Fikadu T. Dagefu, Jihun Choi, Brian M. Sadler, Kamal Sarabandi
©SHUTTERSTOCK.COM/CHESKY
Robust, secure, low-power wireless communications and networking are needed to support emerging applications in emergency and disaster response, robotics, autonomous vehicles, and the Internet of Things (IoT) in physically complex environments, such as indoor/outdoor, dense urban, and other challenging scenarios. These applications require a variety of wireless communications modalities, including high and very-high frequency (HF and VHF) bands. To effectively deploy compact wireless communications systems at these low frequencies, high-performance electrically small antennas (ESAs) are needed. In this article, we present a survey of state-of-the-art low-frequency ESAs, including passive resonant, tunable, multiband, and actively matched designs. While passive designs are subject to classic tradeoffs in size and efficiency, active and platform-integrated techniques offer potential performance-enhancing alternatives. And, though single ESAs tend to omnidirectional beam patterns, multiantenna arrays can achieve directionality, and designs include parasitic, biomimetic, and distributed approaches.
Global trends in robotics, driverless vehicles, and the IoT come with the need for highly reliable and persistent wireless communications. Cellular infrastructure upgrades with 5G (and already-envisioned 6G) are motivating new lines of investigation to address this need. Applications include vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) networking, sensor networks, and incorporating unmanned aerial vehicles (UAVs) and autonomous robotic teams. Beyond fixed infrastructure, ad hoc wireless networks are necessary for emergency and disaster response as well as military operations, with challenges including topology tracking and network management, indoor/outdoor operations, sensing and situational awareness, control, and human–autonomy teaming. These applications and the ever-increasing demand for efficiently exploiting different parts of the scarce electromagnetic (EM) spectrum have led to recent exploration of new ways to use HF/VHF bands. However, the design and use of efficient ESAs at these frequencies is very challenging (see “Low-Frequency Electrically Small Antennas: Practical Challenges and Limitations”). In this article, we survey the state of the art and highlight recent low-frequency ESA designs. We begin with a discussion of low-frequency wireless networking, practical challenges, and a brief overview of this survey.
Ensuring network reliability is very challenging in complex environments, due to increased channel complexity caused by various wave propagation mechanisms, such as reflection, diffraction, multiple scattering, and surface waves, in the case of near-ground antennas. The dominant propagation mechanisms vary dramatically with the wavelength and environment, including the size, density, and dielectric properties of scatterers. In the ultra-high frequency (UHF) and microwave bands, these effects result in multipath signal fading and phase distortion that are highly dynamic with radio and scatterer mobility [1].
In comparison to higher frequencies, the upper-HF and lower-VHF bands have more favorable propagation characteristics due to improved penetration through obstacles and structures, and they have less multipath fading [1], [2], [3], [4], [5]. The small size of typical scatterers relative to meters-long upper-HF and lower-VHF wavelengths minimizes scattering. For example, short-range channels often have a dominant direct path with improved temporal and spatial channel coherence compared to higher frequencies in the same channel. This can be exploited for more robust low-power networking with reduced complexity, which is especially appealing for ad hoc networking in dense urban and other cluttered environments. Low-frequency operation provides persistent connectivity with mobile clients and autonomous robotic agents, including multiuser communications and localization in physically complex, nonline-of-sight scenarios with low-complexity processing [6], [7].
Key challenges for low-frequency wireless applications include environmental noise, EM coupling, and antenna apertures. First, unlike at higher frequencies, man-made and natural noise in the HF and low-VHF bands may be the dominant interference mechanism, potentially more so than thermal noise. The interference spectrum may be nonstationary, non-Gaussian, and impulsive. Fortunately, modern adaptive processing and networking techniques may be called upon to dynamically adjust and optimize network resources. A second fundamental challenge is the EM coupling resulting from interaction with a platform and nearby scatterers. This has resulted in new designs and methods for adaptation (see the “Platform-Integrated ESA” section for examples). A third key challenge is the design of small antennas for power-efficient, low-frequency operation, so they can be readily integrated into handheld devices and small-scale mobile air and ground platforms. This survey highlights recent advances in a variety of small-aperture designs that open avenues for new and emerging applications.
This article is an inclusive survey of low-frequency small antenna designs. We focus on the upper-HF and lower-VHF radio spectrum while also including small antenna architectures that can operate in other bands (that are usually higher and may include upper-VHF and UHF ranges). We classify state-of-the-art small antennas in terms of their functionality and design approach. Low-frequency antenna design balances tradeoffs in size, bandwidth, efficiency, and other factors, such as integration with a specific platform or device. We begin with passively matched, narrowband wire- and microstrip-based dipoles, monopoles, and loop antennas. Successive sections describe tunable, multiband and wideband, non-Foster, and reconfigurable arrangements. In each case, we highlight the tradeoffs in size, radiation efficiency, bandwidth, and platform integration.
