Underwater Antenna Technologies with Emphasis on Submarine and Autonomous Underwater Vehicles (AUVs)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript provides a wide-ranging review of underwater antenna technologies and their applicability to AUV/ROV communication. The topic is relevant and timely. However, the paper requires certain revisions before it can be considered for publication.

1. The abstract lacks specific contributions, comparative conclusions, and quantitative insights; it only restates broad motivations. The main antenna technologies examined, their comparative performance attributes like range, frequency, and efficiency, and the significant research gaps that have been found should all be succinctly summarized in a review paper.

 

2. In Subsections 2.3–2.7, the manuscript elaborates upon MI loops, dipoles, directive antennas, ME antennas, and patch antennas, but does not provide a summary comparison. A suitable review must contain a table that summarizes such metrics of operating frequency, bandwidth, radiation efficiency, gain, communication range, and physical size, among others, also considering environmental assumptions like salinity and depth.

 

3. The conclusion section is incomplete. First, a proper review needs to summarize the main findings, show future research directions, highlight any identified gaps in technology, and provide practical recommendations for AUV designers.

 

4. The manuscript needs to be carefully proofread for clarity due to grammatical errors. Phrases like "swallow water," which should be "shallow water," and "those require each robot," which should be changed to "this requires each robot," are two examples. A thorough language review is necessary.

 

5. There are a number of technical claims, especially in Section 2.1 and Section 2.3, in the manuscript that do not have any proper citation. These include statements on waveguide-like propagation, dipole-wave behavior, and surface-wave theory. Proper peer-reviewed sources should be cited for these claims.

 

6. The manuscript does not discuss regulatory limits and physical constraints related to underwater EM communication, such as frequency allocation and power restrictions. This should be considered following Subsection 2.1 of a review paper that should include these considerations in order to make readers aware of practical and regulatory boundaries for underwater RF systems.

 

7. Subsection 2.6 gives a summary of mostly theoretical aspects of ME antennas without critical evaluation. There is no comparison to traditional antennas on consistent metrics, no discussion of fabrication challenges, and unclear information on radiated power versus efficiency. This section needs to be strengthened by a quantitative comparison in order to enhance its value as a review.

Comments for author File: Comments.pdf

Author Response

Reviewer 1

Comment:
This manuscript provides a wide-ranging review of underwater antenna technologies and their applicability to AUV/ROV communication. The topic is relevant and timely. However, the paper requires certain revisions before it can be considered for publication.

Authors’ Reply:
We sincerely thank the reviewer for the constructive and insightful feedback. We have carefully considered all comments and addressed the majority of the concern raised. Below, we describe the corresponding revisions, which are highlighted in blue in the revised manuscript.

1.1. Comment:
The abstract lacks specific contributions, comparative conclusions, and quantitative insights; it only restates broad motivations. The main antenna technologies examined, their comparative performance attributes like range, frequency, and efficiency, and the significant research gaps that have been found should all be succinctly summarized in a review paper.

Authors’ Reply:
Thank you for this valuable suggestion. We have revised the abstract to clearly emphasize the main antenna technologies reviewed, include comparative performance insights, and highlight key research gaps, providing a more quantitative and informative summary of the paper’s contributions.

Revised text (1-14).

Following the persistent evolution of terrestrial 5G wireless systems, a new field of underwater communication has emerged for various related applications, such as environmental monitoring, underwater mining, and marine research. However, establishing reliable high-speed underwater networks remains notoriously difficult due to the severe RF attenuation in conductive seawater, which strictly limits range coverage. In this article, we focus on a comprehensive review of different antenna types for future underwater communication and sensing systems, evaluating their performance and suitability for Autonomous Underwater Vehicles (AUVs). We critically examine and compare distinct antenna technologies, including Magnetic Induction (MI) coils, electrically short dipoles, wideband traveling wave antennas, printed planar antennas, and novel magnetoelectric (ME) resonators. Specifically, these antennas are compared in terms of physical footprint, operating frequency, bandwidth, and realized gain, revealing the trade-offs between miniaturization and radiation efficiency. Our analysis aims to identify the benefits and weaknesses of the different antenna types while emphasizing the necessity of innovative antenna designs to overcome the fundamental propagation limits of the underwater channel.

