Why Ignoring Antennas is Limiting IoT Device Performance
by Colin Newman, Quectel
The devil is in the details when getting technology to perform at optimal levels. An excellent example is properly integrating antennas to connected devices. Poor siting of an antenna in a device and/or integration issues between the antenna and other components can cause wireless IoT device performance to suffer or fail completely. Antenna selection and integration must be given greater attention with users considering specifying pre-qualified components from the same supplier, for both module and antenna, to streamline a successful design and accelerate time to market. Early decisions need to be made regarding the relative merits of embedded or external selection and then attention can turn to optimizing the antenna integration.
An external antenna can either be a terminal mount which has a connector like an SMA that connects the antenna to the outer housing of your product. Or it can be a cable mount version whereby the antenna is positioned away from the main product and is connected via a cable. External antennas are needed when the device is situated in a low signal area such as a basement, or the electronics are housed inside a metal box. These antennas offer greater performance due to their design and size. They are easy to implement (simply plug and play) and are less susceptible to noise issues because the antenna is far from device electronics and, minimizes TRP (Total Radiated Power) and TIS (Total Isotropic Sensitivity) and RSE (Radiated Spurious Emissions) risk. However, due to aesthetics, many IoT devices these days cannot use such antenna types and therefore the only option is an embedded solution.
Embedded antennas can be split in to two types, SMD (surface mount device) or cable with connector, typically a U.FL type. These offer the smallest size and lowest cost but do require the designer to have more experience and understanding. In particular, this is where the RF layout becomes an important function of the design with regards to performance.
Unless you have experience designing with embedded antennas, it can be a daunting challenge to make them work. Often there is not a quick or easy resolution because the performance issue might not be with the antenna itself and this uncertainty means antennas are often overlooked and left to later in the design process. However, to achieve the best performing design, antenna selection and position in your device should be considered first.
Antennas only work well in certain positions so priority should be given to their placement over and above any other components. If they are wrongly placed, performance is immediately compromised before any other influencing factors come into play.
Deployment criteria also needs to be considered at the early stage in design and how this will affect the antenna selection and position. Many products today are designed for global use. Some countries require regulatory and network certification. Antenna efficiency and performance is key to that success. Where will the end product be mounted or used? If it is a body worn device, then SAR (Specific Absorption Rate) needs to be considered. If it’s a handheld device, then make sure the antennas are not mounted under where the hand will be placed.
IoT devices have evolved over the years to become more complex and smaller in size. Many products today are similar or smaller than a smartphone, and support just as many radios. This puts even more pressure on the design which needs to allow sufficient isolation between each antenna and address compromised performance caused by shorter ground plane lengths on the host printed circuit board (PCB).
Antennas are Being Asked to do More With Less
Digital components and wireless modules have greatly reduced in size while increasing performance over the years. This ‘die-shrinking’ process cannot be applied to antennas as easily because of their physical requirements. Typically, everyone wants their company’s latest device to be the smallest and best performing wireless device on the market. As a result, engineering departments have the task of making this happen within tight physical product constraints.
An external dipole antenna, which is increasingly adopted with IoT devices to ensure optimal connectivity, has two radiating elements. Most embedded antennas perform like a monopole, so the antenna is just part of the antenna structure. With a Monopole antenna, the second radiating element is the host PCB ground plane and radio frequency (RF) layout, and this design element is critical to the antenna’s performance and often the cause of many underperforming designs. Here, the length of the ground plane plays a key role in the overall performance.
The ground plane length ideally needs to be a ¼ wavelength of the minimum operation frequency of the Monopole antenna as follows:
LTE Low Band:
Frequency: 698Mhz, Wavelength=429.5mm => 1/4 wavelength = 107mm
Frequency: 1575MHz Wavelength=190mm => ¼ Wavelength = 47mm
Frequency: 2400Mhz, Wavelength=125mm => 1/4 wavelength = 31mm
As you will see from the antenna efficiency graph (Figure 1), ground plane length affects the sub 1GHz frequency range far more than the with higher frequencies:
An important part of this challenge is that there is no way of fixing or improving this later on, after the design stage. Therefore, Quectel offers to review customers’ RF front end designs at the architecture stage, providing detailed and consistent feedback on what to expect, and helping to limit delay and cost associated with redesigning late in the development cycle.
For the device to perform well, the antenna and RF front-end (RFFE) must match the receiver’s capabilities and specification. If it does not, performance will be compromised. By using a single vendor for both the antenna and RF module, companies can rely on the vendor’s engineering teams, which have already worked together, to make sure there is compatibility between both products when they were developed.
The RFFE is often what makes or breaks a wireless design and this consists of the antenna and RF module as well as a third item, the RF interconnect between the two. No matter how good the performance of the antenna and the RF module, a poor interconnect/RF layout will significantly reduce performance and it cannot be resolved without a costly redesign and delay to the schedule. If multiple vendors are used, challenges emerge as to who is responsible for providing the interconnect support and each vendor can point to the other as the cause of any issues.