The designs are grouped and discussed in the sections below. Passive designs can be characterized by the Chu–Wheeler limit [13], [14], and we use this to compare variants in terms of size and efficiency (see “Performance–Size Tradeoffs for Electrically Small Antennas”). We also describe recent progress in active designs that can transcend the fundamental limits of passive antennas. An important foundational study by Sievenpiper et al. provides an extensive comparison of the theoretical limits of small antennas that appeared by 2010 [15]. It broadly validated the Chu–Wheeler approach for ESAs and designs that are not necessarily electrically small.
Primary and comprehensive treatments of small antennas are also given in texts [16], [17]. We generally consider small with respect to wavelength, and specify the size of a design in these terms. The notion of small also relates to function, integration, and constrained dimensions; for a comprehensive overview, see Fujimoto and Morishita [16, Ch. 2]. We mostly limit our survey to a maximum antenna dimension relative to a wavelength ${\lambda}$ of roughly ${\lambda} / {10}$. For example, at 40 MHz, ${\lambda}_{40} = {7.5}{\text{ m}}$, and ${\lambda}_{40} / {10} = {75}{\text{ cm}}$. However, there is a large variety of small antennas and antenna arrays, and we describe various cases without adhering to a strict definition of small. For convenience, we sometimes refer to these as ESAs, again without a strict definition.
There are many recent surveys related to wireless communications, propagation, and signal processing, including millimeter-wave technology [18], antenna array processing and diversity [19], and cognitive radio [20]. A few surveys summarize state-of-the-art antenna and antenna array design trends, including millimeter-wave applications [21], design strategies for reconfigurable antenna systems [22], and physical layer security applications [23].
Low-Frequency Electrically Small Antennas: Practical Challenges and Limitations
The challenges and limitations of low-frequency electrically small antennas (ESAs) include the following:
Figure S1. Outdoor antenna pattern characterization, where the antenna under test (AUT) and reference biconical and dipole antennas are mounted on a fiberglass tower to minimize ground scattering and other multipath effects.
Performance–Size Tradeoffs for Electrically Small Antennas
Passive antenna designs are subject to a fundamental performance bound derived by Wheeler and Chu [13], [14]. This limit describes the fundamental tradeoff in performance and size and provides a way to compare many different designs. The bound establishes, for a linearly polarized electrically small antenna (ESA) that can be fully enclosed within the smallest possible sphere having radius a, an upper limit on the bandwidth–efficiency product. The bound is expressed as follows. First, a lower limit on the quality factor Q for a lossless linear antenna (i.e., with efficiency ${\eta} = {1}$) is given by ${Q}\,{≥}\,{1} / {(}{ka}{)}^{3} + {1} / {ka}$, where ${k}{(} = {2}{\pi} / {\lambda}{)}$ is the free-space wavenumber and ${\lambda}$ is the wavelength.
Then, the relationship between Q and the fractional bandwidth FBW can be utilized to express the fractional bandwidth as ${FBW} = {1} / {Q}{(}{VSWR}{-}{1}{/}\sqrt{VSWR}{)}$. Here, FBW is the matched voltage standing wave ratio fractional bandwidth as derived by Yaghjian and Best [S1]. Based on this, the fractional bandwidth–radiation efficiency product ${(}{FBW}\,{\cdot}\,{\eta}{)}$ can be computed, providing a theoretical upper limit on antenna performance. For many cases of interest, we can compare the achieved ${FBW}\,{\cdot}\,{\eta}$ against the theoretical upper bound expressed previously. Many existing small passive designs are quite close to the limit; see Figure 1. For a comprehensive background on the work of Wheeler and Chu, see Fujimoto and Morishita [16].
Reference
[S1] A. D. Yaghjian and S. R. Best, “Impedance, bandwidth, and Q of antennas,” IEEE Trans. Antennas Propag., vol. 53, no. 4, pp. 1298–1324, 2005, doi: 10.1109/TAP.2005.844443.
In this section, we survey small, passive, narrowband antenna designs, including variations of wire, microstrip, and loop antennas. We then compare their performance in terms of radiation efficiency and bandwidth. These are listed in Table 1. The Chu–Wheeler limit is plotted in Figure 1 (the red, solid curve) along with the bandwidth–efficiency product values for the designs in Table 1. It is notable that many of these passive designs are close to the fundamental tradeoff of performance versus size.
Figure 1. Passive HF and VHF designs compared with the fundamental Chu–Wheeler limit. In addition to the designs in Table 1, some UHF designs are included [31]–, [32], [33], [34], [35][36] because they also have good performance in terms of approaching the Chu–Wheeler limit and are still considered ESAs if their design is scaled to lower frequencies. FBW: fractional bandwidth.
Table 1. Passively matched wire and microstrip designs.
