By Andrew Colla, Business Unit Manager - IoT
My next IoT based project is calling for an even smaller footprint, but I still need to assure connectivity to the network. How do I design a wearable or miniature product, yet still get my connection to perform effectively & reliably?
IoT design specifications often conflict with the Laws of Physics
· IoT products are being designed to fit into small/confined spaces or be worn on the body
· Deeper penetration (into basements) demands low frequency bands
· Low frequency means longer wavelengths & bigger antenna ground planes
· Last minute embedded antenna positioning can have an extreme impact on performance
· Poor antenna design leads to inefficient performance & adds burden to the battery capacity
Good IoT design practice calls for us to consider the antenna placement & layout first, and then move to designing the rest of the PCB.
In a cellular network, and to facilitate deeper penetration & longer range, Cat-M1 & NB-IoT is deployed on low frequency bands. In Australia & New Zealand, this typically means Band 28 (700MHz) & Band 8 (900MHz). Good antenna design for these low frequencies is paramount for successful deployment in these environments. Adhering to sound design principles & guidelines from the get-go will help to reduce the time-to-market of our product when poor signal strength is encountered at the latter part of the prototype phase.
The Challenge
In antenna design, the size of the ground plane can significantly impact the performance of the antenna, and its size is largely determined by the wavelength of the signal that it is designed to resolve. For optimal performance at 700MHz, design principles (Laws of Physics) dictate that the size of the ground plane should be approx. 100mm.
But many of our IoT projects are being specified to fit into boxes that are often smaller than this. What principles can I apply to my design to circumvent this obvious limitation? While it would be ideal to have the ground plane exceed the bounds of the box, other compensating principles can be applied to achieve reliable & adequate signal strength.
1. Consider at least a 4 layer PCB construction
a. Contain ground planes to layer 1 & 4
i. Maintain tightly knitted connections to prevent floating grounds
ii. Vias between ground planes at regular intervals along the RF feed trace
b. Contain all signal & power traces to layer 2 & 3
2. Ensure all RF feed traces (between Radio & Antenna) are straight & as short as possible
3. Apply RF feed traces as close to PCB edge as possible
4. PCB layout should allow for matching components to form the “pi” network
a. For antenna tuning to optimize performance
b. As close as possible to the antenna feed point
Other Considerations
Other techniques may be applied that still permit adequate performance objectives being met. These techniques do not discount the governing factors described already but could be considered as alternative methodologies to deliver ground plane dimensions that achieve optimal performance on low frequency bands.
1. Extending the ground plane beyond the perimeter of the PCB
2. Active Tuning
a. RF switch that selects different matching circuitry for different frequency
3. Consider alternative antennas
a. Such as FPC (Flexible Printed Circuit) or FLEX Antennas
Conclusion
Embedded antenna design is not a trivial task, particularly if support for low frequency bands is a consideration. It should be explored as early as possible in the design cycle, and it is recommended to engage with an antenna supplier that can provide support and deliver guidance at every design phase. Position and layout of RF components is critical for optimal performance and adhering to documented layout guidelines may eliminate unwanted delays at the eleventh hour, and potentially railroad the product launch schedule.