The demand for wearable electronics and related technologies have grown tremendously in recent years. Some of the key developments that accelerated this growth are miniaturization of wireless devices, advent of high-speed wireless networks, availability of ultra-compact, low-power SoCs and ever-evolving battery technologies. Wearable electronics find numerous applications these days, and most of these applications use different types of antennas to sense, fetch, and exchange data wirelessly to and from a host device or an IoT gateway.

What is a Wearable Antenna?

Wearable antennas are designed to function while being worn. These antennas are commonly used in wearable wireless communication and bio-medical RF systems. Wearable antennas are used within the context of Wireless Body Area Networks (WBAN). In a WBAN, antenna is the key component that supports wireless communication, which include in-body communication, on-body communication and off-body communication. A WBAN connects sensors, actuators and IoT nodes on human body, or on cloths or under the skin, establishing a wireless communication channel. Wearable antennas can be employed on people of all ages, athletes and patients for a continuous monitoring of vital signs, oxygen level (Oximetry) and stress level, among others.

Wearable Antenna Applications

The advent of high efficiency miniature antennas is greatly enabling invasive/non-invasive devices in consumer, healthcare and several military applications. A few examples of consumer-bound wearable devices that use wearable antennas are smartwatches (integrated Bluetooth Antennas), smart glasses (integrated Wi-Fi, GPS and IR Antennas), Body worn action cameras (Wi-Fi and Bluetooth), and small sensor devices in sports shoes (Wi-Fi / Bluetooth) that can be paired with smartphones.

A WBAN device ensures continuous health monitoring of an elderly person or a patient without hindering his day-to-day activities. The implantable antenna sensors are also used for several biomedical applications such as heart pacemakers, cochlear implants and intraocular implants among others. In military, wearable antennas find several applications such as soldier’s live-location tracking, real-time transmission of image and video for instant decentralized communications, etc. These antennas are also used for access / identity management, navigation, RFID applications, etc.

Wearable Antenna Technologies

Compact antennas are an integrable part of wearable devices. Antennas are implemented based on the bandwidth requirements, efficiency, electrical performance, polarization effects, size and and application of the wearable device. Some of the commonly used antenna technologies include microstrip antennas, printed dipole, monopole, printed loops, slot antennas, and planar inverted-Fs (PIFAs) antennas.

Microstrip Antennas

Microstrip antennas are metallic strip or patch mounted on a substrate. Microstrip antennas are simple and inexpensive to design and manufacture due to its 2-dimensional structure. They are easy to fabricate using modern printed circuit technology. Microstrip antennas are of low profile and conformable to planar & non-planar surfaces. These antennas allow linear & circular polarization. These antennas can be easily mounted on rigid surfaces and are available in several forms such as rectangle, square, circle, triangular, elliptical patterns. Most GPS devices use a Microstrip / patch Antenna.

Printed Dipole Antennas

Printed Dipole Antennas are popular due to its low profile, ease of fabrication, low-cost, polarisation purity and wide frequency band coverage. Other major advantages of this antenna are its structure (two arms printed on two sides of a dielectric substrate), large bandwidth and the single-ended microstrip input. Dipole Antennas are relatively large in size, which makes it a little complex to implement in applications with space restrictions. In addition, the degradation of omnidirectional radiation patterns and the likely need of a balun may pose challenges in small form-factor designs. Printed Dipole Antennas are widely used in wireless communication and mmWave applications.

Monopole Antennas

Monopole Antennas are half the size of dipole antenna and are mostly mounted above a ground plane. Due to its relatively smaller size, Monopole Antennas are ideal for applications where a smaller antenna design is required. Monopole Antennas exhibit good radiation performance if they are placed over High Impedance Surfaces (HIS). Monopole antennas are of low-profile, low-cost, and easy to fabricate, which meets the basic requirements for wearable antennas. The simple, lightweight structure of Monopole Antennas make them ideal to integrate into clothes.

Printed Loop Antennas

The Printed Loop Antenna is made of single or multiple loops, in the shape of a circle, square or any other closed geometric shape. The Loop Antenna has a dimension less than a wavelength, which ensures the current throughout the loop remains in phase. These antennas are light in weight and has a simple, compact structure. The Loop Antennas have relatively poor efficiency (very low value of radiation resistance), which results in power loss in the form of heat due to the flow of high current. Two distinct Loop Antennas are available – Large Loop Antennas and Small Loop Antennas. The Large Loop Antennas are used for both transmission and reception whereas the Small Loop Antennas are majorly used for reception. These antennas are ideal for small radio devices, and body worn communication systems suitable for military applications.

Slot Antennas

The Slot Antenna consists of a flat metal surface with fine narrow slots. The Slot Antennas are very versatile and are used typically at frequencies between 300 MHz and 24 GHz. This antenna has omnidirectional radiation patterns and a linear polarization. The slot size (length and width), shape and material characteristics, determine the operating characteristics of the antenna. The simple structure and flexible nature make it suitable for small form-factor wearable applications. Since the antenna can be easily implemented on flexible surfaces like denim, it is ideal for medical and military applications. This antenna provides effective wireless data transmission even when the human posture is changed.