Microstrip-based designs have been studied for several decades. In [11], a microstrip-based ESA devised using cavity perturbation and plate loading techniques is presented for low-VHF applications. The omnidirectional microstrip antenna has a maximum dimension of ${\lambda} / {20}$ and is intended to operate at a center frequency of 41.39 MHz. The volume is 4.8% of a conventional microstrip antenna fabricated at the same frequency, with a measured radiation efficiency of 77.7%. In addition to having the advantage of not requiring a matching network, significant miniaturization is achieved by wrapping the ${\lambda} / {4}$ microstrip line around a cylinder, thereby significantly reducing the antenna profile. In [24], a miniature microstrip VHF antenna design with omnidirectional pattern and vertical polarization providing an efficiency of 1% is proposed. Another miniaturization technique is proposed in [25] by focusing on architectures that use ferrite as a substrate. This requires the development of unique ferrite substrates to improve the bandwidth and efficiency compared to designs that use a ferromagnetic substrate.
Various techniques have also been proposed to significantly decrease the profile of low-frequency monopole- and dipole-type antennas. A disk-loaded folded monopole antenna designed with parallel strip elements is proposed in [26]. The size of this antenna is ${0.1}{ }{\lambda}$ at a center frequency of 389 MHz and a bandwidth of 85 MHz. The measured radiation efficiency varies from 70 to 97% within the bandwidth. A miniaturization technique based on meandered lines to reduce the length of a VHF antenna is presented in [27]. By utilizing a broadband circuit model and simulations, the authors demonstrate that the resonant frequency can be significantly reduced by using a meandered-line geometry as the dipole arms. An optimal number of meandered sections for a fixed dipole antenna profile is proposed, as additional meandering does not significantly lower the resonant frequency.
In [9], a very small form factor is achieved by utilizing multiple vertical elements connected through a T-type, 180º phase shifter that cause the currents in each element to flow in the same direction, resulting in improved radiation efficiency. The phase shifter is realized by using capacitive plates and high-Q coils that further improve the antenna efficiency. The measured gain is 17 dB better than that of a conventional inverted-F antenna of comparable size.
A miniature folded dipole antenna that extends the monopole-based design proposed in [9] is presented in [10]. The overall dimensions of the antenna are ${10}\,{\times}\,{10}\,{\times}\,{15}{\text{ cm}}$ ${(}{0.013}\,{\times}\,{0.013}\,{\times}\,{0.02}{\lambda}$ at 40 MHz), and it weighs 98 g. The measured gain of this antenna is –12.8 dB isotropic (dBi), with a 3-dB bandwidth of 300 kHz. Conventional small antennas with a similar size often have gains well below –20 dBi. In [28], a resistively loaded helical antenna operating in the normal mode is proposed for ground-penetrating radar. By using a distributed resistive loading with a helical geometry, the center frequency of a 1-m dipole was reduced to 50 MHz while maintaining the bandwidth, with a reduction in efficiency of 12 dB. The gain is –22 dBi at the lowest frequency, with the height reduced to 20 cm.
An efficient optimization-based design for nonuniform helical antennas is presented in [12]. In contrast with conventional helical antennas that have a uniform winding radius and pitch, these are jointly optimized to improve the bandwidth. Radial basis functions are used to model the variations of the pitch and radius of the winding, resulting in an efficient design algorithm. Three 25-cm-long helical designs are presented, centered at 25, 50, and 100 MHz. Compared to a conventional uniform helical antenna, the radiation efficiency of the 50-MHz design improved from 60 to 71%, and the bandwidth increased from 65.8 to 76.4 kHz.
A detailed analysis of a multiturn loop ESA is presented in [29], comparing aircore and magnetically loaded versions. The length of the overall antenna is 51 cm, and the diameter of each loop is 16 cm. The intended frequency of operation covers 3–86 MHz. The authors find that the magnetically loaded version has a 10-dB gain improvement for frequencies up to 30 MHz. While the theoretical model presented has advantages in terms of minimal complexity and accuracy, the validity that the claimed enhancement achieved through the approach is limited to a class of small, inefficient antennas, and the technique has not been verified for larger, more efficient ones.
Resonance tunability can also be incorporated into ESA designs. A metamaterial-inspired ESA is proposed for VHF/UHF operation in [30]. A Z-type design is presented, where a parasitic lumped inductor is introduced very close to the monopole antenna feed. The resonance can then be tuned, covering the VHF and UHF bands by changing the value of the inductor and length of the monopole. Unlike other passive designs, this one does not require external matching circuits. As noted earlier, an important challenge is the significant resonant frequency offset that can occur due to near-field coupling with the ground as well as nearby electrically large dielectric and metallic objects. The antenna in [10] has been integrated onto a small ground robotic platform for communications and geolocation experimentation (see Figure 2), at an optimal height of 50 cm above the top surface of the robot, where the resonant frequency offset was found to be small [37], [38]. Resonant frequency and radiation pattern distortion effects due to nearby objects and the coaxial feed cable are also considered in these works.
