Hanne Herssens, Wout Joseph, Arno Thielens
This manuscript presents a survey of the existing literature on on-body antenna arrays. They can be used to enable wireless communication to and from different on-body nodes in a wireless body area network (WBAN) and are also used in medical applications. The design process and the required characteristics of on-body antenna arrays are discussed. On-body antenna arrays should be small, lightweight, and low profile to ensure user comfort. The different types of on-body antenna arrays are discussed as well. On-body antenna arrays differ in the type of antenna element and in the geometric configuration of the array. The gain normalized to the area for the antenna arrays is compared, and it is found that, in the current state of the art, this quantity reduces with increasing frequency from 36 to 0.7 dBi. From our literature review, we draw important lessons that can be used to develop future on-body antenna arrays.
Editor’s Note
This article for the “Bioelectromagnetics” column provides an extensive review of the literature on antenna arrays for placement upon the human body. The review discusses challenges associated with the design of such antennas, covers their design process and associated performance, and provides several examples with a focus on medical applications. The utmost goal is to provide the background necessary for developing future on-body antenna arrays with a performance that surpasses the state of the art.
This column welcomes articles on biomedical applications of electromagnetics, antennas, and propagation in terms of research, education, outreach, and more. If you are interested in contributing, please e-mail me at kiourti.1@osu.edu.
On-body antenna arrays are collections of interconnected on-body antennas. Together, these antennas collectively transmit or receive radio-frequency electromagnetic fields (RF-EMFs) with the aim to communicate data wirelessly, perform microwave imaging, or achieve targeted dielectric heating. On-body antennas have different characteristics, depending on their application, and a wide variety of such antennas exists. They consist of different materials and operate at different frequencies, etc. Several reviews have been made about on-body antennas [1], [2], [3], focusing on different types of on-body antennas. The review in [1] focuses on wearable antennas at 60 GHz, the review in [2] focuses on ultrawideband wearable antennas, and the review in [3] focuses on EM bandgap integrated wearable antennas. None of these prior reviews focuses on on-body antenna arrays, so there is currently no single source in the literature that can be used by antenna designers to obtain the specifications and properties of such arrays. This is the first time a review of on-body antenna arrays is presented, according to the authors’ knowledge.
A typical application of on-body antenna arrays is in so-called WBANs. WBANs have received much attention because of their applications in health care, the military, and sports [4], [5]. These networks connect wireless on-body nodes and can be used, e.g., for continuously monitoring a patient’s health remotely. Motion sensors can also be used to track movement in military and sports applications, e.g., for observing and improving the training of an athlete. A key part of these WBANs is the on-body antennas that enable wireless communication between different on-body nodes.
Therefore, in this article, a systematic review of on-body antenna arrays is presented. The goal of this review is to provide an overview of the existing on-body antenna arrays for WBANs in the literature and discuss future improvements, new designs, and lessons learned. Additionally, some medical applications of on-body antenna arrays will be discussed. The novelty of this review lies in the fact that it focuses solely on on-body antenna arrays in comparison to preexisting reviews, which only focus on some specific on-body antenna types. The outline is as follows. The section “Design Requirements” describes general design requirements for on-body antenna arrays. The section “On-Body Antenna Arrays” presents the results of our review of on-body antenna arrays intended for RF communication. The section “RF-EMF Exposure” considers the RF-EMF exposure of the user by on-body antenna arrays. Section S.3 of the supplementary material discusses the design aspects of antenna arrays for medical applications like microwave imaging and hyperthermia treatment. The section “Lessons Learned and Future Research” provides an outlook on future research on this topic, and the final section contains the conclusions.
This section will outline the main considerations that have to be made when designing an on-body antenna and how they translate into requirements for wearable antenna array design. This section focuses on the design requirements of arrays intended for wireless communication. On-body antenna array design requires optimizing or finding a tradeoff between a set of antenna (array) parameters. The most commonly used parameters are illustrated in Figure 1(a) and discussed in Section S.1 of the supplementary materials. In general, body-worn antennas should be small, lightweight, and low profile [1], [2], [3]. However, the dimensions of an antenna are not freely chosen since they are related to the operating frequency. A higher operating frequency means smaller dimensions; therefore, most on-body antennas operate at microwave and millimeter-wave (mm-wave) frequencies, i.e., between 0.9 and 77 GHz [6], [7]. A lightweight antenna will ensure the user’s comfort. The human body will absorb part of the radiated EM energy by the on-body antenna. Ideally, an on-body antenna radiates while minimizing this exposure. Using an on-body antenna with a ground plane can reduce the backward radiation and consequently reduce the exposure [8].
