The placement of an antenna is equally as important as the hardware choice
With smaller devices in high demand, tech companies are requiring smaller and smaller circuit boards. Now, finding enough space for an antenna, let alone placing it in the optimal location, can be a real challenge.
Modern devices bring conflicting requirements for an antenna. For example, they need to occupy a small space on a circuit board, yet still operate efficiently. At sub-GHz frequencies, making a ground plane smaller can have a major impact on performance. As we examined in our 2019 paper, even a small reduction in ground plane size can negatively affect efficiency.
For devices that need to meet certain thresholds to achieve certification, antenna placement is a crucial step.
Why is antenna placement so important?
Antennas differ from other types of integrated circuits in terms of the conditions they need to perform well. One of the key determinants of where an antenna should be placed on a circuit board is its radiation pattern - something that is unique to each antenna. This means there is no one-size-fits-all solution to antenna positioning - you’ll need to consult an antenna’s data sheet for recommendations. Some antennas need to be placed on the long-side edge of a circuit board, but for others this may not be true.
Placement considerations for embedded antennas
Ground plane size
The first key consideration is ground size - as discussed, the ground plane is an integral part of the antenna. Therefore, sticking to the recommendations of the antenna’s data sheet is vital to ensure optimum performance. However, ground plane space can be an issue. Potential solutions include integrating FPC or case-mounted antennas.
Radiation pattern
As most embedded antennas aren’t precisely isotropic, each should have an optimal orientation. This will require the antenna to be positioned in such a way that the radiated energy is directed towards the source or destination. For wearable devices, it’s imperative that this radiated energy faces away from the user.
Circuit board shape
What constitutes optimal placement is also partially dictated by the circuit board shape. Some antennas need to be placed on a corner – impossible, of course, on a circular circuit board. Others perform best on the long-edge of a PCB.
Co-existing antennas
Many modern devices now house multiple antennas in a single enclosure. For these types of devices, wireless performance can be improved by an antenna with good isolation and cross correlation. For best performance, use antennas that are best placed in different locations, ideally as far apart as possible and rotated 90 degrees from other antennas that operate in similar bands.
Antenna Placement Best Practices
Clearance
- Clearance is required across all layers for antennas to operate well - requirements vary from antenna to antenna.
- Review datasheets thoroughly before making your antenna selection.
Position
- Each antenna will have unique instructions for placement due to varying radiation patterns.
- For instance, some may operate best on a PCB corner, others on the long-side edge.
Antenna feed
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Feed tracks also need to be considered when integrating an antenna.
- The grounded coplanar waveguide model can be especially useful for small designs.
- Feed tracks need to be short, while avoiding 90 degree corners will reduce losses.
Matching network
- Embedded antennas need a matching circuit to perform effectively.
- 3-component Pi matching circuits are suitable - achieving a close match minimising the RF circuitry footprint.
- Matching components should be placed closer to the antenna.
PCB Stackup
Using four-plus layer circuit boards holds a number of advantages:
- Knitting layers together improves isolation and gives a better environment for RF performance.
- Evenly spaced vias can prevent ground loops/paths.
Device application
- Wearable or handheld products require more consideration around their antenna placement.
- User movement often causes the device to suffer detuning effects - a sizable gap between antenna and product housing can mitigate this.
- Additionally, antennas can be matched to work more efficiently while being held or worn.