Figure 2. Examples of small passively matched wire- and microstrip-based antennas. (a) A 40-MHz multielement monopole [9]. (b) A 40-MHz multielement dipole [10]. (c) A perturbed microstrip antenna [11]. (d) Optimized nonuniform helical antennas [12].
As described in the previous section, even the most efficient small passive antennas have a relatively low fractional bandwidth that limits their application. As shown in Figure 1, small passive designs are approaching the fundamental tradeoff between electrical size and performance, and further progress in gain and bandwidth with a small form factor will be limited. To broaden the bandwidth while maintaining the small size and radiation performance, other novel approaches must be pursued. In this section, we summarize bandwidth-enhanced tunable antenna designs. Some platform-integrated designs also focus on bandwidth enhancement, and these are discussed in the “Platform-Integrated ESAs” section.
Tunable antennas are unique in that their instantaneous bandwidth is not necessarily higher than their passive counterpart that employs the same matching approach. However, with electrically tunable antennas, the center frequency can be switched to increase the effective bandwidth. Thus, the goal is to have high radiation efficiency and bandwidth for a given center frequency while also achieving fast switching. The designs described in this section are listed in Table 2.
Table 2. Tunable and platform-integrated HF/VHF designs.
A tunable lower-VHF antenna utilizing a variable capacitor is described in [39]. The proposed antenna, which has a physical dimension that can fit into a 22.75-cm hemisphere, shows performance comparable to a commercial whip antenna. The authors first design a top-loaded, inductively coupled small antenna operating at 39.7 MHz, with a 620-kHz bandwidth (1.6% fractional bandwidth). Then, a variable capacitor integrated at an optimal position in the antenna structure is used for tuning. By varying the capacitor value from 10 to 230 pF, the antenna can be tuned from 40 to 54 MHz without significantly varying the matching and efficiency performance (ranging from 50 to 60%). The measured instantaneous bandwidth was 520 kHz (1.3%) at 40.55 MHz and 560 kHz (1%) at 54.02 MHz.
In [40], a tunable, planar inverted-F antenna (PIFA) is designed to cover 88–2,177 MHz by controlling the capacitance with a varactor across the gap in the slotted PIFA. The length of the antenna was 4 cm, designed over a ground plane spanning ${4}\,{\times}\,{9.5}{\text{ cm}}$. The measured gain of the PIFA at 100 and 470 MHz was –33 and –9.6 dBi, respectively. A miniature self-adjusting loop antenna that tracks the transmit frequency is proposed in [41]. Effectively, the antenna is considered part of the resonant circuit that determines the transmit frequency. This is achieved by utilizing a frequency discriminator integrated onto the antenna that drives a feedback loop to enable automatic tuning. This design is intended to address the shift in the resonant frequency that miniature low-frequency antennas experience due to nearby scatterers. In [42], optimal designs of multiple-tuning circuits are proposed, and examples operating 175 and 300 MHz are presented. Each design described in the paper was shown to have a bandwidth–efficiency product very close to the fundamental limit.
In [43], a composite, low-profile tunable antenna having a lateral dimension of ${\lambda} / {16}$ and a height of ${\lambda} / {150}$ is presented. It consists of nine individual antenna elements arranged in a parallel parasitic configuration to exploit mutual coupling as a way of enhancing its radiation resistance. This, in turn, improves the realized antenna gain. The element design is adapted from [9]. The design also includes a variable capacitor to enable tunability. The resulting composite antenna has a gain enhancement of 10 dB compared to a single-element version and achieves 3.3% fractional bandwidth tunability around a center frequency of 40 MHz.
Miniature antennas are often integrated onto platforms whose metallic and dielectric properties can significantly affect performance. Platform-aware antenna designs have been considered for applications ranging from mobile networking to animal tracking systems. Remarkably, the techniques described here can exploit a platform for improved performance, and so they are very attractive for V2V and V2I civilian and military networking applications. The designs described in this section are listed in Table 2.
In [44], a collar-integrated antenna operating at 154.5 MHz is designed for wildlife tracking, with the goal of providing a ground link in a GPS-based location system. To address the loss in gain that a conformal zigzag antenna experiences, a T-matching technique that uses a shorting pin is employed to improve the impedance matching. The prototype design has a 10-dB bandwidth of 5 MHz (3.2% fractional bandwidth) and 41% efficiency. In [45], a set of antennas operating at 44, 66, and 88 MHz is presented for integration with a helicopter. The designs exploit induced currents on the helicopter airframe to improve the radiation performance and reduce the pattern degradation caused by the rotors. One configuration involves placing small loop antennas on the opposite side of the airframe, which is nominally one wavelength around the fuselage. It is shown that this and similar arrangements provide a reduced gain variation of less than 5 dB, and increased gain over 15 dB compared to a tail-mounted vertical antenna.
