to be a common sight on cordless phones, cell phones, and other wireless devices, antennas are typically totally internal. If you see any vestige of an antenna on a product, it is a small protrusion at most. But that does not make it any less important. If anything, putting the antenna inside the product has worsened its performance. Not having to extend or otherwise fuss with a telescoping whip antenna is great for the user. Getting rid of that external antenna also reduced the repairs on cell phones and other wireless devices. Next to bad batteries, broken antennas accounted for the bulk of repairs. No more.

With cell phones operating in the high UHF and low microwave region, the antenna is only a few inches long. Yet it has to fit inside the cell phone. Some designs incorporated the antenna into the PC board copper. But today, separate antennas optimized for best performance are used. A popular antenna is the planar inverted F antenna (PIFA). It works well enough, but it is also subject to loading by any surrounding objects. Any devices near the antenna, like discrete components or camera modules, tend to couple to the antenna and absorb some of the radiated power—thereby making the antenna less efficient. Another huge problem is the loading of the antenna by the user's hand and head. Holding the cell phone and putting it up to your ear detunes the antenna by coupling energy to your body, thereby reducing efficiency. That also produces an excessive specific absorption rate (SAR), the measure of how much RF is coupled into a person's body. Not a good thing and something to be reduced to the lowest possible level.

This coupling has the effect of reducing the received signal strength and decreasing the amount of radiated signal. The effect in a cell phone is less coverage and more dropped calls. In some phones, poor signal strength is compensated for by increasing the power radiated and that shortens battery life. And in a data transfer situation, less signal strength translates to backed off data rates. All that because of a poor antenna.

Anyway, Ethertronics has come up with their own unique bent piece of metal known as the Isolated Magnetic Dipole. It is a patented design that mostly confines the current flow in the antenna and reduces the coupling and interference to nearby objects. And it has a very low SAR rating. In cell phones the overall efficiency is improved over a PFIA from less than 20% to over 80%, depending the band of operation. It offers multiband operation and is physically a bit smaller than competing types. And it can be more easily fit into designs—even when the antenna turns out to be the after thought of the design.

Antenna design is essentially a black art. You use RF theory in the design, but a final product is more usually the result of lots of empirical efforts like cut-and-try, trial-and-error, and endless experimentation. The result is often some unique design that is patentable like the IMD. Other antenna companies are also coming up with unique designs. Two that come to mind are Sarantel and Skycross. Some of the really innovative designs are coming from companies that are making ultra wideband antennas (UWB). Fractus has an antenna that is based on fractals and manages to give low SWR over the huge bandwidth UWB needs.

With cell phones, and other devices like laptops and PDAs, multiple antennas are becoming the norm. While a single antenna is common today, it is projected that the typical cell phone could end up with 3 to 4 antennas by 2008. Besides the cell phone antenna there will be separate antennas for Bluetooth, Wi-Fi, GPS and who knows what else. NFC and WiMAX are even possibilities. Where will they fit and especially how will you keep one from influencing the others? A real challenge for the antenna companies. And let's hope their innovation will solve this tough problem so we do not have to.

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