Antenna design priorities in IoT applications

This article originally appeared in the Feb'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

Outside of domestic environments, industrial and business use cases are typically more diverse. These may be aimed at automating building systems to improve efficiency and reduce carbon footprint, or their purpose may be to collect data – and lots of it – to help improve process control, business planning, asset management, equipment maintenance, energy and waste management, and even new[1]product conceptualisation and design.

Latest wireless protocols

Wireless technologies offer several inherent strengths for connecting IoT devices. A key advantage is flexibility, which allows devices to be deployed in diverse locations without the constraints of physical cabling. Also, installing new wires in the home, office, or factory can be disruptive. Wireless is often cost-effective, especially for large-scale IoT deployments, and allows easy, inexpensive scalability. Mobility is another advantage, offering a powerful enabling factor in applications like wearables and asset tracking. Additionally, the power efficiency of wireless technologies can be important in battery-operated IoT devices.

Standardised wireless technologies commonly used in IoT applications include NFC, which is ideal for data exchanges of short duration over distances of a couple of centimetres. The energy contained in the RF field emitted by an NFC reader device can be enough to power the receiver circuitry to retrieve and transmit memorised data as requested.

Bluetooth connectivity offers mobility and allows flexibility to engineer the data rate, range, and power consumption to meet the requirements of a given application. It allows point-to-point and mesh connections, and the latest versions also support direction finding and location sensing. Conceived from the outset for mesh networking, Zigbee has similar characteristics. Wi-Fi may be preferred where longer range, higher data rate, or larger connection capacity is required. Several Wi-Fi generations remain in service, up to Wi-Fi 6, which has a theoretical maximum data rate of 9.6Gbps. Wi-Fi 6 also features flexible channel allocation, techniques to reduce interference and waiting times to connect to the network, and beam forming that can improve data transmission efficiency, as well as enhanced WPA3 security.

In IoT applications that need longer range and greater mobility, choices include cellular as well as low-power wide-area network (LPWAN) technologies such as LoRa and Sigfox. As legacy networks are switched off, older 2.5G and 3G data connections are giving way to standards like LTE-M and NB-IoT that use the latest LTE and 5G networks. These are optimised to meet the needs of IoT applications, which typically call for frequent exchanges comprising small quantities of data.

Also, devices such as asset trackers can rely on navigation-satellite constellations (generically termed global navigation satellite systems, or GNSS) such as GPS, Galileo, GLONASS, and BeiDou. Multi-constellation receivers can benefit from a more rugged and robust availability of location data. Some receivers can offer access to special high-accuracy services provided by satellite operators. A tracker can calculate location using the embedded GNSS subsystem and share this information with the host IoT application over a wireless connection such as LPWAN or cellular.

Antenna selection

At its most basic function, an antenna transfers signals between the electromagnetic and electrical domains, leveraging resonance at the RF carrier frequency. This requires the antenna’s effective length to be a specific fraction of the carrier signal’s wavelength. Hence, size is important when considering antenna selection. The size is directly related to the frequency band at which the antenna operates, which depends on the chosen wireless technology and associated operating frequency.

In addition, antenna packaging is a critical issue that affects component selection. IoT devices can be subject to stringent size limitations. This calls for antennas to be small while offering high performance. Sealing is often required, particularly in items like remote sensors and smart meters, which can be exposed to harsh conditions and are expected to remain in service for extended periods.

A portfolio that offers a choice of PCB-mount, internally mounted, and external antennas, optimised for specific frequency bands and wireless technologies often used in IoT applications, can help designers choose the best type for their application. One example is the Amphenol RF antenna portfolio, available from Mouser Electronics, which offers different types and sizes, choices such as soldered or coaxial connections, and parts that are optimised for specific technologies such as NFC and GNSS antennas.

NFC antennas

Several factors influence the choice of an antenna for NFC applications. NFC operates at 13.56MHz, so the antenna must be designed to resonate at this specific frequency to ensure optimal communication. Wire-wound antennas and loop antennas are commonly available as off-the-shelf components.

While the effective antenna length is related to the operating frequency, NFC antennas also have a role in harvesting energy from the RF field emitted by reading devices to power up the IoT device’s embedded microcontroller, memory, and additional hardware that may include a security IC, to gather and transmit the data requested by the reader. Amphenol RF’s NFC antennas range in size from 15 x 19mm to 45 x 34mm. Final selection can depend on variables like the form factor of the device and the desired read range. Typically, smaller antennas are compact but offer shorter read ranges, while larger antennas provide longer read ranges. The available space within the device or application will dictate the antenna size.