In [46], the natural resonant modes of a large mobile platform were excited using ESAs to improve the overall bandwidth of otherwise narrowband HF and VHF antennas. Characteristic mode theory (CMT) is used to evaluate the performance of two different antennas designed for operation on the platform. Specifically, performance metrics, including the maximum antenna bandwidth and its radiation characteristics, are computed using CMT, and an example design for a military ground vehicle is presented. It is shown that the proposed design operating at 60 MHz has more than a fourfold bandwidth enhancement compared to the antenna measured in isolation, with a maximum dimension of ${0.06}\,{\times}\,{\lambda}$. Related approaches were also introduced to realize platform-integrated antennas for applications such as skywave HF and VHF communications [47], [49].
In the designs described previously, the underlying platform is typically an electrically large structure with respect to the frequency of operation. More recently, the applicability of these ideas has been investigated for operation on electrically small platforms, such as small unmanned ground vehicles (UGVs). A platform-based small antenna for lower-VHF operation on a small UGV is presented in Figure 3(c). The antenna is ${0.13}\,{\times}\,{0.09}\,{\times}\,{0.05}{\lambda}$ at 40 MHz and designed to generate a vertically polarized, monopole-like radiation pattern. Simulations of this and similar designs show that the bandwidth of the platform-mounted antenna is more than threefold better than the antenna measured in isolation [48], [50].
Figure 3. Example platform-integrated antennas. (a) A scaled model for a vehicle-mounted HF antenna [49]. (b) The helicopter platform model used in [45] to investigate a design strategy that exploits deliberate antenna–platform interactions to simultaneously improve the radiation characteristics and reduce the rotor modulation effects. (c) A bottom view of a small UGV full-scale prototype with a low-VHF ESA [50]. The design approaches in (a) and (c) rely on characteristic mode analysis and exploit the current distribution on the platform to improve the antenna radiation characteristics. (d) A platform integrated high-gain VHF antenna [51] and (e) its application for reconfigurable distributed robotic beamforming.
Another important technology that is relevant for compact platform-integrated antennas is reactive impedance surfaces (RISs) [52]. RISs can be used as ground planes for small antennas in lieu of metallic conductors, enabling the realization of capacitive and inductive characteristics and achieving a unity magnitude reflection coefficient. Inductive RISs, for example, can be utilized to drastically reduce the resonance frequency of most antenna designs that are inherently capacitive below resonance [53], [54], [55]. Another attractive feature is that RIS designs are often robust and platform agnostic.
In the previous sections, designs were compared with the Chu–Wheeler limit. However, it is possible to exceed the performance tradeoff limits of passive antennas by using active matching with non-Foster circuits. Unlike passive techniques that typically utilize LC circuits for matching and tend to be narrowband, non-Foster circuits enable bandwidth-enhanced operation at the expense of power consumption. These rely on negative impedance converters (NICs) that can be used to realize negative reactive elements [56], thereby enabling the design of matching circuits that operate at higher bandwidths. In this section, we highlight some recent advances in non-Foster designs and describe the potential for enhancing low-frequency ESAs.
Non-Foster elements (i.e., negative capacitors and inductors) have a negative reactance slope as a function of frequency. This enables the cancellation of the high input reactance of an ESA across a wide frequency range by using non-Foster elements as part of the impedance matching network and can go well beyond the limited impedance bandwidth of passive matching networks. A general approach to NIC circuit design involves the use of a cross-coupled bipolar junction transistor (BJT) pairs, where the collector of each BJT is connected to the base of the other transistor [57]. This feedback loop can make the NIC oscillation prone, and achieving circuit stability is a very important aspect of applying it for ESA impedance matching.
Many studies of non-Foster design to date typically begin with a passive unmatched design to which the non-Foster matching circuit is applied, and the two are compared to measure bandwidth enhancement [58], [59], [60], [61]. For example, in comparison with the unmatched case of a passive antenna, an improvement of more than 10 dB in the signal-to-noise ratio across a wide range of frequencies in the lower-VHF band can be achieved by utilizing non-Foster matching circuits with NICs [58], [59]. Depending on the particular design, the power consumption of these circuits can be as low as 20 mW. However, the practical use of existing non-Foster matched designs has been hindered due to their sensitivity to small experimental system variations and nearby metallic objects.
There are several key challenges that require further research, including the design of circuits that enable higher transmit power and achieve stability in practical conditions. Another challenge is related to modeling the steep impedance variation of narrowband resonant antennas and designing appropriate NIC circuits. For approaches that seek to enhance an existing passive baseline design, slow variation of the impedance as a function of frequency makes the design of a non-Foster matching circuit less challenging. This slow variation generally occurs for wideband antennas, which is not the case for many of the designs described in this survey.
