Smart Antenna Choices
Portable trackers offer a very particular set of problems for designers. In this article, Geoff takes a quick look at the design requirements of a portable tracking device, the choice of embedded antennas and some of the points to consider when designing a product where the performance of the antenna in situ could determine the commercial success of the product.
Designing portable trackers means facing some unique challenges, including selecting the right antenna solution. By their very nature, it is difficult to design for Global Navigation Satellite System (GNSS) frequency bands. Particularly in small devices, where a whole host of issues can arise during development. Here, we consider the factors that make a successful design for a tracking device.
Portable trackers need to be small enough to be easily carried around and are often used in applications where they are attached to another item, or they may be used as a portable or wearable device for a living being such as a pet, a child or an adult person. This means that the design needs to be neat and discrete, because the whole idea of the tracker is to provide an inobtrusive way to monitor location. This means that the device is likely to be small, and the antenna that drives it should also be small (Figure 1). If the antenna is to be embedded into the PCB, its location needs to be planned right from the very earliest stages of product design.
There are plenty of antenna options available, including choices of network: LTE, Bluetooth, WLAN and so on, and the satellite options of GPS, GNSS which can pinpoint the location for navigation, mapping, geo-tracking and fitness trackers. For a design example using a Bluetooth radio, see the sidebar “Design Example: Smart Bike Lock” at the end of the article.
In selecting an antenna, you will need to consider the ground plane requirements and proximity loading of lossy bodies (such as the human extremities), to be sure that the design allows enough space for the antenna to radiate effectively. You may want to consider the different forms of antenna as well, which could be a flexible printed circuit, ceramic or an FR4 surface mount chip antenna.
The antenna within a portable device needs to be as omni-directional as possible, so that it can send and receive signals from anywhere in its environment (Figure 2). This might pose a difficulty because it means that the RF design will need to mitigate interference that could affect the antenna from any direction.
Choosing the right antenna and arranging the components correctly within the device becomes particularly important. We have sometimes seen trackers where the RF functionality has been compromised by the operating environment, so it is important to design the RF circuity correctly and carry out tests to be sure how the final device will perform.
Portable trackers are generally small, so the space on the PCB will be limited and the antenna may be placed very close to other components. Components such as batteries and power regulators are known for creating electrical noise, which could interfere with the antenna signal. Some antennas are designed to support co-location with noisy components and other antennas, but others require strict “keep out” areas, which will be specified in the datasheet for the antenna. Designers need to be very aware of how the layout of the other components will affect signal transmission and reception. One way to be sure is to submit your Gerber file for verification by an RF expert, before proceeding too far with the design.
There are also human factors to consider. Some trackers are used to track goods, such as shipping containers or pieces of luggage. However, if the device is a fitness tracker or one that is going to be used to track pets or livestock, it will be placed directly on the body. The challenge is that the body reflects and absorbs radio signals. This could potentially prevent the device from operating in the real world, even if it performs perfectly in a test chamber.
Designers therefore need to consider the environment where the tracking application will be used and create their designs accordingly. Here are some effective solutions: some designs use antennas with ceramic housing to isolate interference, or some even use flexible antennas that can be inserted within textiles and clothing. These can be effective ways to ensure that a tracker functions correctly when placed against the body it will be tracking.
RANGE AND POWER
Consider the effective operational range for the device you are building. A tracker should be able to send and receive signals continuously as it moves around. If the device can easily be moved out of range, it has failed in its mission. It is not easy to guarantee the range for the antenna within a small tracker, particularly when it is integrated alongside other components, as discussed earlier. Real- life use scenarios can be difficult to predict and may reveal oversights in otherwise theoretically “correct” solutions. The best way to overcome this is to use an extensive range of testing scenarios to discover the realistic operational range of the tracker.
For an antenna to maintain a reliable signal in large, noisy environments, it will inevitably draw more power than devices operating in smaller controlled spaces. Also, the volume and frequency of the data transmitted has a direct bearing on battery life.
The battery needs to be large enough to keep a device operating for a reasonable period of time, and there should be a clean way to replace or recharge it. If the means of providing power are not user-friendly, it can be a frustrating user experience. Nobody wants to have to remove and charge their dog’s collar every night when all they were hoping for was an easy way to keep track of their new puppy. One good way to help keep a tracking device charged is to make it compatible with one of the wireless induction chargers.
CERAMIC OR CHIP ANTENNA?
The latest tiny chip antennas for GNSS allow even small devices to receive signals from over 20,000km away. In RF terms, they have taken strides past the traditional ceramic patch antenna. A ceramic patch antenna will usually provide superior performance in open space, with plenty of ground plane, however, they require a large antenna footprint. They are also relatively cheap, but it is unlikely that your design brief will be able to dedicate enough space to accommodate one of these, and the smaller ceramic patch antennas are limited by their efficiency and rarely collect enough RF energy to perform as well as the larger ones.