1.2. Comment:
In Subsections 2.3–2.7, the manuscript elaborates upon MI loops, dipoles, directive antennas, ME antennas, and patch antennas, but does not provide a summary comparison. A suitable review must contain a table that summarizes such metrics of operating frequency, bandwidth, radiation efficiency, gain, communication range, and physical size, among others, also considering environmental assumptions like salinity and depth.

Authors’ Reply:

We appreciate this helpful recommendation. In response, we have added a comprehensive comparison table summarizing the key performance metrics you suggested. In addition, the discussion section has been revised and expanded to provide a clearer comparative analysis across the different antenna technologies.

Revised text (864-973).

Discussion-Challenges-Future Aspects

On Antennas Application in AUVs

Following the analysis of potential candidates for seawater underwater antennas, we are going to delve into their practical integration within AUVs and ROVs. As illustrated in Table 2, which summarizes most of the presented antennas, frequency selection is a critical design parameter. Based on the findings in Section 2.1, the operating frequency must remain as low as possible to minimize propagation losses (due to the rapid attenuation caused by the high conductivity of seawater) and provide communication or sensing over at least short ranges. However, implementing efficient low-frequency antennas on small-scale AUVs presents a significant physical challenge due to the prohibitive size of the required radiating elements relative to the vehicle's hull. Consequently, center frequencies above 10 MHz generally preclude links exceeding a few meters. The primary practical application of such high frequencies is communication within an AUV swarm, where individual units act as relays, as proposed in [12]. In this configuration, compact antennas with sufficient bandwidth can be realized to support the data rates required for larger data packages (such as video). Consequently, coordination among AUVs can be achieved with high data rates, which are crucial for applications such as seabed mining and exploration.

For applications operating above 100 MHz, patch antennas emerge as the most suitable candidates, offering wide bandwidth, higher directivity, and a compact quasi-2D form factor that enables conformal mounting to the hull, thereby minimizing hydrodynamic drag. However, the utility of these antennas is limited to extremely short-range links; at such frequencies, electromagnetic waves experience severe attenuation due to the high conductivity of seawater, rather than intrinsic antenna inefficiency alone. As summarized in Table 2, alternative antenna architectures are generally less attractive due to constraints related to size, data rate, or efficiency. In terms of physical dimensions, MI solutions are often impractical for compact AUVs, attributed to the substantial volume required by the coil structures. Furthermore, MI system performance is highly sensitive to the relative orientation of the transmitter and receiver; even moderate misalignment can drastically reduce the magnetic coupling coefficient, necessitating precise coordination between vehicles. Dipole antennas present similar integration challenges. Although their high aspect ratio will permit placement along the external chassis, any protrusion disrupts the hydrodynamic profile of the AUV. To mitigate this issue, a fin-like antenna structure—similar to those employed in automotive applications—could be implemented as a monopole using the AUV chassis as a ground plane. Nevertheless, both MI coils and electrically short wire antennas inherently suffer from narrow bandwidths, which limit achievable data rates, as well as poor radiation efficiency. While alternative designs, such as the J-pole antenna, may offer improved bandwidth, they introduce additional structural complexity that complicates mechanical integration into the AUV.

A critical challenge in the design of underwater antennas is optimizing their overall efficiency, which is governed by two primary factors: impedance matching to the source and radiation efficiency. Impedance matching is determined by the reflection coefficient at the feed point, arising from differences between the source and antenna impedance. An antenna’s input impedance is heavily dependent not only on its geometry and electrical size but also on the EM properties of the surrounding medium. If an antenna is housed within an air-filled protective radome to prevent galvanic corrosion and eddy currents, the abrupt interface between the air (εr ≈ 1) and the seawater (εr ≈ 81) creates a severe impedance mismatch. This results in significant signal reflection at the boundary, manifesting as a poor voltage standing wave ratio (VSWR).

To mitigate this, researchers have proposed submerging the antenna in a “buffer” medium [15,54,63,64,75]. By utilizing a dielectric buffer—typically distilled or deionized water [15], or a material with a permittivity intermediate between air and seawater [63]—the radiated waves can be generated within a lossless environment before transitioning into the seawater. A particularly effective approach is encapsulating the antenna in distilled water, which matches the dielectric constant of seawater but exhibits negligible conductivity. This isolates the antenna’s reactive near-field from the conductive lossy seawater, thereby minimizing ohmic dissipation while maintaining impedance continuity. Consequently, a robust fabrication strategy requires a compact, liquid-filled sealed enclosure that maximizes matching and prevents direct seawater contact, while facilitating reliable RF and electronic interconnects.

