I’m sure everyone has had the pleasure of their 802.11 network randomly dropping connections. I might even guess that a cordless phone or microwave caused some of the aggravation. Learning that one device’s useful Radio Frequency (RF) signals can be another device’s RF interference is an important lesson.
Pehaps your smartphone was one of the devices that lost its connection when someone used the cordless phone. Now wait a minute, how’s that possible? The smartphone is crammed full of radios, and it works just fine. Come to think of it, so does a wireless notebook. The fact that Bluetooth, WiMAX, 3G, and Wi-Fi can politely coexist in the same device is indeed amazing. How’s it possible to create a friendly RF environment when all those radios are so close to each other?
New antenna technology to the rescue
Being able to co-locate multiple RF systems within millimeters of each other in modern wireless devices is in large part due to reducing RF interference. Many of these “interference-busting” innovations are the result of RF engineers making improvements to antenna systems in the following critical areas:
Antenna efficiency, considered the most important parameter, is the ratio of radiated power to input power. The percentage is a measure of the antenna’s dissipative losses, which are due mainly to mismatched Electromagnetic Field (EMF) characteristics. New materials and engineering expertise have significantly improved this percentage. I remember being happy with 20-30 percent efficiency. Today, engineers are looking at a minimum of 50-60 percent efficiency. This is especially important with wireless devices getting smaller and the demand for longer battery life getting bigger.
RF isolation is all about antenna coupling and is a good thing when antenna coupling occurs between radios that are supposed to be communicating. Conversely, inadvertent antenna coupling between radios that aren’t trying to communicate diverts valuable RF energy to the wrong receiver. For example, if the multiple antennas of an 802.11n device are not properly isolated, a portion of the RF signal meant for a remote device may be captured by another of its own antennas. A normal rule of thumb is to have 15-20 dB of isolation between adjacent antennas.
To further explain this concept, I’d suggest reading an excellent paper by Antenova and the University of London, “Antenna designs for MIMO systems” in which the researchers are trying to maximize RF isolation in different form factors. For example, the following excerpt (courtesy of Antenova) is the test fixture used to mimic possible locations for a MIMO antenna system in a typical notebook lid.
The graph below (also courtesy of Antenova) portrays the results from the tests run on the above fixture and the relationships created between the individual antennas.
As can be seen, there’s definitely interaction between antennas, with the least amount of isolation or greatest antenna coupling occurring between antennas one and two.
Ruckus Wireless is another company that’s doing some exciting work with respect to RF isolation by using beam-forming technology. I felt it important enough to write an article called “802.11n: MIMO really needs smart antennas.” The following image from Ruckus Wireless depicts one of their beam-forming antenna designs that warp RF signals directionally, creating an optimal RF link with the remote device. This process has the added advantage of heightened RF signal rejection from unwanted sources.
Antenna selectivity, or Q, is a critical parameter when a wireless device uses multiple radios. Q is a quantifiable number that signifies tuned bandwidth of an antenna. A lower Q will allow an antenna to have more bandwidth or a wider working frequency range. In many cases that’s good, but in devices that have multiple radios, each antenna should be very selective or have a high Q, meaning the antenna is tuned to resonate at the design frequency of the radio it’s attached to and that’s all. By doing so, the antenna acts like a filter, rejecting unwanted RF signals that could overload the receiver.
So what’s it all mean?
Equipment developers are painfully aware of the fact that every design is a compromise. They make decisions on whether function, form, or performance is most important at every step of the process. My goal with this article is to promote awareness of that subjective process, as doing so allows everyone to decide on whether they agree with the designer’s choice.
I doubt seriously if you’ll easily find vendor literature written about this subject, but it’s available if you’re persistent. I ask for these specifications every time I read about a new phone. For instance, I happen to use the Samsung Blackjack II, and SkyCross developed the antenna systems for that particular phone. I’ve even written an article, “iMAT: MIMO without multiple antennas,” about their use of metamaterials.
Ultimately, I’d like to see antenna system performance become a key selling point rather than an afterthought or a never thought of. This would especially benefit purchasing departments in enterprise settings where lost time and money from inefficient wireless device operation becomes significant due to the sheer number of users.