Planar Inverted-F Antennas (PIFA)

The Planar Inverted-F antenna (PIFA) finds majority of applications in portable smart devices. These antennas resemble an inverted ‘F’, as the names indicates. The low profile and omnidirectional pattern make the antenna popular among wearable product developers. PIFAs can also be printed like microstrips, a technology that allows antennas to be printed on the substrate or circuit board. The Planar Inverted-F Antennas have compact size, dual-band functionality, and very good on-body results (good SAR values), making it suitable for body worn electronics devices.

Impact of Human body on Wearable Antenna and Vice-versa

In WBAN, the close proximity of human body poses significant challenges to the wearable antennas and vice-versa.

• Impact of electromagnetic radiations on human body and
• The reduced efficiency of the antenna due to electromagnetic immersion in body tissue, fragmentation of radiation pattern, impedance variations and frequency detuning.

These factors call for special attention during antenna design for wearable devices. Developers should focus on structural deformation, accuracy and precision in antenna fabrication methods and size during wearable antenna design.

Effects of Antenna on Human Body

Unlike ionizing radiations, the non-ionizing radiations such as microwaves, visible light or sound waves may not have sufficient energy to ionize atoms or molecules in a body, however this energy can increase the cell temperature by moving atoms or make them vibrate. This rise in temperature due to dielectric heating, a thermal effect due to microwave radiation when a dielectric material is heated by rotations of polar molecules induced by the electromagnetic field, may have severe effects in human tissues.

The Federal Communication Commission (FCC) introduced Specific Absorption Rate (SAR) limits for wireless devices to ensure acceptable radiations level in human body. The SAR limit is set to 1.6 W/kg averaged over 1g of actual tissue, while the limit is set to 2W/kg averaged over 10g of actual tissue by the Council of European Union. SAR is a parameter that is used to measure the rate at which RF (radiofrequency) energy is absorbed by human tissues. SAR values ensure that any wearable device or wireless smart gadget does not exceed the maximum permissible exposure levels.

Antennas without a ground plane exhibit higher SAR value since the SAR of on‐body antennas relies on near‐field coupling to the body. Hence, many of the methods to reduce SAR value rely on altering the ground plane. One of the techniques is to use Electromagnetic Bandgap (EBG) structures, or Periodic Conductive Structures to filter electromagnetic waves within certain frequency bands. Similarly, using High Impedance Surfaces (HIS) help block electromagnetic waves within a certain frequency band. High Impedance Surfaces placed behind wearable antennas increase the front-to-back radiation ratio reducing the Specific Absorption Rate (SAR) in a human body. HIS also prevents propagating surface waves and reflects electromagnetic waves with no phase reversal. Another effective method is to integrate Artificial Magnetic Conductor (AMC) ground plane, which serves as an isolator. The SAR reduction techniques such as integration of Ferrite Sheets and Metamaterials are also popular among antenna designers.

Effect of Human Body on Wearable Antenna

The human body also has some effects on wearable antenna when it is in close proximity. The lossy, high dielectric constant characteristics of human body may result in the variation of input impedance, frequency shifts and reduced efficiency of Antenna. It disturbs the communication link between antenna and the external host device.

Based on the application, various techniques can be adopted to address the effect of human body on antenna. One of the key aspects is the placement and orientation of antenna. An ideal position/orientation of the antenna, location and distance from the body significantly reduce the impact of human body on antennas. For high performance devices, automatic tunable circuits and reconfigurable antennas can also be implemented. Antenna designers also implement EBG ground plane and High Impedance Surfaces to address the impact of body on wearable antennas.

Conclusion

Wearable Antenna is among the key emerging technologies, aiding several applications in health care, military, navigation, and entertainment. WBAN technologies, especially wearable antennas provide cost effective solutions for remote sensing and monitoring of several physiological parameters of human body. While considering the advantages of WBAN and wearable antennas, one should also be aware of its impact on a human body. Antenna designers should consider an appropriate RF technology for wearable designs, while ensuring least effect on the efficiency and gain due to electromagnetic immersion in human tissue.

Selecting an appropriate antenna technology, from the numerous models available in the market, for a specific wearable application is a challenging task. Similarly, antenna design is also a complex process requiring high-end simulation tools and experienced RF antenna designers. Mistral’s highly experienced RF team works directly with product developers to evaluate antenna design requirements, antenna characteristics and performance factors. With over 20+ years of experience designing embedded products featuring technologies such as Bluetooth, RFID, NFC and LoRa to multiband GSM, 3G, 4G/LTE, Wi-Fi and UWB systems, Mistral has the expertise to provide custom antenna designs of any complexity that cater to myriad product needs.