Generally, some NFC antennas can be more sensitive to orientation than others, which can require extra care when selecting a specific model and determining its optimal position in the device. It may be integrated into the circuit board or affixed to the enclosure.

Metal objects, electrical interference, and other environmental factors can affect antenna performance. Shielding or appropriate placement may be necessary. Proper impedance matching between the NFC chip/module and the antenna is essential to maximise power transfer and minimise signal loss.

Antennas for commonly used technologies

For technologies such as Bluetooth and Wi-Fi operating at 2.4GHz, as well as cellular and LPWAN technologies, there is a broad selection of PCB-mount, internal, and external antennas. The choice depends on factors like the device’s form factor, size constraints, and the desired range of communication. Chip-size antennas are available for Bluetooth and Wi-Fi 2/3/4 applications in the 2.4GHz frequency bands for industrial, scientific, and medical applications (known as ISM bands).

One example is the Amphenol RF ST0147-00-011-A. Measuring 3.05 × 1.6mm, with a thickness of only 0.55mm, this ceramic-chip loop antenna can handle 2W of RF power. In addition to its small size, the surface-mount antenna is compatible with high-speed automated assembly and is completely contained within the enclosure, allowing easy sealing and a neat appearance.

Amphenol RF also has two ceramic chip antennas for 433 and 915MHz LoRa LPWAN applications. These 1W 5.0 × 3.0 × 0.5mm antennas occupy minimal space on the PCB and have a peak gain of 0.9 to allow communication over long distances using the LoRa protocol.

External antennas tend to be of either monopole or dipole design. A monopole type consists of a single wire that requires a ground plane to reflect the radio waves and help shape the radiation pattern. The pattern is omnidirectional. The dipole type has two conductive elements separated by a gap. These are often half-wavelength antennas, usually longer than a monopole, although the gain is typically greater, and the radiation pattern is bidirectional. The antenna’s gain directly affects the device’s range and coverage. Antennas with higher gain can provide a longer communication range.

Amphenol RF’s ST1226-30-501 and ST1226-30- 001 (Figure 1) are external 5W multi-frequency antennas that operate in 2.4-2.5GHz, 5.15-5.85GHz, and 5.925-7.125GHz frequency ranges, suitable for Wi-Fi applications up to the latest Wi-Fi 6 generation. The gain of these monopole antennas ranges from 2.0 in the 2.4GHz band to 5.1 in 5.925 GHz–7.125GHz.

Cellular is often the chosen connectivity for small devices such as trackers to be mounted on moveable assets like cars or vans, construction vehicles, or portable generators. In these applications, an internal antenna may be appropriate to permit less obtrusive installation or to keep fragile parts out of harm’s way. On the other hand, a larger external antenna may be suited to a device such as a gateway designed to direct data from multiple IoT endpoints into the Cloud via a cellular connection.

The Amphenol RF ST0425-20-401-A is an example of an internal antenna for cellular applications including NB-IoT in the frequency ranges 0.69-0.96GHz and 1.7-5.0GHz. At 90 × 15 × 0.85mm, it comes with a 195mm coaxial cable and plug for connection to the circuit board. It can handle 1W RF power and has a peak gain from 2.1 to 4.2, depending on the frequency band.

GNSS antennas

GNSS antennas come in various styles, such as ceramic patch antennas. As a type they have circular polarisation that ensures high sensitivity to the satellite signals. When designing equipment such as asset-tracking devices with satellite location, designers must ensure that the chosen antenna supports the relevant constellations. The Amphenol RF ST0326-41- 001-A is a SMA-plug antenna suitable for typical GNSS L1-band applications at 1575.42MHz as well as 1602MHz, which is the primary frequency band of the Russian GLONASS constellation. The connecting cable and plug help designers ensure the antenna is placed in a sky-facing position.

Conclusion

Size and packaging are critical issues to consider when choosing an antenna for an IoT application. Large external antennas tend to offer the most favourable RF performance. On the other hand, internal mounting is often preferred to withstand environmental challenges and allow easier use and portability, while surface-mount antennas can offer a solution when size constraints are extreme. Choice is the designer’s friend in the search for the best combination of electrical and physical properties.