Designing cellular antennas into small IoT products

Small products that use cellular protocols like CAT-M and NB-IoT are in demand; indeed, consumers expect wearable electronics such as smart watches, fitness trackers and pet trackers to be very small. These Internet of Things (IoT) products must still work well even though the ground plane is shortened — this article examines the options available to design a cellular antenna into a small area.

For cellular antennas operating below 1 GHz (sub-GHz) frequencies, a ground plane of 100+ mm is usually required to maintain high antenna efficiency. Low efficiency will cause issues gaining network approval for the finished product. So the challenge for the product designer is to create a physical design that meets the limitations of size and space, and still performs well in operation. The Pharaoh antenna, for example, is able to operate on a smaller PCB while still having good performance.

Figure 1: Comparison of ground plane lengths — 120 mm vs 60 mm for the Pharaoh antenna.

The challenge

The challenges faced when selecting antennas for IoT devices today are not dissimilar to those faced by the manufacturers of smartphones when they were first introduced. The challenge is to design a product containing several antennas, some covering multiple frequencies, in a small environment, and still achieve good performance. Over the last decade, smartphones have become bigger because it’s all about the display. IoT products are smaller. They don’t usually have screens, and they need to be small because they are body-worn, like a smart watch or a fitness band.

In addition to this, network providers are pushing everyone to move over to 4G, particularly in the US where 3G is now discontinued. However, the majority of IoT devices only transmit small packets of data, sometimes not every day, and they very rarely transmit any voice signals, so the 4G network is overkill for their requirement. 4G frequencies in the US go down to 698 MHz.

Sub-GHz antennas ideally require at least 100 mm of ground plane, but many IoT devices today are around 50 mm long. Some are also body-worn, which detunes the antenna and reduces performance. There are network approvals to pass which are more stringent in the US than in any other country. Fortunately, the AT&T network has reduced the efficiency requirements for devices smaller than 107 mm in length.

Figure 2: Antenna fitted on a 90 mm PCB.

Design-in of an embedded antenna

Unlike a digital component, an antenna will only perform to the specifications given in the data sheet, when it is correctly integrated into the design. The factors that affect its performance are:

• Other components in proximity.
• The outer casing and its material.
• The position of the antenna within the device.
• The layout of the PCB (power/noise issues).
• The length of ground plane — this is the challenge for the design of a small cellular product.
   

The overall RF layout of the PCB is important to maintain antenna efficiency. It is important that the copper ground plane is not too complex — not cut up with traces or divided between layers. It is recommended to use a four-layer PCB structure, where you have a top and bottom ground plane layer. Both grounds should be tightly knitted together to prevent a floating ground, and all digital signals and power lines should be run in between them.

Attention should also be given to the following:

• Keep the RF trace/feed between the radio and the antenna as short as possible.
• Run vias at short intervals along the feed line connecting the grounds.
• Try to keep the RF trace going straight to the antenna.
• Allow for a PI matching circuit close to the antenna feed point to fine-tune the antenna later.
• Flood any free areas with ground.

Ground plane

A dipole antenna has two radiator arms; the length of each radiator is related to the wavelength of the frequency being received or transmitted. Embedded chip antennas have only one radiator. So, for the antenna to function, the ground plane of the host PCB becomes the second radiator. Therefore, your PCB ground plane length is important.

A half-wave is the resonant length of an antenna. Therefore, an antenna the length of a quarter-wavelength uses the ground plane as the other side of a dipole to form a half-wave.

A full-wave 698 MHz antenna is 430 mm, and a quarter of this is 108 mm; therefore, the ground plane length needs to be at least 108 mm. A surface mount chip antenna contains a track with the length of a quarter-wave.

Figure 3: A cellular antenna within an OBD tracker.

More options to make sub-GHz antennas work in small designs

Copper foil can be added to extend the length of the ground plane. However, care must be taken not to loop this piece of copper around and underneath the existing ground. An example of this is a watch application. In this case, the ground extension can be run into the watch strap.

Another option is the use of a flexible PCB (FPC) antenna which is stuck onto the product housing with a cable and a connector. FPC antennas reduce the requirements for a long ground plane since the cable acts as a ground. The positioning of the FPC antenna and the cable routing are important.

A new option recently introduced is an antenna type designed to operate on a short ground plane. This antenna, called Pharaoh, operates on a ground plane down to 50 x 40 mm, whereas the ground plane length needed for a typical cellular antenna is more than 110 mm. The trade-off is that Pharaoh is larger, at 37 x 13 mm, compared to typical cellular antenna sizes of 35 x 8 mm that require at least 110 mm of ground plane.

Figure 4: Efficiency reduces as the ground plane length is shortened.

Conclusion

Embedded antenna design is never easy, particularly if the environment is challenging and there is the need to support sub-GHz bands. In all designs, it is extremely important to consider the antenna placement and layout first, before moving on to the rest of the design. Why? An antenna only works effectively in a few places on a PCB. Its position and layout are critical to its effective operation. Our advice is to follow the data sheet guidelines and the advice from your antenna supplier carefully. If your product fails network approval, then many months of redesign effort are required before trying again for network certification.

Top image credit: iStock.com/Sitthiphong