The second limiting factor is radiation efficiency, which is linked to the antenna's physical size and field distribution. The fundamental trade-off between an antenna's electrical size and its maximum achievable efficiency/bandwidth is governed by the Chu-Harrington limit [80]. For low-frequency applications, the requirement for miniaturization typically necessitates a severe sacrifice in efficiency. As introduced in Section [Magnetoelectric], ME antennas offer a compelling alternative by exploiting mechanical (acoustic) resonance. This mechanism allows the antenna to resonate at much lower frequencies than conventional electrical antennas of comparable dimensions. However, the radiation efficiency of ME antennas is often compromised by weak electromechanical coupling, with reported coupling coefficients frequently below 5% [72,81].

While surveys indicate that the absolute radiation efficiency of ME antennas can be lower than 10⁻⁷ [82], suggesting they cannot universally replace conventional antennas, they offer distinct advantages in size-constrained, low-frequency applications where they approach the theoretical performance bounds defined by the Chu-Harrington limit [83]. Furthermore, ME antennas are particularly advantageous when integrated as planar structures near a conductive ground plane (e.g., an AUV hull). At electrically small separation distances (< 1 MHz), conventional electric antennas suffer from gain degradation due to destructive interference from image currents, mainly due to the dipole being shorted. In contrast, ME antennas behave as magnetic dipoles, inducing parallel image currents that result in constructive interference and gain enhancement [84]. Therefore, to derive accurate performance benchmarks, ME antennas must be compared against equivalent magnetic antennas (such as loops). As detailed in Table 3, the ME antenna consistently outperforms the conventional loop antenna by at least 15 dB in these scenarios.

However, the fabrication and operational deployment of ME antennas present significant challenges. The realization of high-performance ME devices is hindered by critical material synthesis and processing constraints. A primary difficulty lies in engineering a rigid, high-quality interface between disparate thin films—typically magnetostrictive metals (e.g., FeGaB or Metglas) and piezoelectric ceramics (e.g., AlN or PZT). Since the device relies fundamentally on efficient strain transfer, any interface defects, lattice mismatches, or poor adhesion will severely degrade the electromechanical coupling coefficient [91]. Furthermore, process compatibility poses a significant hurdle; the high thermal budgets required to optimize the crystallinity of the piezoelectric layer often induce deleterious effects such as oxidation, residual stress, or phase degradation in the magnetic layer, particularly when specific magnetic anisotropies must be preserved [92]. Additionally, for polymer-based nanocomposites (e.g., magnetic nanoparticles embedded in PVDF), preventing particle agglomeration during the curing process is critical to ensuring uniform dispersion and minimizing dielectric losses. Finally, fabricating these devices at the operational NEMS/MEMS scale necessitates complex release steps to define suspended vibrating structures, a process often plagued by mechanical fragility and low yield [93,94].

In conclusion, despite their potential, current ME antennas face barriers to practical deployment, primarily due to insufficient gain and radiation efficiency. To address these limitations, researchers have proposed the implementation of densely packed arrays of ME elements [91]. Unlike conventional electromagnetic antenna arrays, which typically require half-wavelength (λ/2) spacing to avoid mutual coupling, ME arrays—operating on acoustic resonance principles—benefit from a significantly more compact footprint. Studies on three-element ME arrays indicate that this configuration increases the effective number of magnetic dipoles, resulting in a threefold enhancement of the induced output voltage [95]. Consequently, establishing similar array-based architectures specifically optimized for the underwater acoustic-magnetic environment is a critical direction for future research. A comparison table containing the benefits and drawbacks of each studied antenna type is presented in Table 4.

 

1.3. Comment:

The conclusion section is incomplete. First, a proper review needs to summarize the main findings, show future research directions, highlight any identified gaps in technology, and provide practical recommendations for AUV designers.

 

Authors’ Reply:

Thank you for this important comment. We have substantially revised the conclusion to better summarize the main findings, identify open research challenges, outline future research directions, and provide practical insights relevant to AUV and ROV system designers.

