Building better M2M devices through antenna optimization

EE Times Article by Dermot O’Shea, Taoglas

026_Taoglas_080415The process of building a better M2M (machine-to-machine) device – one that not only fits the application scenario but also performs well in the field – starts with choosing the right antenna.

Selecting the right antennas for a device, particularly cellular antennas, and testing and integrating them properly, can prove the difference between a device that passes network certification and testing like PCS-1900 Type Certification Review Board (PTCRB) testing, and one that does not. Good radio frequency (RF) optimization and OTA (over the air) performance in wireless devices is critical for these certifications, and incorrect antenna selection and integration can often be blamed when M2M devices fail certification testing. Often, it is a system issue, such as the overall design of the system or how that device is interacting. As with many things, system optimization is the key to success.

This article discusses the important process of antenna selection, how to incorporate antennas in M2M devices and the recipe for connectivity success. First, let’s take a look at selecting the proper antenna.

Antenna Selection Better antennas mean better devices. It’s as simple as that. Understanding the requirements for achieving specific network certification processes are the most important factors when first selecting a cellular antenna. Once the module provider and the carrier have been selected, the next part of the process should be selecting the right antenna for your application.

The first step is for the device maker and antenna company to partner from the start to incorporate their ideas for compliance and RF performance.

The second phase involves the device maker and antenna company cooperating on product design files to work out the advantages and disadvantages of each antenna configuration.

Once the antenna is selected, the customer can start to make prototypes of the device incorporating the required configuration for the recommended antenna. Depending on whether it is a custom design or not, the antenna partner either provides the device maker with a current production sample (off the shelf) or a handmade antenna for the prototype device that enables the device maker to finish their product design. When the product is finished and can connect to a network, the device maker sends the final device to the antenna partner.

Next is the stage where active device optimization happens – this means the ability to increase the device send and receive sensitivity to improve transmissions. When devices are optimized it results in superior M2M devices. The process is what cell phone companies do in their own laboratories before sending devices to final certification. M2M companies in most cases do not have the resources to do that on their own, and they are required to choose antenna partners who can do it for them.

Selecting the right antenna can streamline the whole product design process and result in M2M devices passing certification testing the first time. In general, M2M systems are getting smaller to satisfy scenarios such as body-worn medical devices. Yet, the same device has to get excellent performance and will be compared to a cellphone. When devices are smaller, performance can be compromised, but using an antenna optimized for greater send and receive sensitivity can help compensate for size.

Antenna selection and integration will affect over-the-air (OTA) measurements and can affect radiated spurious emission (RSE) figures. Without high antenna efficiency, certain network OTA requirements, particularly total radiated power (TRP) will not be met. RSE is a common point of failure for M2M devices seeking PTCRB certification. This can be misinterpreted as an antenna issue. Here’s what can happen:

  1. RSE failure can be caused by an antenna impedance mismatch with the module when the device is on and transmitting. The solution is to design the 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.
  2. When the antenna selection is good and efficiency is high, the TRP will be high. This is exactly what you want to achieve for optimal send and receive sensitivity. However, this can also result in the system re-radiating emissions – and 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 emission needs to be identified (the source) and eliminated, or at least prevented from getting to the antenna and being received into the system

Best Practices

Delaying antenna decisions can result in the loss of the device’s window of opportunity in the market and will end up costing hundreds of thousands of dollars in device debugging and/or redesign, not to mention additional testing and certification fees. Following are some best practices when considering RF design and integration while designing M2M products:

  1. Plan for problems – Wireless device design is complex, especially when multi-band cellular is included, and even more so when other wireless technologies such as GPS, WiFi, and 915MHz are present. The presence of batteries and other metals close to the cellular antenna can cause issues in any system. Devices using an embedded antenna are likely to require some level of customization. (Many M2M companies are not experienced in wireless device design to debug design issues, and may not have access to the equipment and resources to acquire this expertise.)
  2. Separate antennas – Keep the antennas as far away from each other as possible to avoid detuning issues.
  3. Size matters. The bigger the antenna, the better the antenna – Size enables antennas to have wider bandwidth, more gain, and better efficiency. 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.
  4. Avoid cables and connectors – Cables and connectors should come with a warning note. They introduce loss and can bring 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.
  5. Target with margin – It is best to target with a 2dB margin. That way, if problems do occur, it does not affect the test plan.
  6. Optimize shielding – Try to 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.
  7. Completely fill your ground plane – It is best to fill in all unused areas of your PCB with ground.
  8. Test the antenna – It is important to perform 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.
  9. Consult with experts – Talk to all the relevant parties regarding your application – the carrier, module provider, antenna provider, test labs and design house. A design review is also recommended before finishing hardware design.

Valuable Advice

The bottom line for ensuring quick development, speedy time to market and ease-free certification for M2M devices is planning ahead. Getting the antenna right is easy if you begin early and integrate antennas into the design as one of the first steps. The end result is that device makers will be happy because they can sell more M2M devices, and carriers will be happy because they will have more connections, all because the antenna was properly optimized for the device in question. Talk to the experts early and frequently; it will save a lot of money in the long run.

About the Author

Dermot O’Shea is co-founder and joint managing director of Taoglas. Having founded Taoglas with Ronan Quinlan in Taiwan in 2004, he is currently responsible for sales, finance and marketing and is based in Taoglas’ San Diego office.  Prior to founding Taoglas, Dermot worked for over ten years in the global electronics industry for companies such as Network International. He is a highly regarded source in the M2M antenna market and today advises automotive, tracking, telemedical and utility companies worldwide on antenna solutions. Dermot is an expert in the wireless antenna arena, he provides high-level counsel on device noise debugging, testing services, device certification and approval management. Dermot has a Science Degree from University College Dublin and postgraduate diplomas from Dublin Business School (Business), Griffith College Dublin (Computing) Waterford Institute of Technology (Enterprise Development).