Feature: RF design
Antenna design for small IoT devices
By Edoardo Genovese, Technical Development Manager RF – Europe, TTI
T
oday’s wireless technologies are at the heart of the smart revolution that’s changing the way we live and work. It’s common for connected “things” to support multiple wireless standards, such as cellular, Wi-Fi and Bluetooth, whilst also integrating GNSS for location and tracking. Here,
we will consider the benefits of customised antennas versus ready- made wireless modules for such devices.
Multiple radios in tight spaces Equipment designers are under enormous pressure to cram multiple radios into extremely tight spaces. Advancements in silicon and packaging technologies allow parts of the circuit to occupy less space on the board. Te antenna, however, is subject to laws of physics that allow only minimal flexibility. Tere are strict constraints on effective length to ensure resonance at the desired frequency, whilst multi-radio designs require adequate spatial separation between adjacent antennas to minimise unwanted coupling. With these limitations in mind, recommended best practice
is to prioritise positioning antennas that are most susceptible to disruption by others. Tis suggests working initially on the cellular antenna (if fitted), followed by GNSS and, subsequently, Wi-Fi and Bluetooth antennas. Clearly, designers have more freedom to optimise these relative positions if antenna placement is considered early in product design and not tackled as an aſterthought.
Antenna selection and placement Choosing and placing an antenna on a circuit board demands a balance between achieving the smallest possible solution and ensuring radiation efficiency in the appropriate frequency range. A planar inverted F antenna (PIFA) is a common choice for cellular radio, since it is resonant at a quarter-wavelength of the carrier signal, which allows for small size. The PIFA should be placed at a corner of the board, since its ground-plane length needs to be one-quarter the wavelength of the carrier signal. A PIFA for a low-band 5G frequency, say 617MHz, will need a ground-plane length of 120mm. It can be tempting to try reducing the length, to allow more space for other components on the board, but this will narrow the antenna bandwidth, leading to sub-optimal performance. Switched-circuit antennas, such as those available from
Kyocera AVX, offer an alternative that optimises the values of matching-circuit components to produce multiple narrowband responses; see Figure 1. The matching-circuit component values can be selected to
cover the entire desired frequency range; see Figure 2. This is a valuable technique to design a board-mounted antenna to cover a wide range of 3G, 4G and 5G frequencies for an IoT application. More recently, loop antennas are being made from stamped
metal and designed for surface-mount assembly. These have high radiation efficiency and maintain stable performance in the presence of nearby metal objects; for example, an RJ45 connector near the board.
www.electronicsworld.co.uk November 2022 27
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