This means that smaller patches measuring less than 17mm × 17mm will typically be unable to achieve omni-directional gain. This will limit their performance in devices that are not operating parallel to the source radiation, in this case, the sky. They are also moderately difficult to amplify without allowing signal noise to impact the performance of the device. In small tracker designs, a surface-mounted antenna will have a smaller footprint, and will usually outperform a ceramic patch at a lower cost (Figure 3).
Ceramic patch antennas need line of sight to the sky, which means they need to be pointed upward wherever possible. Most cars and drones operate parallel to the horizon so this may not be an issue in those use cases, but it does limit their usefulness for trackers, wearables, handheld and other devices that require an antenna with low directivity to perform well for a quick time-to-first-fix.
There are many satellite frequencies within GNSS and many devices support multiple systems, but it becomes difficult to achieve sufficient performance across varying wavelengths with small ceramic patch antennas. If the device is smaller than 25mm2, ceramic patches will only effectively operate on narrow frequency bands. In contrast, a surface mount antenna can work effectively on wide frequency bands. This may not pose a design issue. If the device is to operate parallel to the horizon, on a single frequency, a small ceramic patch antenna may provide sufficient performance.
ANTENNA GROUND PLANE
Due to the size of the satellite wavelengths, an embedded patch antenna will require a greater than normal ground plane size, typically upward of 60mm × 60mm. Surface-mount antennas allow more flexibility, and some overcome this by operating on the corner of the PCB, whereas the ceramic patch antenna must be placed in the center of the ground plane for omni-directionality. It is important to select the most appropriate antenna for your design and minimize the challenges of RF integration.
The ground plan functions like a runway for transmissions to and from the antenna, reflecting signals into and away from the antenna to minimize interference and optimize the radiation pattern. For fixed antennas, the Earth’s surface functions as the ground plane, reflecting radio waves into and away from the antenna. For portable antennas, the ground plane must be part of the PCB itself. This can become a real issue when designing small GNSS tracking devices. Space confines and proximity to other components can compromise the efficiency of a ground plane. Figure 4 shows the M20047, a 1.584GHz general purpose integrated trace RF antenna.
In order for the ceramic antenna to be effective, a ground plane needs to have a radius of at least a quarter wavelength of the radio waves the antenna is tuned to. For GNSS and GPS, which broadcast at either 1575.42MHz or 1227.60MHz, this means a ground plane with a radius of around 5cm. This space needs to be planned in your design early on. It can be problematic if it is not considered early in the design process.
The antenna uses the host PCB ground to effectively radiate. As such, a GND plane must be placed on the host PCB on at least two layers. In the example shown in Figure 5, the only area void of GND is the antenna keep-out area. The solder mask is removed to make the copper visible.
If it proves tricky to allow sufficient space for the antenna and its ground plane, it may help to consider the options of surface-mounted antennas and flexible printed circuit (FPC) antennas. It is crucial that the layout of your PCB does not affect the ground plane. This is a particular issue in GNSS tracking devices, where space is at a premium and components need to be close to one another.
The ground plane will allow effective signal transmission and minimize interference, but the antenna may still receive reflections from glass and buildings in the surrounding area, which could compromise functionality. Portable tracking devices are particularly vulnerable to this. They may be carried into all sorts of environments, including potentially hostile ones, but they still need to be capable of transmitting and receiving reliably, so we would recommend that designers keep this in mind and plan a test program accordingly.
The antenna choice is crucial in the design of the bike lock because the Bluetooth Low Energy (BLE) radio is the only way to communicate with the smart phone app that monitors the lock. In a real-world design along those lines, the design required the Bluetooth signal to be as strong as possible, to maintain the connection between the bike and the smart phone so that the users can monitor the security of their bike from a distance.
BLE solutions typically use a chip antenna co-located on a radio module, but this may not be a reliable solution. Depending upon how the antenna is integrated, it may work next to a plastic surface but not on a metal one. The challenge was to work within the small dimensions of the host PCB and accommodate the antenna in close proximity to the lock mechanism and the bike itself. The antenna was shielded below the electronics, but the integration did not allow a co-located antenna on the BLE radio module.
The antenna was therefore located away from sources of interference on the main board, where it could be protected from knocks and bumps during day-to-day use. For the design, the engineers chose Antenova’s Weii ceramic antenna, which measures just 1.0mm × 0.5mm × 0.5mm, with the matching network tuned for the application. This achieves good efficiency—good enough to operate at a range of a few hundred feet. It uses an antenna topology known as magnetic-dipole, which remains very stable and on-frequency in this application.
Antenova | www.antenova.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • OCTOBER 2020 #363 – Get a PDF of the issue