Figure 1. (a) Illustration of an antenna array in the yz-plane with the beam direction along ${\theta}_{0}$ and ${\phi}_{0}{.}$ The rectangles represent individual antenna elements. (b) Different communication links in a WBAN.
When an antenna is placed close to or on a human body, the antenna parameters will change because of the coupling between the human body and the antenna [9], [10], [11]. The relatively high permittivity and conductivity of the human body [12] can lead to a shift in resonance frequency and result in an impedance mismatch. Therefore, it is necessary to retune the antenna after designing it in free space. For this reason, antennas with wide bandwidths or very high isolations to surrounding dielectric materials are preferred. This isolation can be obtained by introducing a ground plane.
A body-worn antenna will be used to communicate with other antennas inside, on the body, or off the body through various wireless channels [13]. The lossy nature of the human body means that high propagation losses can occur in these channels [14]. To mitigate these losses, antennas with a high gain in these on-body channels can be used. An option to increase the channel gain is to use antenna arrays. An increasing number of antenna elements will lead to a higher antenna gain (see the section “Comparison of On-Body Antenna Array Parameters”) but also to a larger array caused by the physical area occupied by the individual antenna elements and the minimum separation distance between the elements, which is required to achieve decoupling between the elements. The minimum separation distance is related to the operating frequency. A higher frequency will result in a smaller separation distance. This increase in area decreases the wearability of the array. Therefore, a tradeoff needs to be made between the size and the gain of the antenna array. The separation distance of the antenna elements will affect the exposure of the user. Choosing an interelement spacing of ${>}{0}{.}{5}{\lambda}$ will result in grating lobes and will consequently increase the exposure [8].
Single on-body antennas are faced with two disadvantages that can be mitigated by using an array: 1) a single antenna has a fixed radiation pattern, and 2) single on-body antennas have limited gain. Antenna arrays provide a straightforward solution to both issues: 1) by steering multiple antenna elements, an array can have a dynamic radiation pattern, and 2) by combining multiple antenna elements, a higher gain can be obtained. The direction and amplitude of the radiation pattern of an on-body antenna depend on the amplitudes and the relative phases between the elements. Feeding of these elements requires a specific design of a feeding network, e.g., using (microstrip) transmission lines, phase shifters, splitters and combiners, and amplifiers. The design process of such an array consists thus of not only designing the antenna element and the feeding network, but also of designing the array system, i.e., the interelement spacing and orientation, such that radiation patterns can be formed in the desired direction(s).
The choice of fabrication technique and used materials will influence the design of the on-body antenna array. Conformal and flexible antennas on the one hand and rigid antennas on the other require different materials and fabrication techniques. Using textile materials, it is possible to make the antenna flexible and easy to integrate into clothes. This type of on-body antenna array is further discussed in the section “Textile Antenna Arrays.” On-body arrays can also be realized by inkjet printing [15], [16] and screen printing [7], [17] the radiating elements on a thin, flexible substrate. This results in very thin and flexible antenna arrays. A screen-printed layer will have a thickness between 10 and 20 µm, and an inkjet-printed layer will have a thickness of 0.5 µm [15]. They can be printed on plastic or paper substrates of 100–200-µm thickness to achieve a flexible stack. These thinner layers will result in fewer cracks in the metal layer when the antennas are bent.
The required radiation characteristics, i.e., the direction of maximum radiation, channel excitation, and polarization, depend on the application for which the antenna array is used.