Revised text (1041-1055).

In this comprehensive review, we evaluated the performance and integration potential of various antenna topologies for underwater communication, ranging from conventional electric dipoles to emerging magnetoelectric (ME) resonators. Our analysis synthesizes the trade-offs between physical size, radiation efficiency, and hydrodynamic compliance required for AUV applications. The primary findings indicate that for low-frequency, size-constrained telemetry (<1 MHz), ME antennas are superior to traditional loop antennas, offering gains higher by >15 dB and benefiting from constructive image currents when mounted near conductive hulls. Conversely, for high-bandwidth short-range data links (>100 MHz), conformal patch antennas are identified as the optimal solution due to their minimal hydrodynamic drag.

However, significant technological gaps persist. Specifically, the practical deployment of ME antennas is currently limited by fabrication complexities—such as interface fragility in composite materials—and a lack of robust array architectures for underwater environments. Furthermore, a complete physical model explaining the low attenuation of lateral surface waves at extended ranges remains to be fully established. Future research directions should focus on two key areas: (1) developing mechanically robust ME arrays to enhance gain through dipole aggregation, and (2) designing novel excitation elements to efficiently couple energy into lateral and dipole wave modes. Finally, for AUV designers, we provide the following practical recommendations: prioritize magnetic-based antennas for low-frequency operations to leverage hull-induced gain enhancement; employ dielectric buffer layers or distilled water encapsulation to isolate the reactive near-field from lossy seawater; and utilize conformal designs to maintain vehicle hydrodynamics. Addressing these challenges will be pivotal in extending the range and reliability of next-generation underwater electromagnetic networks.

 

1.4. Comment:

The manuscript needs to be carefully proofread for clarity due to grammatical errors. Phrases like "swallow water," which should be "shallow water," and "those require each robot," which should be changed to "this requires each robot," are two examples. A thorough language review is necessary.

Authors’ Reply:

We apologize for these oversights. The manuscript has now been thoroughly proofread, and grammatical, spelling, and clarity issues throughout the text have been corrected.

1.5. Comment:

There are a number of technical claims, especially in Section 2.1 and Section 2.3, in the manuscript that do not have any proper citation. These include statements on waveguide-like propagation, dipole-wave behavior, and surface-wave theory. Proper peer-reviewed sources should be cited for these claims.

Authors’ Reply:

We thank the reviewer for pointing this out. While we aimed to avoid repeated citations within the same paragraph, we acknowledge that some statements required clearer attribution. The references have now been reorganized and placed more appropriately to ensure that all technical claims are properly supported by peer-reviewed sources.

1.6. Comment:

The manuscript does not discuss regulatory limits and physical constraints related to underwater EM communication, such as frequency allocation and power restrictions. This should be considered following Subsection 2.1 of a review paper that should include these considerations in order to make readers aware of practical and regulatory boundaries for underwater RF systems.

Authors’ Reply:

Thank you for this insightful suggestion. We have added a discussion addressing regulatory and practical constraints related to underwater electromagnetic communication. Specifically, we clarify that underwater RF communication is still largely at an early research stage, and therefore no dedicated civilian frequency or power regulations currently exist. We also discuss the limited regulatory precedent associated with very-low-frequency (VLF) submarine communications, as well as the inherently lossy nature of seawater, which significantly limits electromagnetic leakage into the air and potential interference. Power considerations are also discussed, noting that while human exposure is generally not a concern in most underwater applications, this may change in scenarios involving diver communications.

Revised text (245-262).

In contrast with terrestrial and non-terrestrial communication and sensing systems, there are not yet explicitly defined standards or limits for power and frequency. For power, due to the lossy nature of seawater and the lack of human beings in close vicinity, limits for transmitters have not yet been established. For that reason, most works either adopt the power transmitted by a mobile or terrestrial transmitter depending on the implemented protocol (LoRa, WiFi) or use a high-power module in order to ensure the successful reception of the signal at the receiving node. Similar reasoning holds true for frequency allocation for underwater applications; due to the high loss, it is impossible to interfere with the allocated bands of mobile and satellite communications, especially if the application area is in the middle of the sea. The ITU has allocated some Very Low Frequency (VLF) bands for submarine communication (around 3-30 kHz), but these allocations are established for radio services and not specifically for underwater communication or sensing systems [27]. In addition, the lack of regulatory limits is attributed to the limited existence of commercial underwater RF communication systems, leaving the field undirected. However, if more commercial products and services begin to emerge, creating congested environments of WSNs, it is highly likely that regulations will be enforced by international and national organizations. For now, RF underwater communication is an undiscovered and unexploited area of research effort.

