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Jan 5th, 2021

Antenna Design Considerations in IoT Design

As more and more devices are connected into Internet in wireless ways, electronic engineers are confronted with lots of challenges such as how to make radio transmitter assembled to present equipment space and how to design and manufacture devices with increasingly smaller sizes. Moreover, they are striving to meet customers' demands on IoT (Internet of Things) products compatible with ergonomics, applicable accessibility and harmony with environment.

When IoT products are considered, size expectation is one of the most important considerations, apart from which radio properties and price are also commonly considered. Ideally, engineers prefer IoT components with small size, excellent RF (radio frequency) performance, and low prices. However, IoT components usually fail to contain all the advantages mentioned above, so solution providers have to be confronted with challenges.

Fortunately, as electronics industry constantly depends on totally new silicon process technology, recent years have witnessed increasingly smaller size of silicon chips. By integrating MCU (microprogrammed control unit) and RF front end into SoC (system on chip) structure, space issue has been successfully solved for IoT implementation. However, the development trend towards SoC hasn't got the issue solved concerning physical structure of RF transmitter, that is, antenna. We usually leave antenna design to customers or advise them to pick up an easy-to-use antenna module with integrated antenna. Space for antenna is another challenge we have to face up with as we are designing small IoT devices. Space design calls for high efficiency and reliable wireless connection capability.

Why SoC?

As 21st century saw the initial flourishing of IoT, the industry was regarded as M2M (machine to machine). The components contributing to IoT interconnection mainly include GPRS modem, Bluetooth serial cable or Sub-G radio. All the designs take advantage of two leading components for the achievement of connection: MCU and wireless modem. The minimum space sufficient for fundamental IoT functions implementation lies in 50mm in all dimensions, which means that the size of all the devices is just that of a cell phone.

As silicon industry consistently moves towards the technology that integrates MCU and RF functions into the space of one chip, developers start embracing more opportunities. Now, they can make all the functions of IoT devices achieved within the same IC/SoC. Because wireless MCU features obvious advantages, IoT component system starts to convert towards wireless MCU. As a result, engineers are able to design IoT devices with only one type of components and get space saved. Furthermore, they are able to reduce cost due to the low cost of components. As structures of modern IoT devices are ready to be selected, systems based on SoC will be more popular due to advantage in size.

Nevertheless, development trend towards SoC fails to solve the physical structure issue, that is, antenna.

How to Arrange Antenna and How Much Space is Required?

It has to be admitted that antenna has to face up with complexity on multiple dimensions since both size and efficiency have to be considered. Because BOM (bill of material) cost is relatively low, it's common that antenna is designed through PCB tracing for IoT design. However, PCB antenna calls for remarkable size requirement that is usually within the range of 25mm×15mm, leading the volume of IoT product to be large. The antennas also feature another disadvantage when they are applied in modules, that is, they are very sensitive to detuning as a result of shield material and need to be specifically considered in the process of final product assembly so as to reach the optimal working state. In SoC design, as a part of ordinary design, antenna tuning is obtained depending on some specialized knowledge. In the designs, there's no difference between PCB antenna and other antenna.

Antenna manufacturers have long been providing "chip antenna" so as to simplify design work. Moreover, this type of antenna features advantages in terms of size. This category of antennas is primarily provided through the following ways:
a. Antenna uncoupled with GND. This type of antenna calls for clearance range with relatively large size. Typical examples of this type of antenna include unipole antenna and flip-F antenna.
b. Antenna coupled with GND. This type of antenna only needs to provide relatively small clearance range or doesn't need antenna at all.

Both types of antenna feature clearance range or grounding plane and space requirement in terms of PCB size. The space called by RF components in IoT design should also include necessary clearance range since any component or trace mustn't be left here, which means that when designers are estimating the size of IoT equipment, PCB size and clearance range should be noticed compatible with antenna. Moreover, certain space should be maintained between antenna and shield edge.

When IoT devices are designed to be a size of a button cell, antenna efficiency is definitely damaged. When we try to make its size smaller, efficiency to achieve RF performance will be thereafter reduced. The performance of device with less than 10mm in all dimensions won't be achieved until at the frequency of 2.4GHz. For example, Bluetooth connection over 10 meters can be provided to cell phone users, which is accepted by majority.

However, when the size in all directions approaches 20mm, efficiency of RF will be dramatically increased. When it approaches 40mm, the high efficiency of numerous antenna with grounding tuning achieved will climb to the highest.

That thereafter means that communication distance between two equivalent devices should be in the range from 60mm to 400mm in accordance with Bluetooth 4.2 protocol. Once 15.4 protocol (zigbee for example) is applied, the longest communication distance within a visual range can reach 500 meters or above. Thus, designers need to balance PCB size and performance and efficiency of antenna based on differences of applications and targeted size because most chip antennas all regard PCB grounding plane as one section of antenna configuration. Furthermore, antenna/module position also plays a key role in design phase so that designers need to consider clearance range to achieve optimal grounding of module.

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