The three possible communication links in a WBAN are 1) on-body to on-body, 2) on-body to intrabody, and 3) on-body to off-body links. Each of these links uses a different channel [Figure 1(b)]. Therefore, they each have different requirements for the antenna. The antenna arrays used for the on-body-to-intrabody link are out of scope in this review since we focus here on on-body antenna arrays for on- and off-body communication, and the requirements for these antennas differ a lot from the requirements of the antennas used in the other two links [18]. Note that the on-body-to-intrabody link here refers to the wireless communication link and does not refer to the links used for medical applications (discussed in Section S.3 of the supplementary material).
For on-body communication, different on-body devices will communicate with each other [Figure 1(b), in red]. The body surface is used as a communication channel, which can result in a higher attenuation than free space. The propagation during on-body communication is a combination of surface waves and space waves [19]. Surface waves propagate along the interface between air and the body, and space waves propagate in the air surrounding the body. The propagation loss or attenuation of free space waves does not depend on the polarization of the waves. However, surface waves do have a polarization dependency [19]. As an example of this polarization dependency, the lowest attenuation of surface waves for the 2.45-GHz Industry, Science, Medicine (ISM) band is found for a perpendicular polarization to the body surface [10], [20]. In practice, a perpendicular polarization will mean antennas with a high profile [10], [11], [20], [21], which are not comfortable for the user. Additionally, the maximum radiation needs to be along the body surface, and the radiation away and toward the body needs to be minimized.
For off-body communication, the on-body antennas communicate with off-body devices, e.g., via data sent to an external device. In this case, the antenna array will need to have its maximum radiation away from the user as opposed to along the body surface, which was the case in on-body communication. However, it should be noted that in the case of a colocated antenna array on one side of the body, the external antenna can be shielded by the body. In this case, the radiation still needs to go around the body surface.
The high gains required for body-centric communication can be obtained by an antenna array consisting of multiple antenna elements [22]. Usually, the antenna elements are identical. The total field emitted by an array depends on the interelement spacing, the excitation amplitude and phase of the individual elements, and the geometrical configuration of the array. Figure 2 shows common types of antenna arrays: a uniform linear array, uniform rectangular array, uniform circular array, and a distributed antenna array, where the elements are distributed at convenient on-body locations. On-body antenna arrays can also be classified according to their on-body deployment, rather than their geometrical configuration. A (circular) array can be placed in different ways on a human body. The array can be placed in a plane parallel to the body on a specific part of the body (colocated), it can be wrapped around the body, e.g., around the waist or a limb (wrapped), or it can be spread over the entire body or torso (distributed). A linear array can be used to control the beam of the antenna in one dimension, e.g., at elevation angle ${\theta}_{0}$ when the array is placed along the z-axis. A planar, wrapped, or distributed array can be used to control the beam in two dimensions, i.e., ${(}{\theta}_{0},{\phi}_{0}{)}$ [22].
Figure 2. On-body antenna arrays. (a) Linear array. (b) Rectangular array. (c) Circular arrays. (d) Distributed array.
Besides its geometrical configuration and its on-body deployment, an on-body array is also characterized by the type of antenna elements used in the array, the materials used to create the array, or the antenna parameter for which the array was optimized. Therefore, we have subdivided the literature on on-body antenna arrays into the following categories, which are discussed in this section: patch antenna arrays, textile antenna arrays, antenna arrays with improved isolation, parasitic arrays, circular arrays, on-body antenna arrays used for beam steering, and distributed antenna arrays. Section S.2 of the supplementary material discusses other interesting on-body antenna arrays that are not covered in the other categories. Table S.1 (supplementary material) lists these on-body antenna arrays, subdivided based on the targeted communication channels. The array configuration is given together with two important antenna parameters: array dimensions and gain. Note that on-body gain does not imply gain in the direction of the body but indicates the gain when the antenna is placed on the body. The direction of maximal gain varies from one reference to the next.
The patch antenna is the most common type of on-body antenna since this type of antenna has a low profile, which makes it suitable for wearable applications. It consists of a radiating patch placed above a ground plane with a dielectric substrate in between [22]. The ground plane shields the user from the radiation and consequently minimizes the RF-EMF exposure [8]. Additionally, it will make the antenna performance less sensitive to the presence of the human body [23], [24]. Figure 3(a) shows an example of a textile 2 × 1 patch antenna array designed in [25] operating at 2.45 GHz. The maximum radiation of the patch antenna is away from the body if the ground plane is worn touching the body’s surface. This makes it ideal for off-body communication but less ideal for on-body communication. However, patch antennas were also used for on-body communication in [26] and [27].
