Traditionally, small chip antennas used in RF-enabled medical devices have required a designated ground “keep out” area to minimize interference from other components and ensure the ideal radiation pattern for wireless signals. In some cases, this reserved space can eat up as much as 15 × 20 mm of the printed circuit board.
However, with the drive to further miniaturize next-generation medical biosensors and wearables, new alternatives are entering the market that allow the chip antenna to be mounted directly above metal surfaces. By doing so, as much as 10–20 percent of the space traditionally reserved for the keep out area is no longer required, allowing designers to reduce the overall size of the product.
This has major implications for wireless medical devices in which even the miniaturized printed circuit boards (PCBs) along with coin-cell batteries often used are limiting factors in the minimum form factor. Products that could be positively affected by this development include an array of smart devices such as watches, clothing, eyeglasses, patches, pills, and even adhesive bandages.
Mobile Health Biosensors and Wearables
For the medical industry, the future is now when it comes to miniature battery-powered sensor devices that can be located near, attached to, or implanted in the body to monitor physiological signs such as temperature, blood pressure, and pulse rate.
These smart devices will soon monitor everything from fitness to health, environment, lifestyle, and behavior. Biological parameters that can be tracked include vital signs, sleep, emotions, stress, breathing, movement, efforts, posture, gait, body shape, lesions, mental acuity, toxins, blood glucose, ECGs, and medication adherence. The information collected is then wirelessly transmitted to nearby cell phones or remote monitoring stations or through Wi-Fi over the Internet to back-end servers for further analysis, assessment, and decision making.
Collecting data in this manner is expected to facilitate the development of disease models and an understanding of the complex behavior of biological networks. Mobile health data can also be a valuable tool for drug discovery and clinical research. Among the products already incorporating this type of technology are adhesive bandages that contain built-in sensors that measure heart rhythm, respiratory rate, and temperature. These readings can be used, for example, to determine the precise amount of insulin that should be dispensed from wire-lessly controlled insulin pumps worn by diabetics.
Embedded Chip Antennas
To transmit and receive RF wireless signals in the appropriate frequency range, smart devices must contain small RF chip antennas embedded on the PCB or behind the scenes underneath the encasement of the product. These chip antennas radiate and receive electromagnetic waves as other types of antennas, but the most notable difference is their small size. In fact, today's mobile phones incorporate a minimum of four antennas and up to 13 in some models. Smaller wearable devices may only contain one or two antennas.
To work properly, chip antennas have typically been ground plane dependent, meaning they require an appropriately sized and positioned ground plane to form a complete, resonant circuit. While the PCB can serve as the ground plane, the antenna itself must typically be placed on the edge of the board in an isolated section that is free from ground and metal components that would distort its radiation. Without the isolation distance, the performance of the antenna is significantly affected.
The keep out area is fundamental to ensure that the chip antenna can electromagnetically radiate to antenna applications, because everything affects the radiation pattern including the package size, where the antenna is mounted, and its proximity to the human body.
Johanson Technology, for example, has been able to eliminate the requirement for a designated ground keep out area through optimization of materials (ceramics and inks), manufacturing processes, and RF circuit design. Its new 2.4 GHz antenna can now be mounted directly onto the metal ground plane. The antenna measures 2 × 5 mm and is designed for small coin-cell battery operated IoT, 2.4 BLE, wearable, ISM, ZigBee, and 802.11-standard applications where metal or a battery/display covers the entire length or side of the PCB.
Miniature RF ceramic chip antennas are made using low temperature cofired ceramic (LTCC) technology, which has the ability to embed low and high dielectric constants inside the antenna. This enables them to have great detuning resilience and extreme temperature stability (~5 ppm) behavior. These types of ceramic chip antennas offer SMD manufacturability in standard or small form-factor designs and applications (i.e., miniature Bluetooth, Zigbee, ISM, and WLAN antennas) and provide great reliability and versatility while maintaining industry demanding performance. These antennas are easy to tune to unique PCB environment. They offer 2–3 times higher dielectric constant than any FR4, PCB, or standard plastic.
With PCB real estate at a prime, the size and placement of the chip antenna is critical because as everything gets smaller, it becomes increasingly difficult to place more components on the board. Therefore, design engineers are looking to component manufacturers to deliver miniaturized solutions that occupy next to no real board space.
The design of the antenna itself is also critical to its range and performance. With medical devices, radio interference or some other glitch could result in interrupted connectivity. There can be legal ramifications as well. As with any wireless device, products that utilize RF technology, including Bluetooth, to collect or transmit information are subject to regulation by the Federal Communications Commission (FCC). Therefore, it is critical that the device perform at the designated frequency and the design and placement of the antenna is critical to proper tuning.
Despite the critical nature of the antenna, it is often overlooked until late in the design process, at which point optimal antenna performance may not be achievable within the space provided. To assist with chip antenna design and selection, Johanson Technology offers a program where design engineers can send in a miniaturized device, and the company will tune the antenna for optimum functionality.
A chip antenna that can be mounted over a ground plane opens up many applications for products that want to incorporate wireless. To date, the company has received everything from smart shirt buttons to jewelry and other wearables in various shapes and sizes.
Small RF chip antennas embedded on the PCB are ideal for today's smart medical devices. The medical industry is already developing devices that require miniature battery-powered sensors that can be located near the body. With tiny RF chip antennas no longer relegated to PCB keep out area, it allows product designers to further miniaturize wireless biosensors and medical wearables. These chip antennas radiate and receive electromagnetic waves as other types of antennas, but the most notable difference is their small size. And with PCB space at premium, these small antennas open new options for medical device design engineers.
This article was written by Manuel Carmona, RF Business Development & Product Manager for Johanson Technology, Camarillo, CA. For more information, Click Here.