1.7. Comment:

Subsection 2.6 gives a summary of mostly theoretical aspects of ME antennas without critical evaluation. There is no comparison to traditional antennas on consistent metrics, no discussion of fabrication challenges, and unclear information on radiated power versus efficiency. This section needs to be strengthened by a quantitative comparison in order to enhance its value as a review.

Authors’ Reply:

We appreciate this constructive comment. Due to the limited availability of direct comparisons between magnetoelectric and conventional electrical antennas for underwater applications at comparable sizes and operating frequencies, we have drawn upon well-established studies comparing these antenna types in air. As the benefits of magnetoelectric antennas can only be meaningfully assessed when compared against electrically small antennas operating at the same frequency and size, this approach allows for a fair and informative evaluation. The subsection has been expanded accordingly to include comparative discussion and fabrication-related considerations.

Revised text.

See Answer of Comment 2

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper addresses an important and timely topic related to underwater antenna technologies for submarines and autonomous underwater vehicles. It provides a broad and technically informed survey of electromagnetic propagation in underwater environments and reviews a wide range of antenna types, including loop, dipole, directive, magnetoelectric, and planar antennas. The topic is well aligned with the scope of the journal and will be of interest to researchers working in underwater communications and antenna design.

However, the manuscript would benefit from stronger synthesis and critical comparison across the reviewed technologies. While the coverage of prior work is extensive, the paper currently reads largely as a collection of individual summaries. The inclusion of comparative tables or structured discussions highlighting trade-offs in terms of operating frequency, communication range, efficiency, size, and suitability for AUV or submarine deployment would significantly enhance the value of the review.

The authors may also consider broadening the related work discussion to include recent system-level studies on underwater wireless networks and multi-AUV nettworks, which complement antenna-level design considerations. Including such perspectives would help connect antenna design choices with higher-layer system performance and practical deployment scenarios.

The manuscript requires improvement in language quality and clarity. Grammatical errors, inconsistent terminology, and some redundant explanations should be addressed to improve readability. Strengthening the conclusion to better synthesize the main findings and outline concrete future research directions is also recommended.

Furthermore, a more explicit discussion of open challenges and research gaps would enhance the contribution of the review. For example, issues related to scalability, integration of antenna systems with mobile AUV swarms, trade-offs between electromagnetic and optical underwater communication technologies, and practical deployment constraints in real marine environments deserve deeper analysis. Highlighting these aspects would make the paper more forward-looking and valuable to both academic and industrial readers.

Comments on the Quality of English Language

The paper is generally understandable; however, the quality of English requires improvement. The text contains grammatical errors, awkward sentence structures, inconsistent terminology, and occasional typographical mistakes that affect readability. The authors are encouraged to carefully revise the manuscript for clarity and conciseness and to consider professional English language editing to improve the overall presentation.

Author Response

Reviewer 2

2.1. Comment:
The paper addresses an important and timely topic related to underwater antenna technologies for submarines and autonomous underwater vehicles. It provides a broad and technically informed survey of electromagnetic propagation in underwater environments and reviews a wide range of antenna types, including loop, dipole, directive, magnetoelectric, and planar antennas. The topic is well aligned with the scope of the journal and will be of interest to researchers working in underwater communications and antenna design.

Authors’ Reply:

We sincerely thank the reviewer for the positive evaluation of our work and for the constructive comments aimed at improving the manuscript.

2.2 Comment:
However, the manuscript would benefit from stronger synthesis and critical comparison across the reviewed technologies. While the coverage of prior work is extensive, the paper currently reads largely as a collection of individual summaries. The inclusion of comparative tables or structured discussions highlighting trade-offs in terms of operating frequency, communication range, efficiency, size, and suitability for AUV or submarine deployment would significantly enhance the value of the review.

Authors’ Reply:

We appreciate this valuable feedback. To address this concern, we have significantly expanded the final section (Discussion–Challenges–Future Aspects) to provide a structured comparison of the reviewed antenna technologies, highlighting their respective advantages and limitations. A summary table including the proposed performance metrics has also been added to support this comparative discussion.

