7 April 2020
The issues with trace antennas
23 June 2017
Trace antennas laminated on printed circuit boards have long been an option for engineers tasked with WLAN product designs. Times have changed, though. Now devices are shrinking in size, manufactured in complex forms and PCBs are jam-packed with components. Each of these trends has shed light on the hidden costs and restrictions associated with PCB trace antennas.
Chip antennas have now emerged as a favoured option for engineers, due to their design flexibility and ability to negate detuning effects. However, achieving sufficient levels of performance will always prove difficult within compact devices, so does not completely circumvent design-in difficulties.
Furthermore, following guidelines provided in datasheets can appear borderline impossible when a key objective of your brief is to save space. These size-reducing missions can be detrimental to wireless performance, especially where the gap between product housing and the antenna is minimal. To achieve high levels of wireless performance within an inflexible design, it is important you understand the fundamentals of chip antennas.
The chip antenna does not constitute an antenna in its entirety. An appropriately sized ground plane on the host PCB must also be used to create a half-wave or quarter-wave antenna. Chip antennas, therefore, require careful considerations of other design factors, each with the potential to significantly improve or reduce your design’s wireless capabilities.
This means that it is vital you select the right type of antenna with appropriate guidelines for your design. These will be listed on the datasheet. However, you should not base your antenna selection decision purely on the performance parameters promised on a datasheet, particularly for complex, challenging designs. If you do not have a highly flexible design – for handhelds and wearables it is highly unlikely you will have complete autonomy over much of the design – then the parameters measured from the reference board in free space are not a true comparison of performance.
For your design to achieve the best possible wireless performance an intelligent selection decision is required. We’ve developed an Intelligent Antenna Selector to help you decide between antennas for WLAN/BT and location. Odds are your design may not match up perfectly, but the closer you can stay to the guidelines stated on the datasheet, the more likely you are to achieve sufficient performance.
The ground plane serves as half of the antenna design. The dimensions and size of a ground plane will determine the radiation pattern and performance – the datasheet will only provide information about the device on the reference board. Finding the optimum antenna for your design necessitates a careful review of these guidelines to find the best fit for your device.
Finding sufficient ground plane within a compact device can be one of the most challenging elements of your circuit board design. If you cannot meet the recommendations provided in the datasheet, you should carefully review and weigh up the impact on antenna performance. If necessary, consult an experienced Antenova RF engineer to get a better idea of what you can realistically achieve in a small form factor.
If you need to significantly decrease the size of a ground plane, then you will need to thoroughly weigh up the benefits and costs of doing so.
One of the most significant drawbacks of trace antennas is that they can be hugely affected by PCB design. However, whilst chip antennas are less likely to be affected, the placement of components and RF material will play a smaller role in determining the final performance parameters of the device.
Components in close proximity to a chip antenna won’t as significantly detune it, but there will still be some impact on performance. For this reason, you should position any metallic components, such as batteries, LCD screens or metal objects as far away from the antenna as possible to keep it functioning effectively.
In compact devices, having metallic components in nearby proximity to the antenna is almost always unavoidable. In the early stages of circuit board design and architecture reviews, you need to clarify the impact of decisions made at this point with regards to your wireless performance. For example, if the PCB will be surrounded by metal components, then wireless performance will suffer significantly.
Handhelds and wearables regularly suffer from detuning effects caused by the human body. That could in the form of hands, a head or anything else within the operating environment. This requires engineers to tune their antenna to work optimally within the operating environment.
At the point of manufacture, antennas are designed to operate at 50 ohms. In a compact device, however, there will be a significant shift in frequency once the PCB is assembled and housed within an enclosure. You can tune the antenna to work optimally in free space within that enclosure, but for handheld and wearable devices, you need to consider mismatches caused by users.
Handheld and wearable device users aren’t antenna-friendly, and the smaller ‘gap’ between antenna and enclosure the more significant the impact of human operators will be. We recommend you use anechoic testing facilities with phantom body parts to mimic the impact on wireless performance before you finalise your circuit design.
A poorly designed transmission (or trace) lines can result in severe performance degradation. These can limit performance by as much as 50%. It’s therefore important you consider the impact of the dimensions of this transmission line.
We recommend you use a grounded coplanar waveguide, with the optimal gap and width to match your design to 50 ohms impedance. Calculating this, alongside finding room for a sufficiently sized line, is another common difficulty in integrating wireless antenna into compact devices.
Additionally, the length of transmission lines should be minimised. Longer transmission lines are more likely to create losses and pick up noise, which affects the overall antenna performance.
Keeping within budget, on time and delivering on every aspect of your design brief for a handheld or wearable device is challenging. It’s necessary to take a ‘wireless-first’ approach with these designs, as many of these products live and die by their wireless antenna performance. Each and every design decision – from the material of your product housing to the ground plane size – will impact wireless performance in some way.
Having taken the time to review some of the biggest impacts preventing successful WLAN chip antennas, you are already taking the right decision that should ensure your product will perform reasonably well.
To get your design right the first time, then you may wish to contact Antenova for full support throughout the wireless design process. We can help you select the most appropriate antenna for your device, and then assist you test and match your antenna to work within its enclosure and operating environment.
7 April 2020
The issues with trace antennas
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