Figure 3. Textile antenna arrays. (a) A 2 × 1 patch antenna array operating at 2.45 GHz. (Source: [25].) (b) Yagi–Uda antenna operating at 60 GHz. (Source: [61].)
The most popular patch shape is rectangular [8], [16], [23], [25], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51] since there exist approximate formulas [22] for the dimensions of a rectangular patch antenna, which makes their design relatively easy. In [26], a dual polarization 2 × 2 array of circular patch antennas is designed for both on- and off-body communication in a WBAN at 28 GHz.
Besides rectangular and circular patches, there exist more complex patch shapes that are used for on-body antennas. Ring-shaped [52], T-shaped [15], and octagonal [53] patches are chosen to make the antenna element and, consequently, the array smaller. To make the antenna even more compact, slots can be introduced in the radiating elements, which is done in [15], [16], and [25].
In [24], a 4 × 1 antenna array operating at 2.45 GHz consisting of tip-truncated equilateral triangular patch antennas is designed to minimize the coupling between the different antenna elements. The antenna array system is fully integrated into a rescue-worker’s vest. In [54], a circular conformal array consisting of E-shaped patches resonating at 3.4 GHz is designed. The choice of patch shape results in a wider bandwidth in comparison to a rectangular patch. In [55], an 8 × 8 cross-shaped patch antenna array operating at 13.5 GHz is designed for a WBAN. The antenna elements can realize two orthogonal linear polarizations and a circular polarization by exciting different ports. In [56], a hexagonal patch with a slotted-ring antenna array is presented to obtain dual-band operation, i.e., at 2.45 GHz and 3.5 GHz.
Textile antennas are often considered as on-body antennas because of their straightforward integration in clothes. At RFs (0.9–5.8 GHz), conductive textiles are used for the radiating elements and ground planes in combination with a textile substrate [6], [24], [25], [27], [31], [34], [39], [44], [46], [47], [54], [56], [57], [58], [59], [60]. This makes the antenna flexible, which makes it more comfortable for the user to wear. At mm-waves, the radiating elements of the antenna are much smaller, and a higher accuracy is needed to fabricate them. Therefore, a thin and flexible metallic foil was used instead in combination with a textile substrate to make a 2 × 2 patch antenna array [23] for short-range off-body communications and a Yagi–Uda antenna [61] for on-body high-data-rate communications at 60 GHz. The use of a textile substrate still makes the antenna flexible. Textile substrates often have low real and imaginary parts of the dielectric constant, which reduces the losses and improves the impedance bandwidth [52].
When designing a textile antenna, flexibility needs to be considered. The textile can be twisted, folded, and bent. This deformation can affect the antenna performance. In [23], a 2 × 2 patch antenna array is studied under bending and crumpling conditions. It was shown that the antenna array remained matched in the 57–64-GHz band under bending in the H-plane. When the antenna array is bent in the H-plane, the radiating edges of the patches remain unbent. Therefore, the resonance frequency remains unchanged. The free space gain decreased from 9.0 to 7.9 dBi when the bending radius decreased from 15 to 5 mm. The array also remained matched under crumpling conditions, but the radiation pattern was strongly affected. The effect of the deformation on the antenna performance was also studied in [7], [24], [31], and [47]. From these studies, it can be concluded that the antenna performance under various bending conditions can remain unaffected when choosing appropriate materials. Therefore, in most cases, the antenna array can be designed in its planar form.
In [59], the flexibility of the textile is used for beam steering. A planar log-periodic dipole array (LPDA) consisting of 12 dipoles is designed for body-worn applications to operate between 0.9 and 3 GHz. Frequency-dependent beam steering is obtained by bending the dipole array at specific lengths.
