Embedded systems design is garnering a lot of attention these days, but for many designers, the embedded antenna remains somewhat of a mystery. Indeed, antennas have always had a bit of RF mystique around them, and probably will continue to do so.
In a bid to demystify the process, this article takes a closer look at the key considerations for designers as they select and design in their next embedded antenna, which could be responsible for targeting mobile voice, video and/or data.
For designers of portable and embedded systems, the evolution from single band/narrowband to multiband/wideband has been rapid, with antenna designers reporting demand for penta-band coverage in the cellular bands (850 MHz to 2 GHz).
Complicating the drive to multiband systems is the desire for components to be the smallest size possible--and the antenna is no exception.
"Devices are shrinking, and end users want sleek devices without an antenna protruding from the side," observed Paul Tornatta, managing director and vice president of operations for Melbourne, Fla.-based antenna vendor SkyCross USA.
In the past, design activity seemed to be concentrated in single bands moving higher in frequency. Now, by contrast, the push to high-frequency systems is taking place simultaneously with a growing interest in low-frequency systems.
DriveCam product from Ethertronics is a custom quad-band embedded antenna that operates over the GSM 850, GSM 900, DCS and PCS bands for sending video data over the cellular network. |
"These low-frequency requirements turn out to be more challenging than main antenna design and integration because of physics limitations," said Jeff Shamblin, vice president of advanced technology at Ethertronics (San Diego). "Efficiency rolls off more rapidly, due to the antenna's being electrically small at the lower-frequency bands."
What's the best way to attack these low-frequency bands?
Shamblin said Ethertronics is developing new passive and active antenna configurations and is also working with "new materials to cope with the requirement for a physically small internal antenna with a specific efficiency. We have some solutions in the works."
For the embedded antennas used for mobile data and video, important specs include power consumption, efficiency, return loss and isolation.
• Efficiency. When electrical power is supplied to an antenna element, efficiency measures how much of it is converted into electromagnetic power. In real-world applications, some loss of power is expected because of resistive and dielectric elements. High efficiency translates into better signal reception, which reduces the number of dropped connections and improves the system's ability to support fast data transfer rates.
Depending on the application, efficiency can be traded off to improve other specs.
"Because mobile video is a receive-only service, efficiency is often sacrificed for size," said Tornatta of SkyCross. "Wireless data requires both transmitting and receiving, so efficiency is a critical specification for both the signal strength and the battery life."
Shamblin agreed that efficiency is important, as he summed up some of the trade-offs facing antenna designers.
"Achieving the efficiency and bandwidth for the internal media antenna in a small volume, along with power consumption [which will affect battery life], are the main design concerns," he said.
• Return loss is a measure of the power reflected or returned compared with the power transmitted.
Tornatta said SkyCross antennas are all designed to have a return loss of –10 dB or less, meaning that at least 90 percent of the electrical energy generated by the radio is transferred into electromagnetic wave energy, rather than being reflected back.
According to its data sheet, Ethertronics' Prestta quad-band embedded mobile phone antenna also boasts return loss of better than –10 dB across 0.5 GHz to 2.5 GHz.
• Selectivity and isolation. If the design is going to be equipped with multiple antennas, then selectivity becomes very important.
"Currently, we see six to seven antennas in some cell phones and mobile devices, which requires co-location of these antennas in a small volume," Ethertronics' Shamblin said. "So, besides the efficiency that will impact the power consumption of the system, we have to consider the selectivity of each of the antennas to avoid crosstalk and interference."
Many wireless data protocols require receive diversity or multiple-in multiple-out (MIMO) architectures. In those instances, isolation--the measure of radiation traveling to a neighboring antenna (coupling)--is particularly important, so the best case is to have high isolation and low coupling.
"It is most desirable to have the radiation patterns as different as possible so that each feed or antenna covers a new spatial region to achieve pattern diversity and signal independence," said Tornatta of SkyCross. "This makes the way the device is 'pointed' with respect to towers or base units less crucial."
With the impending release of the 802.11n WLAN specification, and a market already featuring "pre-n" products, antenna designers have had to consider the impact of MIMO designs requiring multiple antennas. For example, SkyCross has released its iMAT product line, which enables a single antenna structure to behave like multiple antennas through the use of multiple feed points.
"Each feed accesses the antenna as if it consisted of multiple antennas, each with high isolation, low correlation and high per-feed antenna efficiency," said Tornatta. "The resulting improvement in antenna gain and receiver sensitivity significantly enhances device performance and network capacity."
Ethertronics, on the other hand, addresses this issue with its proprietary Isolated Magnetic Dipole (IMD) technology, which aims to confine current flow on the antenna and optimize isolation.
"The potential for interference among the multiple antennas in the cell phone must be addressed during the design phase to ensure a high-quality, robust antenna suite," said Shamblin.
According to the company, the proprietary IMD antenna technology controls near-field emissions in order to ensure that designs meet specific absorption-rate requirements in different markets.
By its nature, the approach also can reduce interaction with surrounding components on the circuit board as well as the end user's body, according to Ethertronics.
So, what are the major design challenges facing antenna designers for next-generation designs?
Shamblin defined them as "achieving the required bandwidth in a small-volume and cost-effective form factor, and minimizing performance roll-off at band edges for wideband applications."
"Size is the most challenging area of development for new an- tennas," said Tornatta. "For mobile video, the space challenge is driven by low frequencies and wide bandwidth."
Clearly, embedded systems designers who are looking for their next antenna can benefit greatly from advances in antenna technology. Both custom and standard antenna designs are available, and the final trade-offs for the decision will likely be time-to-market, cost and performance.
Janine Love (firstname.lastname@example.org) is site editor of RFDesignLine.