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05 September 2010
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Top 3 considerations when selecting an antenna
By
Jeff Shamblin, Ethertronics
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Courtesy of
RF Designline
May 12, 2008
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As wireless phones continue to evolve and increase their functionality beyond basic mobile communications, added functions such as digital cameras, location based services, Bluetooth capability, mobile Internet, and other features increasingly complicate the already difficult antenna selection process. All of these applications require hardware components that occupy prime real estate within today's small phones, which ultimately leaves less space for the antennas and a bigger headache for RF engineers. In today's world where the smallest, thinnest and most versatile functioning mobile devices are ranked supreme, device manufacturers are finding themselves at a cross road where performance meets size and making the wrong design tradeoff can have severe RF performance implications.
When selecting an antenna for any given mobile device, there are three main factors that device manufacturers should address to ensure optimal performance. These characteristics encapsulate the most crucial factors for end user satisfaction, which all rely on the selected antenna.
#1: Efficiency
The most important quality an antenna offers is its efficiency. Despite color, industrial design, and any other aesthetic quality a phone may offer, if it drops too many calls or suffers from impaired data rates, it will not be a commercial success. As mobile devices shrink, many industrial design engineers are also shrinking the size allocated to antennas to coincide with this trend. It is essential, however, to ensure that the reduced size does not sacrifice overall performance. For the main antenna, it is best to aim for a minimum of 50% efficiency at the desired bandwidth. For supporting or supplemental antennas, a target of 60% or better efficiency for GPS functions and 40% for Bluetooth is reasonable. Diversity antennas can work well with efficiencies in the range of 30 to 40%.
#2: Isolation
Another aspect to keep a watchful eye on is the isolation between antennas. On a circuit board where as many as five or more antennas may co-exist within a small and confined space, it is absolutely critical to make sure that the antennas have the least amount of interference amongst each other. If this situation is not addressed in the design phase, the device risks overall poor performance due to antenna coupling effects. The more isolation the antenna has, the closer device engineers will be able to place it next to other electronic components. This helps free up valuable space on the circuit board and can also help reduce the overall size of the device. A well-designed antenna for a small form factor wireless device will remain tuned when components are placed within 3 mm. Fifteen to 20 dB of isolation should be a target for isolation between the multitudes of antennas in today's devices. Chart 1 shows an example of isolation measurements for a combination GPS/Bluetooth antenna with high isolation of 29dB for GPS and 24dB for Bluetooth.
1. Example of isolation measurements for a combination GPS/Bluetooth antenna with high isolation.
#3: Selectivity
Finally, the natural third factor engineers must consider when selecting an antenna is the antenna's selectivity. In addition to increasing the antenna's isolation, good selectivity also acts as a cost-effective method to suppress interference between multiple applications. The selectivity of an antenna must allow it to be fully-functional across the frequency spectrum of interest without losing reception near the ends of the bands. At frequencies outside the band of operation of the antenna, the antenna response should roll off quickly, providing some filter characteristics. When looking at an antenna's selectivity, look for consistent reception throughout the entire bandwidth with a sharp-cutoff out of band. Some antennas tend to only perform at specific frequencies within a given bandwidth, which can also lead to poor service. Chart 2 shows an example of two GPS antennas whose efficiency is plotted against frequency. Again, it is best to select antennas such as the blue one in the chart, which has a sharp roll-off outside the operational band of the antenna as opposed to the other red antenna in the chart, which may have good efficiency but no roll-off outside of the desired band of operation.
2. The blue line shows an antenna with good performance: a sharp roll off outside the operational band.
While the antenna may not seem to be a key selling point or determining factor of any particular mobile device and its commercial success, its performance capabilities definitely have the ability to make or break the device in the marketplace. A phone's ability to communicate (without suffering dropped calls or low data rates) is the ultimate criteria by which consumers will judge the merits of a phone. This is why it is absolutely critical to evaluate the overall antenna performance in terms of efficiency, isolation, and selectivity before integrating it into a device simply because "it fits." While time-to-market and cost also play a part in the ultimate decision for antenna choice, those factors are only part of the picture. If a company is able to produce a timely, multi-functioning, low-cost handset, the performance of the device as it relates to reliable communications will ultimately decide its commercial success.
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