According to a recent Transparency Market Research report, the IoT medical device market is set to see a surge by 2026. These devices may span “vital signs monitoring devices, imaging systems, respiratory devices, implantable cardiac devices, infusion devices, anesthesia machines, neurological devices, hearing devices, fetal monitoring devices, and ventilators.”
With this perspective surge, designers in the medical device industry may notice significant hardware innovations, even at the passive levels. For instance, passive manufacturer Abracon recently updated its selection of dielectric resonator antennas, affirming its commitment to solutions for IoT medical devices.
Abracon’s chip selection guide covers multiple classifications, including Wi-Fi, cellular, and medical. Image used courtesy of Abracon
Commonly known as chip antennas because of their small size (as small as 2 mm x 1.25 mm), dielectric resonator antennas offer certain benefits over planar printed PCB antennas. In this article, we’ll compare the two and discuss how each may play a role in an evolving embedded landscape.
Design Benefits of Chip Antennas and PIFA
Both chip antennas and planar inverted-F antennas (PIFA) have their own design complexities that will require an RF designer’s expertise to navigate.
Chip antennas can be successfully tuned with a passive network of inductor and capacitor components, producing a well-matched transceiver system. Comparatively, PIFAs require significantly more simulation and potentially additional re-spins of the board to optimally tune the RF chain.
Several other practical considerations for selecting a chip antenna include the operational frequency with lower frequencies potentially making PIFA structures untenable and ground plane clearances dictating the space constraints on the rest of the PCB.
What Parameters Determine Antenna Geometries?
IoT devices are varied in their application, and RF design requirements such as antenna pattern, gain, and directivity vary based on the use case.
Both PIFA and chip antennas possess an omnidirectional antenna pattern, which is ideal due to the mobile nature of an IoT device. Image (modified) used courtesy of 5G Technology World
Antenna polarization (either vertical, horizontal, or circular) strongly affects the power reception. An omnidirectional pattern generates energy in a uniform “doughnut,” allowing both receiver and transmitter to be at various incident angles to each other.
Another parameter, permittivity (also called dielectric constant), has a major impact on determining what the final dimensions are to match a quarter wavelength. The wavelength is inversely proportional to the dielectric constant for a given frequency.
PCB layout size constraints can be eased considerably with a small ceramic chip antenna compared to a monopole or PIFA. Image used courtesy of Abracon
When printed on FR4 with a dielectric constant of 4.4, a printed monopole will measure ~23 mm in length. However, ceramic chip antennas have significantly higher dielectric constants that allow for much smaller geometries and potentially lower losses at very high frequencies.
Antenna Considerations: Form Factor, Frequency, and Range
Using the Blumio blood pressure monitor as an example, a designer must weigh the considerations of cost and frequency of operation against both the fixed form-factor size of the wearable device and the wireless range between transmitter and receiver.
The Blumio radar-based blood pressuring monitoring development kit, natively wired to measurement equipment, can be designed to include wireless capability. Image used courtesy of Blumio
A design scenario might unfold as follows: a development kit is expanded to include wireless capability, which will communicate with a vitals monitoring station near the patient. Since the device and its receiver are co-located, a designer may select an operational frequency in the MBAN region (2360 GHz to 2400 GHz). Due to the higher frequency of operation, multiple antenna structures become viable contenders.
There is a cost tradeoff to be made between the non-recurring engineering (NRE) cost to develop a well-tuned PIFA and the large-scale production cost associated with the additional passive components required for a matching network and the chip antenna.
Delving deeper into this conversation, the designer also needs to address the end market. A larger production cost may be tolerated on a smaller run—say, a medically-oriented product family (pro: chip antenna); however, a commercial product family intended to be sold in millions of units would benefit from a smaller bill of materials (pro: PIFA).
Final Thoughts on Competing Antenna Parameters
Chip antennas, like planar PCB antennas, have pros and cons that affect the design process. There are considerations beyond the RF parameters of selectivity, sensitivity, efficiency, and performance to take into account as well.
Some of these considerations include the design houses’ expertise in RF, the application production volume, and antenna geometry limitations due to form factors, among others.
Choosing an antenna structure for minuscule medical IoT designs can be a challenging process. If you’ve recently worked on such a project, what choices did you consider? Let us know in the comments below.