How to get better wireless performance for mobile devices with small PCBs

The demand for smaller wireless devices is growing, for use in consumer applications such as wearables, medical devices and trackers as well as in industrial applications such as lighting, security and building management.  It follows that smaller electronic devices will require smaller PCBs, which mean that the antennas must work with shorter ground planes, and if they are battery operated, power is also a factor – because the device must not consume too much power.

This presents quite a challenge for the product designer. The end design will need to be submitted for formal network and government approval before the new product can be used on the carrier networks, and the design is likely to fail if the antenna doesn’t perform correctly, or if the device creates radio interference by re-radiating noise from the device. It follows that it is even harder to get carrier approval for a smaller product, because it is more difficult to achieve wireless performance that is good enough to pass transmit and receive minimum levels. This is particularly true in the USA where a design must meet strict criteria to gain network approval.

It is a fact that for electrically small antennas to operate at frequencies below 1GHz, they ideally need ground plane lengths of 100mm or more to achieve good performance and efficiency. If the antenna efficiency should drop, it will cause issues with power consumption and achieving network approval for the finished product. This means that the challenge for a product designer is to create a design where there is enough space for the antenna to perform correctly, and still fit all the components into a smaller PCB.

This is especially true for antennas operating at frequencies below 1GHz, which are typically used for products such as IoT devices, product trackers, fitness devices and other similar small devices.

Wearable devices and medical devices that are used close to the human body present a special challenge. The human body restricts RF signals, so the designer should consider how the antenna will radiate, and be certain to place the antenna in such a manner that the human body will not obstruct the signals.

Wearable devices can be as small as 50mm or even less. And some of them may use more than one antenna!

There are several factors that affect the performance of the antenna in a small device, and this article will address them in turn. The first and most important is the ground plane, which in many cases is essential for the antenna to radiate. But this is not all, the designer should place the antenna correctly and consider the other components and the position of these in relation to the antenna, to ensure that nothing noisy or metallic lies in the antenna’s path.  Finally, the casing for the device can make a difference, and we will outline the main materials to avoid.

Embedded Antennas – how they work

A dipole antenna uses two radiators to operate, but an embedded chip antenna has only one. For an embedded antenna, a surface of the PCB becomes the second radiator. This explains why, if the length of the PCB is too short, the antenna will not operate efficiently.

The resonance of an antenna is directly related to its wavelength. The antenna must resonate at whole number multiples or fractions of the wavelength, with the shortest resonant length being a quarter of the wavelength.

A full-wave antenna at the 916MHz frequency would need to be approximately 327mm long, which is not practical for an embedded antenna, but a quarter-wave version is practical at a ground plane length of 87.2mm. This will be coiled up across the copper traces and layers that are hidden within a tiny surface mounted chip antenna.

Antenna designers get around this limitation by using the ground plane as the missing half of the half-wave dipole, so a quarter-wave monopole antenna radiates against the ground plane. Therefore, the most popular embedded antennas in small wireless devices tend to be quarter-wave monopole antennas.

Ground plane length

For an embedded antenna to work efficiently, the ground plane must be at least a quarter wavelength of the antenna at its lowest frequency. Accordingly, at the lower frequencies the design will be much easier when the ground plane is 100mm or greater.

The performance of an embedded antenna is directly related to the length of its ground plane, so allowing for the ground plan to be the correct length is the greatest challenge for smaller designs.

Figure 1 shows the trade-off between ground plane length and antenna efficiency from 794 MHz on the left to 2.69 GHz on the right.


Figure 1. (Source: Antenova Ltd)

These results show clearly how the antenna efficiency drops for small ground planes at frequencies below 1GHz. These results were obtained for a 3G/4G chip antenna operating at frequencies 791-960MHz, 1710-2170MHz, 2300-1400MHz and 2500-2969MHz.

Generally, the ground plane would need to be 100mm or more for a device using the frequencies below 1GHz. In the USA, the 4G frequencies use bands as low as 698MHz or even 617MHz as with T Mobile’s B71 band requiring a ground plane even longer than 100mm.

Positioning the antenna on its PCB

Next, we should consider the position of the antenna on the PCB and its placement in relation to other components.  The antenna should be placed in the best position in the overall RF layout and PCB stack-up to allow it to radiate effectively.

Each individual antenna is designed to work efficiently in a few places on a PCB. This is often the corner or an edge, however each antenna is different, so it is important to select an antenna that fits into the design and place it according to the manufacturer’s recommendation for that antenna.

Figure 2 shows how the antenna is placed with its clearance area in a small device such as a wearable product or watch.


Figure 2. (Source: Antenova Ltd)

Figure 3 shows a suitable antenna placement for a watch design. The design maintains the recommended clearance specified above and below this antenna, which is shown in red.


Figure 3. (Source: Antenova Ltd)

Do not place noisy components, such as a battery or an LCD close to the antenna section. Antennas are passive components that receive energy and will pick up noise radiated from the noisy components, and transfer that noise to the radio, degrading the received signal. The antenna should also be placed away from the human body to improve RF performance, this is the distance marked in blue in Figure 3 above.

