While there is plenty of talk about evolving radio standards and emerging IoT connectivity services, one critical aspect of the IoT is often overlooked – the antenna that translates radio-frequency energy to and from an electrical signal.
To some extent, antenna designers have had their own way when it comes to the physical design of products. Antennas have tended to be the shape they are because that is what the physics allows, and everyone else has had to work around this constraint. But IoT devices are testing this premise – you just can’t put a 1980s CB-radio whip antenna on a fitness tracker, or a big stub antenna on a connected thermostat, and expect to sell any.
Engineers are problem solvers, and it turns out there are multiple ways to accommodate the laws of physics when the pressure for more discreet antennas becomes great enough.
For example, LSR has developed a flexible single- (2.4GHz) or dual-band (2.4/5.5GHz) WiFi antenna technology that can be used in all sorts of the unlikely ways that IoT products are now demanding. The FlexPIFA antenna is designed to offer consistent performance when installed on a variety of non-conductive surfaces of varying thicknesses, as well as near metals or even the human body (neatly addressing the fitness tracker issue). It can also be installed on convex or concave curved surfaces without losing performance, and is supplied with an adhesive backing that is designed to survive humidity exposure and multiple thermal cycles.
The 50-ohm antenna is 40 x 11mm, and one end of it sits on an adhesive pad that acts as a standoff for a solder connection to a 100mm MHF4L or U.FL cable. It offers a peak gain of +2dBi, and an expected average gain of more than 1.5dBi. The antenna’s polarisation is linear, and its voltage standing-wave ratio measurement is less than 2.0:1 over the single-band version’s 2400 to 2480MHz operating range.
As you’d expect, these are the basic measurements for the antenna in its single-band form when it is deployed flat. A datasheet for the part demonstrates the complexity of making good RF measurements for an antenna when its performance will be modified by the material it is mounted on, its radius of curvature, and even whether it is being bent in a concave or convex direction.
Every application is going to be different, but the datasheet offers guidance to help designers get the best out of the part. For example, the main element of the antenna should be kept clear of any non-metal objects (such as plastics) above it by at least 10mm, while its two long sides should be given at least 2mm of clearance.
The best material on which to mount the antenna is 1.5mm-thick polycarbonate, although it will tolerate being mounted on other materials (at a possible cost to its performance). As you would expect, it’s best to route the antenna’s cable straight away from its element, rather than, for example, running it over the top.
The datasheet also discusses the impact of nearby metal on the antenna’s performance. Ideally, the antenna should be kept away from conductive materials, in which it could induce currents that would create interfering radiation of their own. Other objects, such as LCDs, could distort the radiation pattern, while materials that absorb electromagnetic fields should be avoided.
IoT device designers may also appreciate the following list of issues to be avoided when placing an antenna within a design, such as:
- wire routing
- speakers or sounders
- metal chassis elements and frames
- batteries and the current they drive through product wiring
- screening foils and metallic coatings
- proximity to the human body
Another important design constraint of this flexible antenna is its bend radius, which should not be less than 16mm when bent in a convex direction. Going smaller than this might result in the antenna peeling off the surface. The antenna should not be flexed in the concave direction to a bend radius of less than 25mm, because this will press the ground plane of the antenna closer to the main element and so reduce performance.
The FlexPIFA antenna can be mounted on a conductive surface, although this will detune it and so reduce its gain, because of the increase in the size of the antenna’s effective ground plane.
The antenna should not be mounted where there will be metal less than 10mm above the main element, because this will both detune the antenna and limit its radiation pattern. For the same reasons, metal should be kept more than 5mm away from the two long edges of the antenna. Ideally, designers should include a 1.5mm-thick polycarbonate spacer between the antenna and any conductive surface on which it is mounted, to improve its performance and tuning.
The FlexPIFA antenna can be mounted close to the body, but designers need to recognise that the body is an excellent absorber of 2.4GHz RF signals. They should place the antenna so the ground plane is closest to the body, with the main element pointed away from it. In handheld devices, the antenna should be mounted so that it is not covered by the hand. If the antenna has to be mounted so its main element is close to the body, designers should try to keep at least a 10mm separation, perhaps with a spacer (again, ideally of polycarbonate). Often, a product can be adapted so this standoff distance is integral to its design.
Good WiFi connections demand good antennas. The FlexPIFA from LSR demonstrates that it is possible to create such antennas, even in some challenging circumstances.