Feature: RF design


PIFA. Abracon’s ProAnt brand, for example, has high sensitivity even if its orientation can’t be controlled to face the sky. Another great benefit is that its resonant frequency is relatively unaffected by nearby metallic components such as a chassis or enclosure. Hence, although larger than a ceramic alternative, the stamped metal PIFA can be positioned conveniently with no need for ground clearance. In addition, other components unrelated to the antenna circuit can be placed on the other side of the PCB. Te antennas are surface mounted and compatible with automated placement. As a PIFA, corner mounting is preferred, and the antenna can be placed on opposite side of the cellular antenna; see Figure 3.


Wi-Fi/Bluetooth Aſter selecting cellular and GNSS antennas and ensuring their satisfactory positioning, consider the Wi-Fi and Bluetooth side of things. Typically, a loop antenna is preferred for optimal performance, with greatest immunity to detuning by the presence of nearby antennas and components. Tey are well suited for wearable products such as smart watches, and can be ceramic or stamped metal. For operation in the 2.4GHz frequency range, the half-wavelength dimension is 62mm.


Best performance optimisation When the antennas have been selected and their location nominally determined, further work is needed to finalise their position to ensure optimal performance. Adequate isolation between antennas is critical, quantified by the S2,1 relationship between adjacent antennas; see Figure 4. Te typically tight space constraints in portable and mobile devices


mean some level of interaction is unavoidable. Coupling between any two antennas causes some of each antenna’s efficiency to be lost, since a portion of its power is coupled to the adjacent antenna’s instead of being radiated. In the worst case, excessive coupling can cause interference. A GNSS receiver can be particularly susceptible to the influence of an antenna close by. In this case, an S2,1 of at least -15dB or, better still, -20dB, is recommended. It is also vital to investigate any detuning effects caused by objects


in the near field of any antennas in the system. Specific types, such as PCB trace and wire antennas are more susceptible to detuning. Although loop antennas in particular are generally robust and can maintain performance under non-ideal conditions, the effects of objects such as the plastic enclosure can shiſt the antenna’s resonance away from the desired frequency and, ultimately, reduce the signal strength at the receiver. Other hazards include the effects of cables and nearby metal objects or surfaces, which can couple with antennas and reduce their efficiency. Te effect of the cover glass of a display can be particularly acute. Stamped metal antennas tend to be more resistant to these effects. On the other hand, internal flexible printed circuitry (FPC) or


PCB cable antennas are not optimised to operate in free space. Instead, placement next to a PCB or housing is expected. Some types are available with an optional foam layer to provide extra decoupling between the antenna and an adjacent surface – particularly a plastic, glass or metal surface – to optimise efficiency.


The benefits of wireless modules Instead of developing a custom design from scratch, there is a wide range of wireless modules available that are pre-certified for a given application. Most contain the antennas mentioned, but there are others from Panasonic and Murata that offer more, such as ZigBee and Tread. Providing added flexibility for connectivity in IoT battery-


driven applications where low power consumption is crucial, combination wireless modules exist. Tey enable simultaneous yet independent operation of, say, Wi-Fi when high data rates are required and, when not needed, Bluetooth Low Energy. Comprehensive evaluation kits support rapid prototyping


of wireless modules, streamlining the layout and optimisation processes and thus helping designers quickly achieve an implementation that performs well. With minimal fine tuning, a solution capable of passing mandatory product tests can be achieved. An additional benefit is that a module’s lifecycle can be


longer than that of a custom antenna design – consider the risk of one of the antennas becoming obsolescent – which can be useful for wearable home-healthcare smart devices. Last but not least, wireless modules reduce production time, procurement and inventory, resulting in further savings.


Due consideration to antenna design Today’s personal electronics and IoT devices typically rely on multiple wireless standards for networking, data exchange and context sensing. With multiple radios coexisting in a confined space, antenna selection and layout is more complicated than for a simple single-radio system. Tese options will help designers achieve the best trade-off between PCB size, antenna radiation efficiency and receiver sensitivity (which influences overall energy efficiency), cost and ease of manufacture. As with any radio design, testing in a realistic environment


– such as ensuring the board is correctly installed in the intended enclosure and in proximity to any objects that might affect antenna performance – is essential to verify system functionality and prevent failures later in the development process. Overall, giving due consideration to antenna design at an early stage can help avoid repositioning challenges and costly delays later in the project. On the other hand, multiple vendors offer wireless modules


for the widely-adopted protocols, such as Wi-Fi or Bluetooth. Tese help free up valuable design resources and alleviate the headache of specialist RF engineers having to balance overall performance and efficiency. Tey also come fully tested and pre-certified, enabling a faster route to market. TTI Europe stocks a wide range of antennas and wireless


modules from Kyocera AVX, Abracon, Molex, Panasonic, Pulse, TE, and others. Our dedicated team of RF specialists can also help with technical advice and guidance, for both custom RF design or the use of ready-made wireless modules.


www.electronicsworld.co.uk November 2022 29


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