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Successful Design Strategies for Embedded Antennas

Thu, 07/08/2010 - 12:50pm
Pertti Visuri, Ph.D., Airgain Inc.

Consumer demand for internet and video content throughout the home has expanded the market for in-home wireless networking devices. A new generation of wirelessly enabled products is emerging. In addition to standard networking devices such as wireless routers and gateways, today, media phones, gaming devices, set-top boxes, digital televisions and more are being designed with wireless functionality in mind. For this new breed of wireless communication device, consumer satisfaction is directly correlated to wireless performance, an area where antenna design plays a crucial role.

In first generation wireless devices, external antennas were the norm and their integration was frequently an afterthought for device designers. As designers shifted to more demanding design requirements, smaller form factors and embedded antennas; antenna integration became more complex. Now, to ensure maximum performance in these new wireless devices, the antenna system needs to be among the first considerations in the product design.

Sub-Optimal Antenna Placement
Often the problems that a device maker attributes to the antenna are simply the result of sub-optimal antenna placement. This can include insufficient space in the device allocated for the antennas, positioning the antenna where other components obstruct the signal pattern, or inadequate separation from other antennas in the device.

To compensate, device designers sometimes resort to adding power amplifiers to increase transmit power and low noise amplifiers to minimize noise from other board components. However, with a well thought out antenna system, the same performance can be achieved without such additional cost.

Fundamental Design and Integration Factors
A superior antenna system begins with the antenna. Selecting an antenna style that optimizes antenna gain and efficiency can maximize the device transmit power, leading to increased range and throughput, as well as more reliable connectivity. Once this is defined, proper integration of the antenna within the industrial design (ID) must take into account a variety of factors to ensure optimal device performance.

In our experience, four fundamental design and integration factors should be considered when designing for embedded antennas. These include board-level issues, housing and industrial design considerations, antenna selection, and antenna placement.

Noise is present on every functioning circuit board. The sources of noise can include the SDRAM and DDRAM processors, bus and interface, radio interference and coupling via other board components. At the board level, the design goal is to reduce the antenna’s exposure to such noise through proper placement.

In the industrial design housing, care must be taken to limit space constraints between the antenna, the circuit board, and the outer casing. Proximity to key board components and other internal structures or metal housing parts must also be considered. Any of these features can obstruct the antenna signal pattern and create interference which also impedes performance.

Successful Design Strategies for Embedded Antennas
click to enlarge

Figure 1.
The antenna design itself is a key consideration. Attention must be paid to where the resonance currents in the antenna primarily flow, as well as the polarity of the signal and overall antenna gain patterns. The most effective way to determine whether the right antenna choice has been made is to measure the antenna gain and wireless throughput early and often in the engineering integration process.

Antenna Placement is Critical
All three of these considerations — board design, housing design and antenna choice — effect the final consideration: antenna placement. Placement within the device is critical, and any number of problems can arise from placement too near or far from metal parts, metallic paint, or the plastic casing. These can include loss of antenna gain, distorted gain patterns, loss of efficiency, and antenna coupling. A mismatch between the antenna and its feeding circuit can also cause de-tuning, whereby the signal no longer radiates in the desired central frequency.

Antennas must also be mounted close to the plastic casing or onto a conducting ground surface, and because antennas are so sensitive to placement, it’s the first place to explore when a device is not performing as expected.

Unfortunately, many device manufacturers have introduced products to market only to discover too late that their antenna integration may have resulted in dramatically reduced system performance.

That is not to suggest that innovative device designs are not possible. Interesting housing shapes and vertical, horizontal or ceiling mounted devices can all be made to work well with embedded antenna solutions as long as the antenna integration is considered early in the design process.

Next-Generation Devices
However, as next generation devices become ever more compact, the antenna system design becomes much more complex and challenging.

For example, antenna placement within 1 to 3 mm of an ABS plastic case may shift its central resonating frequency by several hundred megahertz. If this outcome is taken into account early in the device design phase, an antenna design can be incorporated that includes the mounting, casing and attachment as a system that will properly resonate in the central frequency of the band.

Successful Design Strategies for Embedded Antennas
click to enlarge

Figure 2.
The best method to avoid antenna integration issues is to involve your antenna partner early, preferably when the PCBA, ID and RF specs and requirements are being developed. A good antenna partner can work with the design team to give advice on the ID design, provide engineering samples and integration assistance, and test gain patterns and throughput to fine tune the antenna as the design concept is refined (see Figure 2).

Proper and thorough testing is also essential throughout the engineering integration process. In an indoor environment, minor changes in device location and orientation can elicit dramatic changes in wireless performance. To get reliable and repeatable test results of over-the-air throughput performance, device designers and their antenna partners need to perform hundreds of measurements to average out random variations. While the testing is time-consuming, it can be automated by using computer controlled turntables.

Conclusion
Wireless performance will be a key attribute driving customer satisfaction in next generation in-home networking devices. Working closely with antenna experts at every step of the device design process helps achieve excellent wireless performance at relatively low cost.

Pertti Visuri, Ph.D., is president and CEO of Airgain Inc. Dr. Visuri holds a Ph.D. in Engineering Physics and a business degree from the Helsinki University of Technology and is a graduate of the Executive MBA program from the University of Michigan. For more, see www.airgain.com

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