For on-body antenna arrays, the dimensions of the array should be as small as possible. A small array area implies small spacing between the array elements and, consequently, a potentially increased coupling. Therefore, the articles listed in Table 1 have focused on improving the isolation such that a smaller spacing than ${\lambda}{/}{2}$ between the elements of the on-body antenna array can be taken. The isolation is improved by introducing C-shaped slots [36], an “8”-shaped stub [52], a parasitic structure [62], a shorted strip and two slits [63], and a line patch, rotating one patch by 90° [56]. In [48], a combination of a defected ground structure and split-ring resonators is used as an isolator to improve the isolation under bending of the array (Figure 4). The mutual coupling varies between –27.1 and –31.3 dB under different bending conditions with an isolator and between –17.9 and –23.2 dB without an isolator.
Figure 4. On-body antenna array with split-ring resonators and a defected ground plane to improve the isolation. (a) Front view. (b) Back view. (Source: [48].)
Table 1. On-Body Antenna Arrays with Improved Isolation.
The obtained isolations and the used spacings for the previous articles are summarized in Table 1. In general, several prior investigations have demonstrated that high antenna isolation can be achieved with a spacing less than ${0}{.}{5}{\lambda}$ in wearable antenna arrays, and [63] suggests that these techniques could also be used on the human body. These results are promising for the development of future, compact on-body antenna arrays.
A parasitic array consists of one or more driven antenna elements and several parasitic antenna elements [22]. The currents in these parasitic elements are induced by mutual coupling with the driven element not connected directly to the source or load. In most antenna arrays, this coupling is avoided since this will degrade the performance of the array. However, in parasitic arrays, mutual coupling is used to improve the directivity and gain of the antenna. These parasitic elements can also be used to direct the beam in a wanted direction. A parasitic array is sometimes not considered an antenna array since it consists of only one driven element. However, we did include a brief discussion of these arrays in our review because they can be important building blocks in certain WBAN applications.
A parasitic array can act as a reflector or as a director. A reflector radiates more power away from the antenna, and a director directs the power in its direction. When the parasitic elements are shorted to a ground plane, they act as a reflector, and when they are not shorted, they act as a director. This is used to beam switch in a (sector) top-loaded monopole antenna array [21] and a parasitic array of patch antennas [7]. Here, the reflectors and directors are fixed.
The reflectors and the directors can also be controlled. This is done for parasitic arrays consisting of dipoles using variable capacitors [6], [64] or p-i-n diodes [57], [65] and with patch antennas using p-i-n diodes [66]. In [6], the beam can be steered in three directions, i.e., 0°, 45°, and 315°. This is illustrated in Figure 5.
Figure 5. Simulated radiation pattern of the beam steering parasitic antenna array designed in [6]. (Source: [6].)
In [61], a Yagi–Uda antenna is designed to operate at 60 GHz for on-body high-data-rate communications [Figure 3(b)]. A Yagi–Uda antenna has a maximum radiation parallel to the body surface when it is placed parallel to the body. This makes it suitable for directive on-body communication.
In [67] and [68], an antenna array of four printed parallel Yagi–Uda antennas and a substrate integrated waveguide (SIW) Yagi–Uda antenna are designed for 60-GHz on-body communication, respectively.
In [20], a parasitic array is designed for on-body communication at 2.45 GHz for body-worn devices. The array consists of a printed monopole with parasitic elements placed parallel to the driven monopole. The parasitic elements are added to enhance the launching of the surface waves. The antenna has a low profile, which is often not the case with vertically polarized antennas, and is also capable of launching a sufficiently strong surface wave.
In general, the current literature on on-body parasitic arrays demonstrates that parasitic elements can be used to enable (on-body) beam steering and enhance the on-body channel gain. This approach can be beneficial in comparison to using multielement active arrays in terms of system complexity and energy consumption.
The human body will influence the radiation pattern of an antenna. A single antenna placed on the front of the body will have poor backward radiation because of the shielding of the human body. This can be mitigated by placing another antenna on the back of the body. This is investigated in [58]. A circular array can thus be used to achieve an omnidirectional radiation pattern, which is desirable for both on-body and off-body communication [27], [37], [44], [58], [69], [70].