The approach in [62] and [63] begins with a small passive design that is close to the Chu–Wheeler limit and then applies non-Foster matching to enhance the bandwidth and gain while maintaining the electrical size. The result is a small and efficient antenna that exceeds the Chu–Wheeler limit for its given size and wavelength. The design is challenging due to the rapid variation of the impedance near resonance. An accurate impedance model is employed that captures the real and imaginary part of the input impedance, and then an adaptive/tunable circuit is realized that significantly reduces the capacitive reactance of the impedance around the resonant frequency. An iterative approach using circuit and full-wave EM simulations is employed to determine the optimal impedance bandwidth. The procedure focuses on achieving full stability in the presence of device variations. An example is provided in Figure 4, displaying measured power spectral density with a stepped tone input. A more than threefold bandwidth enhancement is achieved with the non-Foster design [62], [63] compared to the original passive antenna presented in [10].
Figure 4. The transmission efficiency of a passively matched folded dipole ESA [10] and a corresponding actively matched version showing bandwidth enhancement [62], [63]. The efficiency is plotted relative to an unmatched monopole of the same size.
The potential for enhanced low-frequency ESA bandwidth and efficiency during transmission is noteworthy. However, while there has been significant recent progress on active matching techniques, there are open challenges, including sensitivity and susceptibility to the oscillation of active matching circuits and designs accommodating higher transmit power. Also, the question of whether non-Foster matching techniques have a significant advantage during reception is not fully understood because the bandwidth enhancement may be accompanied by an offsetting rise in receiver noise [61].
Next, we consider some additional designs, including multiband, wideband, and reconfigurable antennas, listed in Table 3. While some of these antennas are not optimized to provide the best performance at a single frequency, these designs offer unique advantages, as they seamlessly enable multiwavelength operation with a single antenna. In [64], a low-profile, wideband omnidirectional antenna is designed operating from 30 MHz to 3 GHz, with a height, width, and length of ${\lambda} / {29},{\lambda} / {5}$, and ${\lambda} / {6}$ at 30 MHz, respectively. The realized gain ranges from –13 dBi at 30 MHz to 3 dBi at 3 GHz, with up to a 6-dBi gain at some frequencies. Measurements at different frequencies show an omnidirectional pattern for the most part, except in the mid-VHF range, where distortions were observed.
Table 3. Wideband and multiband HF/VHF designs.
Another wideband design covering the VHF band is described in [65], with a height and diameter of ${\lambda} / {200}$ and ${\lambda} / {15}$ at 30 MHz, respectively. The peak gain ranges from –22 dBi at 30 MHz to –9 dBi at 300 MHz and provides a broad hemispherical pattern. A conical monopole-type wideband antenna is presented in [66], with a center frequency of 111 MHz and a 32.2-MHz bandwidth. The height and diameter of this VHF antenna are 24.1 and 36.8 cm ${(}{0.113}{\lambda}$ at 94.9 MHz). A similar electrically small slotted cone antenna operating in VHF/UHF was proposed much earlier in [67]. A genetic algorithm (GA)-based optimization was described in [68], yielding a low-profile broadband antenna covering 41 MHz to 2 GHz, with a height and diameter of ${\lambda} / {50}$ and ${\lambda} / {12}$. The realized gain ranged from –15 dBi at 41 MHz while maintaining at least a 5-dBi gain from 210 MHz to 2 GHz. In [69], a set of conformal spiral designs for operation in VHF and UHF are described. The authors show improved impedance matching at lower frequencies by using dielectric and inductive loading. An example design has a 46-cm diameter ${(}{0.06}{\lambda}$ at 40 MHz) and provides a gain of –16 dBi at 40 MHz and 4 dBi at 600 MHz.
Several researchers have also proposed multiband designs, where lower-frequency bands are included while maintaining a small size. The work presented in [70] describes a design approach for a miniature multiband antenna, where GA-based optimization is used to strategically place stubs at different locations on a monopole-type antenna to enable multiband operation while maintaining a low profile. One of the designs is 1.7 m long ${(}{\lambda} / {6}$ at 30 MHz) and has gains of 4.94, 4, and 7.88 dBi at 30, 80, and 108 MHz, respectively. In [71], a multiband, low-profile, inverted-F HF/VHF antenna operating at 27, 49, and 53 MHz is specifically designed for integration with a vehicle. The height is ${\lambda} / {25}$ at the lowest frequency and provides fractional bandwidths of 1, 0.33, and 0.14% at the three frequencies, respectively. The radiation efficiency is above 40% in all three bands. In addition to the multiband passive designs, a compact multiband active antenna design consisting of a normal mode, triple-band, helical passive antenna and a triple-band low-noise amplifier is proposed in [72]. The active multiband antenna shows a measured gain of 0, 5, and –5 dBi at 196, 550, and 1500 MHz, respectively. The active design yields more than a 10-dB gain improvement compared to a passive version for frequencies below 1 GHz. The size of the helical antenna is 5 cm (${\lambda} / {34}$ at 176 MHz).
