Antenna Placement

Where to place your antenna for optimum performance

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.

Mission Impossible? How well can an embedded antenna perform on a tiny PCB?

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Why is antenna placement so important?

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.

Embedded antennas: the key considerations

Ground plane size
Radiation pattern
Circuit board shape
Co-existing antennas

Ground plane size

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.

Radiation pattern

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.

Circuit board shape

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.

Co-existing antennas

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.

Best practice for antenna placement

Clearance

Antennas require clearance across all layers to operate well. Clearance requirements vary with each antenna. This makes it essential to review antenna datasheets carefully before selecting one.

Position

The radiation pattern of every antenna is unique. For this reason, antennas have recommended placement instructions detailed within datasheets. For some, the optimum location is on a PCB corner, for others, it may be the long-side edge.

Antenna feed

Feed tracks are another important consideration while integrating an antenna. The grounded coplanar waveguide model is particularly useful in small designs. However, they should be kept short and not contain 90 degree corners, as to minimise losses.

Matching network

Embedded antennas will require a matching circuit to operate effectively. 3-component Pi matching circuits offer a means of achieving a close match while minimising the footprint of RF circuitry within a design.

PCB Stackup

There are a whole host of advantages to using four plus layer circuit boards. Knitting these layers together provides better isolation and creates an environment conducive to RF performance. Use evenly spaced vias to knit these layers together, this helps prevent ground loops/paths. Antenna integration can become more of a challenge on smaller PCBs, particularly at sub-1GHz frequencies.

Device application

Products used in the hand or worn bring an additional set of considerations to antenna placement. Leaving a sufficient gap between the antenna and the product housing can help counteract the detuning effects caused by users. Additionally, antennas can be matched to work more efficiently while being held or worn.