In [37], a circular array consisting of patch antennas is designed to operate at 5 GHz for long-time health monitoring. The performance of the circular array is investigated with single, four, eight, and 16 patch antennas distributed uniformly across a cylinder that represents the human trunk. Again it is seen that the human body hinders the propagation of the EMFs. In [70], a cylindrical array of dipole antennas is designed to operate at 5 GHz. The beampattern is investigated as a function of circular arrays present in the cylindrical array.
In [27], a circular array consisting of patch antennas resonating at 5.8 GHz is designed (Figure 6). It is investigated how to achieve omnidirectionality and avoid radiation nulls with a circular array. This is also investigated in [44].
Figure 6. Circular antenna array operating at 5.8 GHz. (Source: [27].)
Only a few on-body circular antenna arrays are mentioned in the literature (six circular arrays versus 54 linear/rectangular arrays), despite their potential. We believe that circular arrays have a substantial potential since they can be used to achieve an omnidirectional radiation pattern, and they can be integrated into a belt or a wristband, which simplifies rooting between the antennas.
An antenna array can be used to steer a beam in a desired direction [22]. This is useful in both on-body and off-body communication. To obtain a beam in the desired direction, the fields of the separate elements need to interfere constructively in that direction and destructively in the other directions. Since a user is mobile, the direction in which on-body or external nodes will be found constantly changes. Therefore, it is useful that the direction of the beam can be adapted. This is commonly referred to as adaptive beamforming.
The direction of the beam depends on the phase difference between the different antenna elements in the array. This phase difference can be fixed, e.g., by the length of the feed lines, or it can be changed by using one port per antenna element. This is not ideal for an on-body antenna since it will increase the size of the array. As mentioned earlier, parasitic elements can also be used to steer the beam depending on whether they act as a reflector or a director. This has the advantage of requiring only one port instead of multiple ones. The reflectors and directors can be fixed [7], [20], [21], [61], [67], [68] or controlled by using, e.g., varactor diodes [6], [57], [64], [65], [66], [71].
Distributed on-body antenna arrays are arrays in which the antennas are distributed over the body in convenient places, not in a particular geometric configuration [Figure 2(d)]. These antenna arrays are often not explicitly designed for improving the gain. Therefore, these on-body arrays are not considered in the analysis of the next section, “Comparison of On-Body Antenna Array Parameters.” The positioning of these antenna elements depends on the application of the antenna array, e.g., as a personal distributed exposimeter [72], [73], [74] and for investigating (dynamic) channel models [75], [76], [77], [78].
Figure 7(a) shows an overview of the obtained maximum antenna gain in free space as a function of the area of the antenna array for the different on-body antenna arrays. The distributed antenna arrays were not considered since the area for these would correspond to the human body area, and the array is usually not used to increase the gain. If only the dimensions of a single antenna element were given, a spacing between the antenna elements of ${0}{.}{5}{\lambda}$ was assumed, and approximate dimensions of the array were used based on the values given in the articles. It was also assumed that the gain values were given in decibels relative to isotrope. The smallest fabricated array (see the blue triangles) is the SIW Yagi–Uda antenna designed in [68], operating at 60 GHz with an area of 1.0672 cm2, and the largest is the 4 × 1 patch antenna array designed in [24], operating at 2.45 GHz with an area of 864 cm2. From Figure 7(a), we can conclude that the measured gains are situated between 0.42 dBi (a 2 × 1 patch antenna array [45]) and 15.7 dBi (the 4 × 1 printed Yagi–Uda antennas [67]) for the fabricated antenna arrays. Figure 7(a) also shows higher gains, i.e., 16.04 dBi (a 4 × 4 patch antenna array [35]) and 22.8 dBi (an 8 × 8 patch antenna array [55]), but these gains are only achieved with simulations and not verified with measurements.
Figure 7. (a) Free space gain as a function of area. (b) Free space gain as a function of number of elements. (c) Normalized free space gain as a function of frequency. A: area; N: number of elements.
