As circuit boards get smaller and more compact, finding sufficient space for an antenna becomes more of a challenge. Even when antennas are already placed, the first place many designers look to reduce space is in the antenna’s ground plane.
At sub-GHz frequencies, a reduction in ground plane can make a huge difference to performance. Even a small drop in plane size can reduce efficiency dramatically. For devices which need to meet certain thresholds to achieve certification, this is a key factor.
Unlike other sorts of integrated circuits, antennas must abide by the laws of physics in order to function. Every antenna has a unique radiation pattern, which, among other factors, will determine where the optimum position of an antenna will be on a circuit board. Generally, the best place to position an antenna is along the long-side edge of a circuit board. Data sheets will outline placement recommendations for each antenna.
Ground size is an important factor when using an embedded antenna. The ground plane constitutes a crucial part of the antenna. This makes following the recommendations outlined in the datasheet imperative for performance. If ground plane space is an issue, then consider integrating an FPC or case-mounted antenna.
Most embedded antennas are not precisely isotropic. For this reason, there will be an optimal orientation of the antenna. To enjoy the best performance, the antenna should be positioned so the radiated energy heads towards the source/destination. In the case of a wearable or handheld, this also needs to be away from the user.
The shape of the circuit board also has a bearing on the optimal placement of the antenna. Some antennas require placement on a corner – which of course is not possible on a circular circuit board. Others perform best on the long-edge of a PCB.
It is no longer uncommon for a device to house multiple antennas within a single enclosure. In these scenarios, choosing an antenna with good isolation and cross correlation can markedly improve wireless performance. To ensure these antennas can operate well, select antennas that are best placed in different locations, ideally far away from one another.
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