Printed circuit boards (PCBs) do more than just provide a surface for installing embedded antennas. Like pole, screw, and magnet mounts for external antennas, PCBs directly affect the performance of embedded antennas and thus the quality and reliability of the applications using them.
For example, whether it’s cellular, Wi-Fi, GNSS, or Bluetooth, the PCB serves as the antenna’s ground plane. The size of the PCB ground plane, as well as the antenna’s location within that space, play a major role in device and application performance.
But finding that ideal location is tricky because PCBs are packed with other components, such as CPUs and batteries. In addition to competing for scarce PCB space, those components can undermine antenna performance if they’re too close.
Read on to learn more about how to successfully navigate the top PCB considerations, including avoiding common pitfalls and using testing to verify that the antenna and the rest of the device will meet performance requirements in the real world.
Select RF Technologies
Most embedded surface-mount (SMT) antennas are monopoles or planar inverted F antennas (PIFA) and rely on the ground plane. Thus, the choice of RF technologies — such as cellular, GNSS, Bluetooth, Wi-Fi, and so on — sets the stage because their frequencies directly affect the ground plane size requirements.
The antenna’s electrical length is a quarter wavelength. For it to work effectively at a given frequency, the antenna’s total electric length needs to be a half wavelength. The ground plane length will compensate for the missing quarter wavelength.
Knowing the radio technology/technologies will help determine the ideal PCB size for optimal antenna performance. For example, for cellular, if the antenna needs to support B12 (700 MHz), then the ideal PCB size should be at least quarter wavelength, which is ~108 mm.
Consider Antenna Options Early in the Device Design Process
Smartphones, wearables, and IoT sensors are examples of devices that typically use embedded antennas. They have something else in common: They’re small, which means their PCB space is inherently tight.
This highlights the importance of considering antennas early in the device design process. Waiting until the form factor is finalized and/or after all the other components have been assigned board space severely limits where the antenna can go. It also can limit the antenna options if off-the-shelf products can’t meet performance benchmarks when installed in the only remaining place on the PCB.
Granted, the antenna can’t dictate a device’s form factor or always trump the needs of other components. But making the antenna’s PCB requirements a top consideration from the start helps avoid the expense of fine-tuning an off-the-shelf product, ordering a custom antenna, or redesigning the board to shuffle components around. This also avoids delays in time to market and time to revenue.
Size Matters
The ground plane plays a key role in determining the antenna’s gain, which measures the antenna’s directionality as it relates to the strength of the signal that it’s transmitting or receiving. That means the ground plane is an important consideration for every RF technology, such as GNSS, 4G LTE, 5G, Wi-Fi, Bluetooth, and more. (For a deeper dive, see “Understanding Antenna Gain and How It Affects Device Performance, Reliability, and Competitiveness.”)
A top ground plane consideration is its length, which should be at least one quarter the size of the wavelength. For example, the North American 4G and 5G bands range from 600 MHz to 3.5 GHz. At 699 MHz, the minimum ground plane length is 107 mm. At 2400 MHz, it’s 31 mm. For more information, see “Understanding Ground Planes for Cellular and GNSS Devices.”)
The size of the keep-out area also is critical. This the space on the PCB surrounding the antenna that must kept free of anything metallic — such as other components, screws, and copper traces — so they don’t undermine the radiated signals.
It’s also important to look beyond the PCB keep-out area. Metal and plastic parts such as cables, batteries, screens, and the device enclosure all affect the antenna’s ability to radiate and/or receive effectively. For example, Taoglas recommends keeping the antenna a minimum of 20 mm from the battery.
The smaller the antenna, the easier it is to shoehorn it into the ideal location. An example is the new Taoglas PCS.62.A antenna, which is designed for sub-6 GHz cellular applications. At just 38 x 10.3 x 3 mm, this ultra-compact antenna increases design flexibility simply because it takes up less board space.
Finding the Sweet Spot
The device’s RF technology/technologies help determine both the type of antennas that can be used and where they should be located on the PCB. For example, patch antennas are common choice for devices that use GNSS, such as wearables, asset trackers, and drones. These should be placed in the center of the PCB, which should be at least 70 x 70 mm in order to optimize performance.
Whether it’s cellular, GNSS, or another technology, each Taoglas antenna comes with a datasheet that provides detailed integration guidance for the ideal location and ground plane size. For patch antennas, the optimal location is in the center of the PCB, while non-patch antennas need to be at the PCB edge (follow the keep-out area recommendation).
An example is the PCS.62.A, whose integration guide is available at https://www.taoglas.com/datasheets/PCS.62.A.pdf. Pages 31-34 provide detailed information about how each ground plane size affects VSWR, gain, efficiency, and other performance metrics.
For more information, see “Best Practices for GNSS Antenna Integration” and “All Together Now: Best Practices for Cellular Antenna Integration.”
Follow the Datasheet
Taoglas antenna datasheets also provide guidance for the matching network, which ensures that reflected signals don’t damage the transmitter while maximizing performance and efficiency. For example, the PCS.62.A requires a 50-ohm (Ω) transmission line and a Pi matching network.
The baseline matching network in the antenna datasheet is a starting point. Those components can be tweaked later if needed, such as if testing identifies aspects that need to be fine-tuned to meet performance requirements. During PCB design, it’s recommended to leave space for additional matching components that in case they’re needed for future tuning. This is another example of why antennas should be considered early in the device design process.
Finally, the transmission line should be designed to have a 50 ohm impedance for perfect signal transmission. The design also should ensure that an RF reference ground is used to protect against electrical and RF noise and against detuning effects from other components.
Test to Verify
Taoglas’ engineering services can create and run antenna simulations for clients when they’re early in the design process. Once clients have developed prototypes, Taoglas also can test and debug them with a vector network analyzer (VNA) and anechoic chamber. Test results provide the insights necessary to tune the antenna and/or tweak the PCB design to achieve performance goals.
For example, Taoglas’ ISA.12: PCB & Gerber Design RF Review service helps device OEMs successfully navigate the complex process of designing and implementing an RF transmission line. Taoglas engineers provide expert guidance for steps such as calculating an appropriate impedance range, designing for stack-up and production tolerances, minimizing parasitic losses, and selecting the ideal transmission line type.
Another example is Taoglas’ CSA.30 Cellular OTA TRP Testing service, which ensures that the antenna has been integrated properly so it can pass carrier certification.
If you’re developing a product that requires wireless communication and need assistance in selecting the right antenna and its placement, the Taoglas Antenna Integrator—part of the AntennaXpert suite of tools—can be an invaluable resource. This tool helps you optimize RF performance, determine component placement, and minimize PCB size. Additionally, it simplifies mechanical integration and reduces development time. You can learn more by clicking the button below.
