4 keys to selecting antennas for small IoT devices

Wireless connectivity is a critical enabler for many IoT applications, particularly those that need to be mobile, or where cabled data communication is not cost-effective or practicable. Popular wireless protocols include Bluetooth®, ZigBee®, and Wi-Fi®, which operate in the 2.4GHz or 5GHz frequency band and can sustain high data rates over relatively short range. Standards such as LoRa and Sigfox support connections over longer distances, at lower data rates, operating in sub-GHz bands (868MHz in Europe).

Designing an RF subsystem based on any of these protocols demands specialist skills, often most readily sourced by adopting a wireless module or a reference design that comes with protocol stacks already integrated and critical hardware-layout issues solved. While these can greatly simplify design challenges, achieving a working solution is not an easy plug-and-play enterprise.
 

1. The system is the antenna

Among the challenges that must be dealt with, selecting and integrating a suitable antenna has a significant impact on the RF-subsystem and consequently affects the system performance in terms of communication range, overall power consumption, and battery life.

The antenna is often required to fit inside the device enclosure, which usually demands small size and careful positioning to ensure optimal performance. On the other hand, a suitable antenna should also have high efficiency and adequate bandwidth covering the desired frequency range.

Improving any of this triad of parameters is achieved at the expense of the other two. Hence implementing a suitable antenna calls for compromise, using the datasheet figures only as a guide to performance in the final application. Factors such as the antenna connection, positioning relative to other components inside the enclosure, and the properties of the enclosure material and any coatings that may be applied to it, also have noticeable effects.  

For these reasons it is best to consider antenna design issues as early as possible in the project, on the basis that, from an overall RF performance standpoint, the entire system is the antenna.
 

2. Integrating a 2D or 3D antenna

A discussion of antenna basics will show that a centre-fed dipole antenna should have length equivalent to one half of the wavelength of the signal, to achieve resonance. A combination of a smaller monopole antenna, one-quarter wavelength long, close to a ground plane connected as a return path, has a comparable effect; ideal for space-constrained IoT devices. The ground plane effectively creates an image of the monopole conductor (Figure 1), and so must be adequately sized.

A simple quarter-wavelength wire antenna could be arranged in free space above the PCB, fed by a transmission line designed to match the RF-amplifier output impedance. In practice, miniaturised PCB antennas are better suited to volume production. Typically featuring an inverted-F, L-shaped, or meandered antenna pattern, they are reliable, easy to use, and adjustment-free.

A PCB antenna can be attached to the main circuit board, above the ground plane. The ground plane must be the top layer of the main circuit board, and the design must respect the “keep out” areas specified in the antenna datasheet.

PCB antennas are intended to be mounted at the edge of the circuit board. If space constraints inside the enclosure demand more flexibility in positioning the antenna, a PCB antenna can be connected to the main board using a miniature coaxial cable and U.FL connector. A stamped metal antenna is a suitable alternative, particularly if high-volume, high-speed assembly is a priority. If space is extremely limited, three-dimensional antennas made using moulded interconnect device (MID) technology can be considered.

Although an antenna can be selected to have the same impedance as the RF-amplifier output, loading by nearby components on the circuit board and the material properties of the device enclosure may require a matching network to be inserted. This can be a simple single series capacitor and parallel inductor for a single-band antenna, or a more complex network for a dual-band application (Figure 2).

An impedance matching circuit may be needed to maximise power transfer

3. Ensure timely delivery of custom antenna

A standard antenna capable of meeting the system performance requirements usually offers the most cost-effective solution, benefiting from easy and fast off-the-shelf availability. On the other hand, if size constraints are extremely tight or performance targets extremely high, a custom antenna may be needed. To ensure timely delivery, consulting suppliers early is recommended. The supplier needs to know the frequency bands bandwidth, efficiency, link budget, and any multiband requirements.
 

4. Plan for RED and EMC compliance

Wireless IoT devices to be marketed in Europe must comply with the recently introduced Radio Equipment Directive (RED) as well as the EU Electromagnetic Compliance (EMC) Directive. As far as EMC is concerned, high-frequency circuitry such as switching regulators powering various parts of the system, or an unintentional radiator such as a current loop, can cause problems. If EMC is left as an afterthought, common techniques applied as a quick fix – such as board-level shielding – can significantly impair the antenna’s performance.
 

Conclusion

A variety of standard antennas that are designed for frequencies appropriate to IoT applications, and also are compatible with the extreme size and power constraints that apply to typical IoT endpoints, is available. Nevertheless, successfully integrating an antenna is not a simple plug-and-play exercise, and requires careful selection, awareness of the effects of other components in the system, including the enclosure, and fine-tuning of the antenna feed and positioning.

It is worth keeping an eye on technical developments, too. In the quest to usurp the trade-off between miniaturisation, efficiency, and bandwidth, researchers are investigating advanced materials such as graphene – leveraging its excellent conductivity to print thin and flexible antennas, dielectrically loaded polymers, and textile antennas for wearable applications.

To learn more about selecting and integrating an antenna suitable for small-size IoT devices, download the TE Connectivity wireless white paper, or if you would like to discuss your design in more detail, click the Ask an Expert button to get in touch with one of our technical specialists in your local language.

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