Reconfigurable designs that leverage the interaction among distributed radiating elements provide another approach to significantly enhance antenna performance and agility. In [74], a bandwidth enhancement approach exploiting the EM coupling within a formation of elements is proposed. The intended application is a swarm of UAVs flying in formation (Figure 5). It is shown that a cluster of three elements can provide a 7.7% fractional bandwidth that is more than a sevenfold improvement compared to an individual element. This technique effectively provides an electrically large antenna composed of disconnected radiating elements. A compact, reconfigurable, coplanar waveguide-fed slot antenna operating at four frequency bands ranging from 59.5 to 1,000 MHz is presented in [73]. The planar antenna has a lateral dimension of ${0.06}{\lambda}$ (at 59 MHz), or 30 cm on each side. The design includes electronically controllable p-i-n diode-loaded slots to control the flow of current and hence the antenna characteristics. The design shows fractional bandwidths of 0.8, 23, 14, and 2% at center frequencies of 59, 356, 469, and 871 MHz, respectively. It also has a realized gain of –23, 0, 3, and 3 dBi for the four frequencies in increasing order.
Figure 5. Robotic parasitic VHF ESA arrays improve radiation characteristics. (a) A multi-UAV antenna system exploits inter-UAV mutual coupling to increase bandwidth [74]. (b) A full-wave directional beam pattern simulation of a two-UGV Yagi-type parasitic array [75].
A common requirement of almost all the wideband and multiband designs presented in this section, including [64], [65], [66], [67], [68], [70], [71], and [73], is an electrically large ground plane because the architectures are monopole, conical, and body-of-revolution broadband-type antennas. These designs are good candidates for integration with large vehicles and for fixed applications. This makes them attractive for wideband V2V and V2I communications and heterogeneous 5G and IoT applications that may require a wide multiband operating spectrum. On the other hand, some designs are intended for handsets and require only a small ground plane, such as the helical design presented in [72]. Also, the composite antenna described in [74] utilizes distributed radiators without requiring one large ground plane.
Low-Frequency Electrically Small Antennas: Future Directions and Open Problems
Some desirable goals and areas for further investigation include the following:
[S2] K. Sarabandi, J. Choi, A. Sabet, and K. Sabet, “Pattern and gain characterization using nonintrusive very-near-field electro-optical measurements over arbitrary closed surfaces,” IEEE Trans. Antennas Propag., vol. 65, no. 2, pp. 489–497, 2017, doi: 10.1109/TAP.2016.2633949.
As this survey illustrates, small antenna designs have achieved improved efficiency and bandwidth and successfully approached the Chu–Wheeler limit, and many innovations have been applied to enhance their bandwidth, tunability, and platform integration. However, decreasing size and the push to the Chu–Wheeler limit generally lead to an omnidirectional radiation pattern; e.g., see [15]. Consequently, achieving a directional beampattern with a single ESA may be fundamentally limited. In this section, we focus on low-frequency multiantenna systems that employ ESA elements. We discuss three fundamentally different approaches to multielement beamforming: 1) parasitic arrays, 2) biomimetic arrays, and 3) distributed beamforming arrays.
Parasitic arrays employ one or more excited elements in conjunction with parasitic elements near the excited elements to shape and improve the overall radiation performance of an antenna system. The classic Yagi–Uda parasitic array exploits mutual coupling to improve the gain of an antenna and enhance directionality. The Yagi–Uda has been extensively studied, and empirical free-space designs for half-wave types are well known. Recently, an ESA-based Yagi–Uda array was proposed, with each low-frequency ESA mounted on a UGV [75], [76]. UGVs may be autonomously configured to form a Yagi–Uda array. Element location and orientation can be optimized using full-wave simulations, the ground dielectric constant, and a GA. Experiments with software-defined radios have demonstrated the efficacy of a multi-UGV Yagi–Uda array at 40 MHz, providing directional beamforming with low distortion in cluttered urban environments.
Directional gain in classical phased-array beamforming relies on the number of elements and interelement spacing, e.g., a uniform linear array with ${\lambda} / {2}$ spacing between adjacent elements. However, the array size may become prohibitively large at HF and lower-VHF wavelengths, and classical beamforming techniques suffer when the element spacing becomes small due to beam broadening and mutual coupling.