However, by using a single supplier (such as Quectel), you can be assured that the supplier already knows how their two components need to be connected and a single point of engineering support for both components can be provided. A further benefit is that, if your product requires network certification, the antenna, module and interconnect all play a vital part in achieving approval. Again, using a single supplier means their engineering team has experience with the whole RF front-end and can help with all these aspects as well as providing their own pre-compliance testing.
Here are some key RF PCB layout points to consider:
- 50ohm CPWG transmission line design
- Ensure a clean, uninterrupted ground beneath the RF transmission line to allow a proper return path for the RF current
- Provide adequate vias to link ground planes together
- Minimum 4-layer PCB to reduce interruptions of the GND
- Maintain the shortest possible length for the RF trace
- Avoid bends in the RF trace. If bends are unavoidable, make a curved bend.
- Avoid stubs and test points on the RF trace.
- Do not place any other traces close to and parallel to the RF trace
With success in IoT reliant on rapid time-to-market and cost optimization, this approach is a simple way to achieve both rapid development and cost efficiency. In addition, building a closer working partnership with a supplier means greater efficiencies can be achieved in comparison to working with two separate companies, possibly in different parts of the world.
You only know how well your wireless device is working once your design is complete and prototypes have been built and tested so any performance issues or failure to achieve network certification need to be resolved quickly and with the minimum of modifications. Reducing additional engineering costs and accelerating time-to-market by working with a single engineering support team that provides the key components here greatly helps.
These 5 important antenna considerations will maximize the performance, optimize the cost, and minimize the form factor of connected IoT devices:
1. Recognize that all parameters are crucial and interact together. Place equal importance when designing on return loss, radiation pattern, efficiency, gain, and antenna polarization. By assessing all of these with equal attention you will be able to find the optimal balance and set the basis for a successful antenna design.
2. Focus on the architecture and position of the antenna within the device. Antennas must be positioned with care. Proximity to other components and metals will cause interference, limiting performance. An easy way to ensure optimal positioning is to follow the reference design in the antenna vendor’s data sheet. The main challenge here is to balance the best position for the antenna with competing needs of other components along with the overall size of the product.
3. Prepare to address detuning challenges. All embedded surface mount device antennas are vulnerable to being detuned by the customer’s design. The host PCB, the dielectric housing, the battery or LCD, and the surrounding components can all cause detuning. Detuning effects can be compensated for with a matching circuit that is placed on the host PCB, close to the antenna feed point. It is recommended to utilize a matching circuit of at least four components (Figure 3).
4. Establish certification criteria at the start. Certification takes three main forms: regulators, telecoms industry and operator certifications. Some or all of these must be complied with for the device to be approved for use, depending on region. It is important to understand the requirements of each because they can have impacts on device design. For example, T-Mobile’s utilization of the 600Mhz band in the U.S. requires longer ground plane lengths for embedded antennas. Therefore, set a performance target at the start of the project and always design with some margin to ensure certification can be achieved.
5. Select IoT modules and antennas from the same supplier. The module can’t work without the antenna and vice-versa, so purchasing both from the same supplier simplifies the design process, gives you engineering support from one team, enables easier pre-compliance testing and accelerates time to market.
Benefits of Seamless Antenna Integration From One Supplier
There are definite benefits from closer collaboration between suppliers and customers that not only strengthen the working relationship and mutual trust between the two, but this also delivers tangible and intangible efficiency benefits. From a financial aspect, having fewer suppliers means fewer financial transactions and relationships to manage whilst maximizing volumes and minimizing spend. Improved inventory control and quality of products and services also help you to streamline processes and be more efficient.
In development, having fewer suppliers helps forge a closer working relationship, and build trust and loyalty which can be especially valuable in times of component allocation issues. That close partnership helps provide early access to new supplier developments and roadmaps so that, as a customer, you are ahead of new product launch dates and in some cases, vendors can provide input to tailor the device to meet your needs. Quectel certainly takes this approach of collaborating across multiple aspects of device design and helping customers to build a smarter world.
The benefits of a successful antenna integration with one trusted supplier yields key benefits from an operational standpoint:
- Streamlines a successful design and accelerates time to market
- Ease of installation and enhanced connectivity with compatibility between vendor products and antenna
- Optimal performance once in use and maximum cost efficiency
- Single point of engineering support for all connected products
- Support for network certification and product compliance for RF front-end products
- Trust building and collaboration between vendor and supplier
- Fewer suppliers mean fewer financial transactions
- Less relationships to manage while maximizing volumes and minimizing spend
- Improved inventory control and quality of products and services
My suggestion is to select one vendor for your antenna and module integration to reduce device design complexity and accelerate time to market.
About The Author
Colin Newman, Director of Antenna Business Development for Quectel, has over 25 years of technical and marketing expertise in broadband and other related industries driving strategic company growth within international marketplaces. Quectel, is a global supplier of cellular and GNSS modules and antennas for wireless technologies like 5G and IoT devices.