Geoff Schulteis, RF Antenna application specialist with Antenova, provides tips and advice for developing wireless devices for the medical sector, and explains how to select the most suitable antenna topology, and to achieve the best wireless performance and range from the device.
Designing a wireless device for medical use brings a unique set of challenges. Medical equipment needs to perform to a high level of reliability, but when it is used close to the human body, this can introduce challenges to the operation of the antenna. This article discusses the choice of antennas available and provides some tips for integrating antennas into devices that will be used close to the body.
Sometimes the very nature of the device can impact RF performance. Devices that are worn on the body can easily detune, as the body can block RF signals. In these instances, the safest option is to place antennas away from the human body to avoid signal loss.
The topology of an antenna determines how it will perform in terms of efficiency, bandwidth, radiation pattern and gain. In designs for small devices, space on the circuit board is always limited, but the smallest antenna is not always the best choice. Here we look at some of the options available.
Trace antennas offer unprecedented levels of control, are relatively inexpensive and can be reproduced quickly at scale. However, they can use more space on a circuit board, and they may have issues with de-tuning. Seemingly small changes to circuit board materials or component layout can drastically detune a trace antenna. Their integration is relatively difficult, and they cannot always be optimised for your device.
Ceramic patch antennas are popular for GPS and positioning applications, but in handhelds and wearable devices they are being superseded by other types of antennas. This is probably chiefly because patch antennas are highly directional and need to be pointed upwards to the sky to function effectively.
Small ceramic patch antennas can be expensive, and do not guarantee the same levels of performance as other kinds of antenna due to the shortage of ceramic material available to transmit and receive RF energy. In small electronic devices, ceramic patches typically only support narrow frequency bands, so their performance is limited.
Planar Inverted-F Antennas (PIFA) have become the de-facto wireless solution. Since being popularised in 1998, they have become ubiquitous in handheld devices, wearable electronics, and other small connected devices. Their chief advantage is their ability to perform well in a limited space. The 3D topology of these antennas provides good SAR properties (Specific Absorption Rate of the RF energy) while being resonant at sub quarter wavelengths.
They offer high performance in a small area and are easy to integrate because they include a simple matching circuit. They are widely available and relatively inexpensive, and as they are provided in tape-and-reel, they are suited to low cost manufacturing.
PIFA antennas offer the design advantage of being able to function on top of the PCB ground plane, which means that other components can be placed below the antenna. They give the product designer more options for layout.
Electrically Small Antennas (ESAs)
ESAs, or electrically small antennas, are much shorter than their designated wavelength. Whereas some antennas radiate at one
quarter or one half of the length of their ground plane, ESAs can operate at as little as a tenth of a wavelength.
Recent innovations have improved the performance of ESAs dramatically, bringing better gain, wider bandwidths and field patterns. ESAs offer the advantage of a tiny footprint. Some of them measure as little as 20 mm, and they are relatively immune to interference and detuning effects. Also, their capacity can be scaled relatively easily by using beam steering.
Where mission-critical wireless performance is required, terminal antennas offer high levels of performance which is ideal for health monitoring equipment and health devices that require constant and reliable connectivity. They are used externally.
Terminal antennas are larger and operate outside the housing of a device. They offer outstanding performance in free space as they do not need to counteract the effects of intra-device detuning, and integration is easier because they do not need to be optimised to operate within the housing of the device.
Magnetic loop antennas
A magnetic loop antenna couples to the magnetic field wave in the region near the antenna. This small configuration works perfectly in ultra-small devices that require high levels of performance in a small space. Looped antennas are highly efficient, with good performance. Also, the coupling between a loop antenna and the PCB is reduced, which enables the antenna to operate with a smaller ground plane.
Magnetic loop antennas, due to their topology, are stable against de-tuning, which makes them ideally suited for wearable and handheld devices, where PCB space is at a premium and performance is critical.
Achieving best performance and range
It is crucial to consider antenna placement early in the design process and to ensure that individual components do not interfere with each other but perform correctly together.
The position of the antenna is key to RF performance, so it should not be a last-minute decision. The proximity of other components can have a huge impact on the way the device transmits and receives data. This needs to be understood early in the design process, otherwise the device will fail testing and the whole design will need to be re-thought.
Determining the location of the antenna upon the circuit board should be the first step, but it will also need space for the ground plane that allows it to radiate.
The ground plane size can have a huge impact on the efficiency of the device, so it is essential to follow the requirements stated in the antenna datasheet, to be sure that the antenna will function correctly and efficiently.
Antennas usually require a clearance area, where there should not be any other noisy components, high speed data line or ground. This varies from one antenna to another. Some antennas are designed to work on the corner of a PCB, to minimise the ground plane requirement, and these may come in left and right versions.
Conductive surfaces such as metal shield cans, data and power connection point, LCD displays, or plated housings can adversely detune an antenna and make a device unreliable. Signals from other antennas can also interfere with the device’s main antenna, so it is essential to keep antennas separate from each other, to avoid interference. The same can be said for batteries, metallic components, switches/ buttons, and LCD/LED displays.
Finally, the antenna, its transmission line, and the radio, must all operate at the same impedance (typically 50 ohms). This ensures that the antenna will perform efficiently – without any signal loss. Any differences can be resolved using matching circuits, such as π-matching circuits, to bring antenna and radio elements to the same impedance.