24 Apr 2017

Designing for the IoT: Overcoming the antenna challenge

Tom Nordman of Silicon Labs provides some practical ideas for the perennial issue of antennas for IoT applications.

We’ve quickly become accustomed to things around us being connected to the Internet. From light bulbs to fitness trackers and even our cars – not to mention all the connected devices found in industry – this Internet of Things (IoT) is opening up new ways to improve our lives and work.

But the quest to create a truly connected world is giving rise to fresh challenges for those designing the equipment. This is largely down to the demands to produce devices that are simultaneously small, cheap and ergonomic, while also offering robust radio frequency (RF) connectivity.

Not too much to ask, right?

The antenna challenge

Achieving all these goals is no mean feat, not least when it comes to the communications antenna. When designing this part of their product, engineers face lots of questions. How much space does the antenna require? What type of antenna is best? Should they use a module with an integrated antenna, or go for an external one?

These aren’t simple questions to answer. There are many variables to consider, including size, RF efficiency and detuning issues. Making the right decision is particularly important when an antenna architecture design may be used in more than one type of housing.

PCB trace antennas

A popular choice in IoT designs is the PCB trace antenna (such as the inverted-F), which is a relatively cheap solution. However, this type of antenna has a large footprint, typically around 15 mm x 25 mm, which has implications for the overall device size and is at odds with the IoT obsession with miniaturisation. Moreover, when used in a module, PCB trace antennas are sensitive to detuning caused by housing materials. If they’re to work effectively, designers must give them special attention when it comes to overall product assembly.

 IoT PCB Antenna

Figure 1 IoT PCB Antenna

Chip antennas

To simplify designs, many antenna manufacturers offer ”chip antennas,” which come in two variants: those not coupled to the ground plane (which need a large clearance area, or one free from components, traces and the ground), and those coupled to the ground plane (which don’t require a clearance area, or only require a small clearance underneath the antenna).

When producing their size estimations, IoT device designers must remember to include the antenna and its necessary clearance areas, as well as leaving sufficient distance to the edge of the housing.

Size-efficiency trade-offs

Given the need for many IoT devices to be very small – sometimes just the size of a coin cell battery – there’s a trade-off when it comes to the efficiency of the antenna. The smaller the unit, the less efficient RF performance will be.

Devices measuring up to 10 mm in each dimension start delivering performance in the 2.4 GHz band. This enables Bluetooth connectivity with a smart phone up to 10 meters away. For most personal IoT devices, this range is perfectly acceptable.

Increase these dimensions to nearer 20 mm and RF efficiency jumps, now offering a range between 20 m and 40 m, depending on ambient conditions.

Double the dimensions again to 40 mm, and antennas that tune with the ground plane size hit optimal performance. Using Bluetooth 4.2, two identical pieces of equipment can communicate over a range between 60 m and 400 m. This figure can exceed 500 m (in line of sight) when using zigbee and other IEEE 802.15.4 protocols.

Because most chip antennas make use of the PCB ground plane as part of the antenna setup, designers must think about RF performance and efficiency in relation to the overall size of their PCB.

External antennas

Many IoT designers consider external antennas, such as those built into the housing using a U.FI connector. However, few IoT devices end up pursuing this route, preferring a built-in chip antenna instead. There are several reasons for this. External antennas can make the overall device design less practical and less attractive, and can even break if dropped. They also push up the bill of materials and assembly cost. Moreover, external antennas offer little in the way of RF efficiency gains over well-designed chip antennas.

That said, if the device housing is made of metal, the resulting impenetrable faraday cage will make an external antenna a necessity.

Avoiding detuning

To avoid antenna detuning, designers must pay particular attention to the device housing and the materials surrounding the antenna. If the antenna touches metal or plastic, it will detune, so it must not make physical contact with such materials.

Remember, different types of antenna offer different levels of detuning sensitivity: a ground-coupled antenna is much less sensitive than a monopole-type antenna, for example.

The System in Package (SiP) approach A system in package (SiP) combines a system-on-a-chip (SoC) device with an antenna and other components, providing an ultra-small-footprint solution for miniaturizing IoT devices. Given this high level of integration, designers no longer need to think about RF issues, provided they follow the layout guidelines correctly.

For example, Silicon Labs’ BGM12x SiP module measures 6.5 mm square and includes an ARM Cortex MCU, flash memory and RAM, as well as Bluetooth low energy technology and a built-in antenna.

SiP modules overcome the aforementioned detuning challenge, because the antenna sits within the substrate and is detuned to the proximity of the housing. This gives designers much greater flexibility over where to place the SiP module in their end products, which helps minimize the overall device size, making the BGM12x perfect for wearables and other equipment that needs to be small and lightweight.

Conclusion

Designers of IoT devices face many challenges when it comes to creating small, lightweight and low-cost devices that deliver high-quality RF performance. The antenna is a key part of the design, and product developers must consider size, RF efficiency and detuning issues when choosing the right kind of antenna to use. Recent innovations, such as the Silicon Labs BGM121 wireless SiP module, make the RF design process significantly easier and enable designers to create connected devices that are smaller, lighter and better-performing than ever before.

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About the author

Tom Nordman is the Director of Marketing for Silicon Labs’ wireless modules, overseeing product platform marketing strategies. Tom joined Silicon Labs through the acquisition of Bluegiga, which he co-founded in 2001 and where he served as global Vice President of Sales and Marketing. Prior to Bluegiga, Tom worked as global customer service manager and sales director for a global Internet security company. He holds a degree in electrical engineering from Helsinki Institute of Technology and a degree in marketing from Espoo Business College.

Silicon Labs is a leading provider of silicon, software and system solutions for the Internet of Things, Internet infrastructure, industrial control, consumer and automotive markets. We solve the electronics industry's toughest problems, providing customers with significant advantages in performance, energy savings, connectivity and design simplicity. Backed by our world-class engineering teams with unsurpassed software and mixed-signal design expertise, Silicon Labs empowers developers with the tools and technologies they need to advance quickly and easily from initial idea to final product.

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