However, nature offers an apparent contradiction in the hearing mechanism of a fly, Ormia ochracea [77], [78]. Despite a very small baseline (the diameter of the insect’s head) with respect to the acoustic wavelength (e.g., 3.4 m at 100 Hz), the fly nevertheless can discern a relatively accurate acoustic angle of arrival that is well beyond what is predicted by classical two-element plane wave angle estimation theory. This arises because of a mechanical coupling in the fly’s hearing and has inspired a biomimetic approach to antenna doublets that takes advantage of the EM coupling that occurs with small element spacing. The coupling approach is appealing at lower frequencies, where small spacing may be inherent to an application.
Recent results from the antenna, radio-frequency (RF) circuits, and signal processing areas reveal how an analog circuit inspired by the mechanical coupling of the fly’s hearing can improve angle estimation performance in some regimes [79], [80], [81]. Behdad, et al., propose design approaches for a biomimetic doublet with enhanced phase difference amplification compared to a two-element array having the same baseline and without an external coupling circuit [82], [83], [84]. A tradeoff study taking into account the phase difference enhancement and total output power of the array system has also been presented. A compact passive ESA array with a baseline of ${\lambda} / {15}$ in conjunction with a biomimetic RF circuit was designed and prototyped at a center frequency of 20 MHz, and through-the-wall direction of arrival was demonstrated [6].
A generalized design approach to two-element biomimetic arrays was recently developed, focusing on extracting the maximum possible power for a given phase difference amplification [85]. The design procedure and performance in the presence of various types of system-induced noise have been investigated [86], and the Cramer–Rao bound (CRB) on angle estimation is derived while accounting for the spacing, degree of mutual coupling, and parameters of the coupling circuit design. The CRB demonstrates the fundamental gain in angle of arrival estimation that is available by taking advantage of the mutual coupling and characterizes optimal coupling circuit design.
As noted in the preceding, conventional phased-array systems require relatively large antenna element spacing to achieve high directional gains. This is impractical at low frequencies for handheld devices, UGVs, and UAVs. An alternative approach is to employ a multiagent system with a single ESA per agent and form a distributed array. This is possible for collaborative multiagent autonomous systems, such as UGVs operating in dense indoor/outdoor environments. An experimental UGV-based, reconfigurable, distributed beamforming system is shown in Figure 3(e). Each UGV is equipped with a software-defined radio and low-VHF ESA [87], [88]. A leader–follower radio protocol at 900 MHz provides synchronization with subnanosecond accuracy, and distributed beamforming at 40 MHz has been achieved. The details will be reported elsewhere.
This article surveyed the state of the art in small, low-frequency antennas, focusing on upper-HF through lower-VHF bands. Interest in electrically small low-frequency antennas has increased, motivated by the desire to integrate compact and lightweight low-power, low-frequency, and multiband systems into handheld and mobile platforms, where size and efficiency are critical. This survey included passive, non-Foster, tunable, multiband, and reconfigurable antennas, highlighting tradeoffs in size, radiation efficiency, bandwidth, and platform integration. Recent breakthroughs in ESA array processing were also described, including parasitic, biomimetic, and distributed configurations. These advances are enabling new applications in V2V, V2I, autonomous UGV and UAV, disaster response, IoT, and other areas (see “Low-Frequency Electronically Small Antennas: Future Directions and Open Problems).
Fikadu T. Dagefu (fikadu.t.dagefu.civ@army.mil) is a research scientist at the U.S. Army Research Laboratory, Adelphi, Maryland, 20783, USA. His research interests include physics-based channel modeling, miniature antennas, wireless communications, and exploitation of robotic teaming for enhanced wireless networking. He is a Senior Member of IEEE and serves as an associate editor for IEEE Antennas and Wireless Propagation Letters.
Jihun Choi (jihchoi@umich.edu) is a postdoctoral fellow at the U.S. Army Research Laboratory, Adelphi, Maryland, 20783, USA. His research interests include antenna miniaturization, antenna measurement techniques, wave propagation modeling in highly cluttered scenarios, and low-power high-frequency/very high-frequency radio system design. He is a Member of IEEE.
Brian M. Sadler (brian.m.sadler6.civ@army.mil) is the U.S. Army senior scientist for intelligent systems at, and a fellow of, the U.S. Army Research Laboratory, Adelphi, Maryland, 20783, USA. His research interests include networking, processing, and control for autonomous multiagent intelligent systems. He is a Life Fellow of IEEE.
Kamal Sarabandi (saraband@umich.edu) is the director of the Radiation Laboratory and the Rufus S. Teesdale Professor of Engineering in the Department of Electrical Engineering and Computer Science, University of Michigan at Ann Arbor, Ann Arbor, Michigan, 48109, USA. His research interests include microwave and millimeter-wave radar remote sensing, metamaterials, electromagnetic wave propagation, and antenna miniaturization. He is a member of NAE, a Fellow of IEEE, AAAS, and NAI.
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Digital Object Identifier 10.1109/MAP.2021.3127559