WIRELESS & IoT
Shrinking wireless designs and moving to 4G/LTE
Figure 1: Wrist mounted wearable example
Geoff Schulteis senior antenna applications engineer at Antenova explores the latest advances in Wi-fi, WANs and 4G/LTE
C
ertain antenna design principles apply in all wireless design projects, but
designing for LTE cellular networks is a little more challenging than designing for the other networks. Designs for the Internet of Things or smart
cities may use the ISM networks, in particular LoRaWAN, or possibly the cellular networks: 3G, NB-IoT, 4G/LTE or LTE Cat M1 for mobile devices. Most IoT products are small and are used with sensors to control lights or other services, therefore they have small PCBs. Consumer products such as wearable
electronics or trackers for pets or luggage are also based on very small PCBs, but these typically use the Bluetooth frequencies at 2.4GHz or 5GHz.
The challenge of shrinking designs The challenge is always to create a physical design that meets the limitations of size and space, and still performs well in operation, and the antenna needs special consideration. Its placement in the PCB stack-up, and the space around it, can help to improve the performance of the antenna in the finished design. The laws of physics impose some limits on
smaller wireless designs due to the way that antennas work. Surface Mount Device (SMD)
10 OCTOBER 2021 | ELECTRONICS TODAY
antennas use a ground plane to radiate, and the size of this is related to the wavelength of the operating frequency. The ground plane needs to be a certain length, but if the design is small with a shorter ground plane, the antenna may not radiate RF power at the optimum level.
Making antenna choices The first decision will be the type of antenna to use. The smallest antennas for cellular networks are the SMD antennas, which might be as small as 20 millimetres in length (one example is Antenova’s part number SR4C033 for 791-960MHz cellular frequencies) which is clearly attractive for small designs. Another useful choice might be the FPC, a flexible printed circuit antenna, which can be placed away from the PCB in the design, maybe cunningly fitted into a tiny space in the design. The performance of an antenna in the real
world situation will not be the same as the results shown on the datasheet, which are based on its performance in free space. A number of factors affect the antenna’s performance: the length of ground plane, other components close by, the outer casing of the device, the position of the antenna within the device and its position on the PCB.
SMD antenna integration You should give careful attention to the details of the overall RF layout if antenna efficiency is to be maximised. It is important that the copper ground plane is not too complex – not cut up with traces or divided between layers. For multi-band frequencies we recommend a minimum four-layer PCB structure with a top and bottom ground plane layer. Both grounds should be tightly knitted together to prevent a floating ground. All digital signals and power lines should be run in between them. This kind of configuration will allow for the whole of the ground plane part of the antenna to radiate effectively. Our advice is as follows: keep the RF
trace/feed between the radio and the antenna as short as possible; run vias at short intervals along the feed line connecting the grounds; try to keep the trace straight and as close to the host PCB edge as possible; allow for a PI matching circuit close to the antenna feed point to fine tune the antenna later, and flood any free areas with ground. Remember to allow a PCB clearance area for antenna parts, and check the datasheets for full guidance. It is not easy to guarantee the range for the antenna when it is integrated alongside
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