When designing an on-body antenna array, there will be a tradeoff between the size of the array and the obtained gain. More elements will increase the gain but will also increase the size of the array. This can be seen in Figure 7(b). A good on-body antenna array will therefore be small in size with an acceptable gain. The antenna array consisting of 64 patches [55] shows the highest gain, i.e., 22.8 dBi. The lowest gain, i.e., 0.42 dBi, is found for the 2 × 1 patch antenna array presented in [45]. For two, four, eight, and 12 elements, the minimum obtained gain is 0.42 dBi (2 × 1 patches [45]), 2.1 dBi (2 × 2 patches [26]), 0.5 dBi (circular array of dipoles [79]), and 4 dBi (a planar LPDA [59]), respectively. For two, four, eight, and 12 elements, the maximum obtained gain is 11.45 dBi (2 × 1 patches [25]), 14.8 dBi (4 × 1 patches [24]), 14.8 dBi (4 × 2 patches [33]), and 12.2 dBi (a parasitic array of dipoles [65]), respectively.
As mentioned before, a good on-body antenna array has a high gain but is small in size. Therefore, a higher value of Gain/A will lead to a better on-body antenna array. However, it is difficult to compare the areas of arrays operating at different frequencies since a higher frequency will result in smaller antenna elements, and, consequently, more elements can be placed in the same area or a smaller antenna array is obtained using the same number of elements. A solution to this is to use the normalized area, i.e., ${A}_{\text{norm}} = {A}{/}{\lambda}^{2}{.}$ A good on-body antenna array should be small relative to its wavelength and have a high gain. Figure 7(c) shows the normalized gain as a function of frequency, with ${Gain}_{\text{norm}} = {Gain}{/}{A}_{\text{norm}}{.}$ Almost all measured normalized gains are less than 10 dBi. There are only two arrays with a measured normalized gain above 10 dBi. The first array is the patch antenna array designed in [52]. An “8”-shaped stub was placed between the antenna elements to improve the isolation. As a result, the elements could be placed closer together, which leads to a smaller area and a higher normalized gain. The second array is the parasitic array consisting of sector top-loaded monopoles presented in [21]. The sector top loading of the monopoles reduces the ground plane size, which results in a smaller area. However, this also increases the height of the array, which is not practical. The other normalized gains above 10 dBi are simulation results and therefore represent ideal cases. From Figure 7(c), we can conclude that the normalized gain reduces with increasing frequency from 36 to 0.7 dBi. This trend is shown by the black curve. There is a research opportunity to develop on-body arrays in mm-wave frequencies with a normalized gain comparable to that currently found at lower RF frequencies.
The human body will absorb part of the radiated EM energy by the on-body antenna array. The absorption depends on the antenna design and the frequency of the EM waves; e.g., a higher frequency generally results in a more localized absorption because of the smaller penetration depth. The exposure can be quantified in terms of the specific absorption rate (SAR). Table 2 lists the peak SARs averaged over 1 and 10 g for the on-body antenna arrays studied in this review that have listed such values. These SAR values scale linearly with the input power into an antenna array and therefore put a limit on the maximal power that can be emitted by the arrays. The local SAR limit (averaged over 10 g) for the general public is 2 W/kg for frequencies below 6 GHz [80]. The on-body antenna arrays designed in [8], [30], and [33] operate at 60 GHz and will result in a more localized absorption. Therefore, the quantity used to describe the exposure in these articles is the peak power density averaged over 1 and 20 cm2. The exposure limits for the general public for these quantities are 200 W/m2 and 10 W/m2 for the power density averaged over 1 cm2 and 20 cm2, respectively. Note that for frequencies above 6 GHz, exposure should be described in terms of the absorbed power density $({S}_{ab})$ averaged over 4 cm2 and additionally for frequencies above 30 GHz averaged over 1 cm2 according to current exposure guidelines defined by the International Commission on Non-Ionizing Radiation Protection [80].
Table 2. Peak SAR averaged over 1 and 10 g and peak power density averaged over 1 and 20 cm2 of the on-body antenna arrays.
