Antennas are a key component in the M2M (machine-to-machine) devices that are used for remote monitoring and tracking in South Africa. Since products in rural, remote and border areas must remain connected, efficient connectivity and durability are a must.
According to network operators in the USA, antennas are the single most common point of failure (certification or performance failure) for M2M devices. For this reason, antenna selection and integration are vital processes in wireless product development. Getting it right the first time can save a lot of time and money during the design cycle.
This article is aimed at wireless device designers and manufacturers and discusses the importance of effective antenna integration and RF design, including some of the basics on how to plan for laying out an efficient wireless device for the South African market.
The four-step process for creating the ultimate M2M device
The bottom line is that better antenna choice and integration will give you a better device. Here’s a step-by-step plan for ensuring your antenna choice and integration runs smoothly.
1. Understanding your target specification and application is the most important factor before selecting any antenna. Selecting the right antenna can streamline the whole product design process and result in M2M devices having the desired performance or achieving product certification first time.
A deep understanding of where and how your device will be used and installed is necessary to successfully deliver to your customers’ needs.
M2M systems are getting increasingly smaller to accommodate applications such as covert tracking devices and body-worn medical devices. Add to that the increase in functionality and added wireless technologies like Wi-Fi or 868 MHz.
However, these same devices must achieve excellent performance despite the smaller sizes and the need to embed devices in places such as under the dashboard of a car.
2. Teamwork. After the module and airtime providers have been selected, you are ready to choose an antenna for your device. When a device maker and antenna provider team up at the beginning and plan out product design files together, this puts the whole project on the right path.
Together, they can work out the advantages and disadvantages of different antenna types and configurations. From this vantage point, it becomes an easier process to select the correct antenna for your device.
3. Off-the-shelf or custom design. Once the device maker is ready to start making prototypes of the device and incorporate the required configuration for the recommended antenna, it is time to decide between a custom or off-the-shelf antenna.
The antenna provider needs to give the device maker a current production sample (off-the-shelf) or a custom, handmade antenna for the prototype device. This will enable the device maker to finish the product design.
When the product is finished and can connect to a network, the device maker sends the final device to the antenna partner to check active device performance and antenna matching etc.
4. Device optimisation. Superior M2M devices have likely implemented RF and antenna optimisation to enhance their sending and receiving sensitivity. The majority of M2M device design companies would not have the resources to do that by themselves, however a capable and experienced antenna partner can do it for you.
When devices are smaller and use embedded antennas, performance will be compromised, but using an antenna designed specifically for a device can help compensate. Antenna selection and integration will affect over-the-air (OTA) measurements and can affect radiated spurious emission (RSE) figures.
Without high antenna efficiency, devices will not remain connected in low-signal areas. Added to this, certain overseas network OTA requirements will not be met.
RSE is a common point of failure for M2M devices seeking PTCRB or GCF certification. This can be misinterpreted as an antenna issue. In reality, what happens is that RSE failure can be caused by an antenna impedance mismatch with the module when the device is on and is connected.
The solution is to design your antenna for an active device, not just a passive device. There must be a good impedance match when the device is on and transmitting and when connected to the network/base station simulator.
When antenna selection is effective and antenna efficiency is high, the total radiated power (TRP) will be high. This is exactly what you want to achieve for optimal transmission sensitivity. However, this can also result in the system re-radiating emissions. With the increased power, RSE failures can result.
It is not good practice to detune the antenna or bring down antenna efficiency to resolve this issue. The source of the emission needs to be identified and eliminated, or at least prevented from getting to the antenna and being received into the system.
Avoid the pitfalls
Selecting the wrong antenna can end up costing significant time and money in device debugging and/or redesign, not to mention additional testing and certification fees. It causes delays to market and headaches for the company.
Wireless device design is complex, especially for multi-band cellular devices, and even more so when other wireless technologies such as GPS, Wi-Fi and 868 MHz are included. Added to this, the presence of batteries and other metals close to a cellular antenna can cause issues in any system.
Devices using an embedded antenna are likely to require some level of customisation if performance is to be kept high. The following antenna guidelines will help you avoid most of the problems associated with device failures.
* Keep antennas as far away from each other as possible to avoid interference issues.
* Use bigger antennas if you need wider bandwidth, more gain and better efficiency. The bigger the antenna, the better the antenna. The more space allocated for a cellular antenna, the easier it will be for the antenna designer to deliver a successful solution. The same rule applies to antenna clearance and ground plane.
* Use cables and connectors carefully. Cables and connectors should come with a warning note – they introduce loss and can cause impedance mismatches. This is unavoidable if external antennas are required, but an edge-mounted connector can be used with a transmission line to route the signals to the module. This is more effective than a cable jumper.
* Target for a 2 dB margin. That way, if problems do occur, your extra breathing space could mean it does not affect the test plan or design cycle.
* Implement physical shielding on the printed circuit board (PCB) as much as possible. The simplest way to achieve this is to place metal cans over active circuitry.
* Fill your ground plane completely and fill in all unused areas of your PCB with ground.
* Test the antenna and undertake proper antenna testing (return loss and efficiency) during the initial design and prototype stages. At the final stage, it is vital to measure antenna efficiency and perform OTA and RSE pre-scans.
* Talk to all the relevant parties regarding your application, including the carrier, module provider, antenna provider, test labs and design house. A design review is also recommended before finishing hardware design.
And finally…
As an M2M solutions provider, you may not have all the wireless device design expertise at your fingertips or have the ability to debug design problems. It may also be difficult to get access to the equipment and resources to acquire this expertise.
Awareness of the pitfalls is the first step to solving the problem. If you only do one thing when you design your M2M device, focus on making the right decisions about your antenna.
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