For best results, the antenna’s location in a wireless medical device must be addressed early in the design process.
Geoff Schulteis, Antenova
Radiofrequency (RF) tests are conducted in an anechoic chamber on phantom body parts. (Image courtesy of Antenova)
The antenna is one of the most important components within a wireless device, so it must function efficiently. Because the antenna’s position is important to good wireless performance, its placement within the device should not be a last-minute decision.
The antenna’s signal may also be compromised by the proximity of other components, affecting how well the device transmits and receives data. It is best to plan component and architecture layouts for good radiofrequency (RF) performance. Here are some factors to consider.
Location is everything
Deciding the location of the antenna upon the circuit board should be the first step, before laying out any of the other components. Chip antennas are the most popular choice for surface-mounted designs, hence their short name, SMD antennas. SMD antennas require a ground plane to radiate. The shape and dimensions of this must be considered, as each antenna has its own unique radiation pattern and requirements.
The clearance areas around the antenna need to be part of the design, as placing other components too close to the antenna can affect the antenna signal. Batteries, metallic components, LEDs and LCD displays can all create interference if they are placed too closely.
Wearable devices can be tricky to design because the human body can block radio signals and cause an antenna to detune. The optimum position for the antenna in a wearable design may be on the side of the device facing away from the body to avoid signal losses.
Ground plane specifications
Take note of how the antenna’s ground plane is designed. SMD antennas use a ground plane to radiate, and this must be the correct size and length. Follow the ground plane requirements stated in the manufacturer’s data sheet to be sure the antenna will perform as it should in-situ.
Some SMD antennas are available in left and right versions and antenna designs for corner positions are available. One of these options may enable you to keep the antenna away from the person and accommodate the ground plane requirements better into a design.
Isolation
Often a design has more than one antenna, such as a Wi-Fi antenna and a Bluetooth antenna. Where this happens, the signals from one antenna may interfere with or detune the other, so it is important to keep them separate from each other to avoid interference.
Transmission lines and matching
The antenna, its feed trace and the radio transceiver must all operate at the same impedance (typically 50 ohms) to ensure that the energy transfer from the radio to the antenna performs efficiently. If the impedance should vary along this path, it can be resolved with matching circuits, such as π-matching topology, which can be tuned using lumped element inductors and capacitors to bring the antenna and radio to the same impedance.
Prototyping and testing
Bear in mind that even the smallest of last-minute deviations can affect the performance of a device. RF performance will also be affected by the environment and other devices in the vicinity. A metal housing is not recommended for a wireless device, and within hospitals there may be areas with no signal at all. Generally, these areas are rooms below ground or those that are screened for x-rays.
RF specialists conduct passive tests in an approved anechoic chamber with phantom body parts to simulate how the device will perform in the real world. The tests will determine the antenna efficiency, gain, return loss, impedance and 2D- and 3D-radiation patterns.
The design must then pass statutory tests for safety and regulatory compliance to meet Federal Communications Commission standards in the U.S. and CE standards in Europe. The final step in testing will be to conduct pre-certification “over-the-air” tests to measure the performance of the device and ensure it will provide a good user experience while assessing its readiness for commercial deployment.
Geoff Schulteis is an RF antenna application specialist with Antenova Ltd. He specializes in over-the-air testing for wireless devices.
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design and Outsourcing or its employees.