The arrangement of the RF feed and the ground connections are critical to the function of the antenna. With small embedded antennas in small PCBs, the copper tracks etched on the PCB may form an integral part of the antenna so care should be taken to follow the manufacturer’s specification or reference design.

Overall RF layout and PCB stack-up

You can maximise the performance of antenna by giving careful consideration to the layout of the RF elements in the design. The copper ground plane should not be cut up with traces or arranged over more than one layer, then the ground plane portion of the antenna will be able to radiate more effectively.

It is essential to keep components such as LCD or batteries clear of the antenna area in the PCB layout, as these can interfere with the way the antenna will radiate.

For multiband frequencies, we suggest a PCB layout with a minimum of four layers.

Figure 4 shows how the top and bottom layers provide ground planes, while the digital signals and power which need to be away from the ground plane, run in the space between these.


Figure 4. (Source: Antenova Ltd)

Tuning the antenna for performance

For those cases where the ground plane is shorter than ideal, a designer can look at other techniques to increase the performance of an embedded antenna.

One way is to tune the antenna for its country of operation.  The 4G frequency range is a wide one, spanning from 698MHz to 2690MHz, but each different world region uses just a portion of this band, and an antenna can only operate on one frequency at a time. This means that when a product is to be used in one geographical region, it can be tuned to operate in a narrower section of the frequency band. This will boost the performance of the antenna.

Another technique is to include an active tuning network, effectively an additional RF switching circuit, which will help to get over the bandwidth reduction caused by a smaller ground where the host PCB is less than 75mm.  A PI matching circuit is added close to the antenna feed point, to fine tune the antenna and boost up performance. The design of the matching circuit will usually need some assistance from an RF specialist.

Figure 5 shows a matching circuit on an antenna evaluation board.


Figure 5. (Source: Antenova Ltd)

Designing the transmission line

Once the material for the PCB has been chosen and its thickness and dielectric constant are known, a co-planar transmission line can be designed using one of the commercially available RF trace design software packages. This will use the PCB thickness, the copper layer separation and substrate dielectric constant to calculate the optimal width for the transmission line and the appropriate gaps on either side to achieve a co-planar transmission line of 50 Ω.

All transmission lines should be designed to have a characteristic impedance of 50Ω, and the other parts of the RF system, such as transceivers or power amplifiers should also be designed with an impedance of 50Ω.

Antenova offers a free RF transmission line calculator tool to help designers determine the size of the transmission line.

Other factors

There may be more than one antenna, operating at different frequencies on the same PCB but placed at close proximity. If the antenna is a receive only system, such as a GPS receiver, it could be de-sensed by a nearby transmit antenna such as a 4G radio reducing the accuracy of the GPS system. Care must be taken to separate these antenna systems either by physical distance between the antennas – making sure the antennas are orthogonal to each other – or by notching the ground plane to remove the ground currents shared between the antennas.

In multiple input, multiple output (MIMO) systems, the design will require more than one antenna, which should be placed with one relative to the other so that they can co-exist. Then they can be matched to the same frequencies. It is imperative that the antennas be placed to ensure that the isolation and cross correlation are within acceptable limits. As mentioned above, care must be taken to separate the antenna in the device either by physical distance between the antennas, making sure the antennas are orthogonal to each other or by notching the ground plane in between the antennas to remove the ground currents shared between the antennas.

Figure 6 shows proximity configurations for diversity.


Figure 6. (Source: Antenova Ltd)

Figure 7 shows opposed configurations for diversity.


Figure 7. (Source: Antenova Ltd)

The outer case should not contain metal close to the antenna, but certain metalized coatings are acceptable because they do not conduct energy effectively. Metal objects near the antenna can cause the frequency of the antenna to shift lower in frequency. It can also reduce the amount of bandwidth the antenna is designed to operate with. Another issue with metallic objects near the antenna is that the metal objects block the signal in the direction the metal is placed reducing the overall radiation pattern and possibly causing the signal to degrade enough to lose connection with the base station.

Conclusion

If the product design is to include an antenna, especially if it is using a small PCB, we would recommend selecting the antenna first, and placing this first on the PCB. It is easier to do it this way rather than to slot an antenna into an otherwise finished design. Thinking about the antenna first is usually the fastest way to achieve a design where the RF element performs as it should.

This will increase the chance of obtaining network approval for the device. The antenna needs to operate efficiently if it is to gain approval and the rules are tough. However, AT&T did make allowances for devices smaller than 107mm and lowered the threshold of efficiency for these smaller devices.


Geoff Schulteis is Senior Antenna Applications Engineer with Antenova Ltd. Geoff has more than 20 years of experience in designing, integrating and testing antennas, and he currently heads technical support for Antenova’s customers in North America. He is an antenna engineering professional with more than 20 years’ experience designing, integrating and testing antenna systems for consumer products from R&D through manufacturing and commercial deployment.

 

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