There exists a variety of on-body antenna arrays in the literature. These on-body antenna arrays can differ in the type of antenna element, e.g., patch antennas and slot antennas. They can also differ in the geometric configuration of the array, e.g., a linear array or rectangular array. These on-body antenna arrays have different characteristics, but there are general considerations that apply to all on-body antenna arrays. The antenna arrays need to be compact since they are body worn. The characteristics depend on the communication link in the WBAN, i.e., on-body communication or off-body communication. The on-body antenna arrays designed for off-body communication had their maximum radiation away from the body to communicate with an external device. This differs from the on-body antenna arrays designed for on-body communication. On-body communication makes use of surface waves. Since it is known that a monopole effectively launches these surface waves in the 2.45 GHz ISM band, several articles have tried to obtain a monopole-like radiation with their antenna arrays since a monopole is not practical because of its high profile. One way to do this is to still use monopoles but to top-load them with a disk to reduce the height [21]. Another way to do this is to use dipoles in a colocated circular array [79], [82]. The most common on-body antenna array is a patch antenna array. Most of the patch antenna arrays are designed for off-body communication as the antennas have their maximum radiation away from the body.
A higher frequency resulted in a lower normalized gain for the antenna arrays found in the literature. Future research can include designing arrays at mm-waves with the comparable normalized gains that are found at lower frequencies.
An interesting on-body antenna array is a circular array wrapped around a limb or the waist. These are useful since the human body can shield different antenna elements from each other. The circular arrays can thus be used to achieve an omnidirectional radiation pattern. They are also practical since they can be integrated into a belt or a wristband. Only a few of these circular arrays are mentioned in the literature (six circular arrays versus 54 linear/rectangular arrays), despite their potential. This type of array could potentially be used to achieve uniform on-body coverage for situations where the on-body nodes in a WBAN are spread over the entire body. This requires further research.
An overview of the existing on-body antenna arrays, their performance, and their potential in the literature has been presented. This has not been done until now. These antenna arrays operate at frequencies between 0.9 and 77 GHz. The design process of the on-body antenna arrays is discussed as well as the required characteristics.
The performance of the on-body antenna arrays is discussed and compared. The antenna arrays have an area between 1.07 and 864 cm2 for frequencies between 0.9 and 77 GHz. The measured gains are situated between 1.5 and 15.7 dBi. The highest obtained gain with simulations is 22.8 dBi, but this has not been verified with measurements. More antenna elements will lead to a higher gain but also to an increase in area of the array. An antenna array consisting of 64 patch antennas has the highest gain, i.e., 22.8 dBi. The lowest gain, 0.42 dBi, is found for a 2 × 1 patch antenna array. The normalized gain was also discussed since it is difficult to compare antenna array areas for different operating frequencies. It was found that the normalized gain reduces with increasing frequency. Additionally, it was found that almost all measured normalized gains are less than 10 dBi.
Two medical applications, i.e., microwave imaging and hyperthermia treatment, of on-body antenna arrays have been discussed. The general design requirements, i.e., compactness and a low profile, also apply since they are wearable. Additionally, a tradeoff needs to be made between the penetration depth of the EMFs and the resolution of the image (for microwave imaging) or the size of the focus spot (for hyperthermia treatment).
This article has supplementary downloadable material available at https://doi.org/10.1109/MAP.2023.3262144, provided by the authors.
Arno Thielens is a postdoctoral fellow of the Research Foundation Flanders under Grant agreement 1283921N.
Hanne Herssens (hanne.herssens@ugent.be) is with the Department of Information Technology, Ghent University-IMEC, 9052 Ghent, Belgium. She is a Ph.D. student at the Waves Research Group, Ghent University. Her research interests include electromagnetic exposure, on-body antennas, and body-centric wireless communication.
Wout Joseph (wout.joseph@ugent.be) is with Ghent University-IMEC, 9052 Ghent, Belgium. He is a professor in experimental characterization of wireless communication systems and a principal investigator. His interests are electromagnetic field exposure assessment and propagation. He is a Senior Member of IEEE.
Arno Thielens (arno.thielens@ugent.be) is with the Department of Information Technology, Ghent University-IMEC, 9052 Ghent, Belgium. He is a professor at Ghent University and a senior postdoctoral fellow of the Research Foundation Flanders. His research focuses on bioelectromagnetics. He is a Member of IEEE.
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Digital Object Identifier 10.1109/MAP.2023.3262144