Revised text (864-973).

Discussion-Challenges-Future Aspects

On Antennas Application in AUVs

Following the analysis of potential candidates for seawater underwater antennas, we are going to delve into their practical integration within AUVs and ROVs. As illustrated in Table 2, which summarizes most of the presented antennas, frequency selection is a critical design parameter. Based on the findings in Section 2.1, the operating frequency must remain as low as possible to minimize propagation losses (due to the rapid attenuation caused by the high conductivity of seawater) and provide communication or sensing over at least short ranges. However, implementing efficient low-frequency antennas on small-scale AUVs presents a significant physical challenge due to the prohibitive size of the required radiating elements relative to the vehicle's hull. Consequently, center frequencies above 10 MHz generally preclude links exceeding a few meters. The primary practical application of such high frequencies is communication within an AUV swarm, where individual units act as relays, as proposed in [12]. In this configuration, compact antennas with sufficient bandwidth can be realized to support the data rates required for larger data packages (such as video). Consequently, coordination among AUVs can be achieved with high data rates, which are crucial for applications such as seabed mining and exploration.

For applications operating above 100 MHz, patch antennas emerge as the most suitable candidates, offering wide bandwidth, higher directivity, and a compact quasi-2D form factor that enables conformal mounting to the hull, thereby minimizing hydrodynamic drag. However, the utility of these antennas is limited to extremely short-range links; at such frequencies, electromagnetic waves experience severe attenuation due to the high conductivity of seawater, rather than intrinsic antenna inefficiency alone. As summarized in Table 2, alternative antenna architectures are generally less attractive due to constraints related to size, data rate, or efficiency. In terms of physical dimensions, MI solutions are often impractical for compact AUVs, attributed to the substantial volume required by the coil structures. Furthermore, MI system performance is highly sensitive to the relative orientation of the transmitter and receiver; even moderate misalignment can drastically reduce the magnetic coupling coefficient, necessitating precise coordination between vehicles. Dipole antennas present similar integration challenges. Although their high aspect ratio will permit placement along the external chassis, any protrusion disrupts the hydrodynamic profile of the AUV. To mitigate this issue, a fin-like antenna structure—similar to those employed in automotive applications—could be implemented as a monopole using the AUV chassis as a ground plane. Nevertheless, both MI coils and electrically short wire antennas inherently suffer from narrow bandwidths, which limit achievable data rates, as well as poor radiation efficiency. While alternative designs, such as the J-pole antenna, may offer improved bandwidth, they introduce additional structural complexity that complicates mechanical integration into the AUV.

A critical challenge in the design of underwater antennas is optimizing their overall efficiency, which is governed by two primary factors: impedance matching to the source and radiation efficiency. Impedance matching is determined by the reflection coefficient at the feed point, arising from differences between the source and antenna impedance. An antenna’s input impedance is heavily dependent not only on its geometry and electrical size but also on the EM properties of the surrounding medium. If an antenna is housed within an air-filled protective radome to prevent galvanic corrosion and eddy currents, the abrupt interface between the air (εr ≈ 1) and the seawater (εr ≈ 81) creates a severe impedance mismatch. This results in significant signal reflection at the boundary, manifesting as a poor voltage standing wave ratio (VSWR).

To mitigate this, researchers have proposed submerging the antenna in a “buffer” medium [15,54,63,64,75]. By utilizing a dielectric buffer—typically distilled or deionized water [15], or a material with a permittivity intermediate between air and seawater [63]—the radiated waves can be generated within a lossless environment before transitioning into the seawater. A particularly effective approach is encapsulating the antenna in distilled water, which matches the dielectric constant of seawater but exhibits negligible conductivity. This isolates the antenna’s reactive near-field from the conductive lossy seawater, thereby minimizing ohmic dissipation while maintaining impedance continuity. Consequently, a robust fabrication strategy requires a compact, liquid-filled sealed enclosure that maximizes matching and prevents direct seawater contact, while facilitating reliable RF and electronic interconnects.

The second limiting factor is radiation efficiency, which is linked to the antenna's physical size and field distribution. The fundamental trade-off between an antenna's electrical size and its maximum achievable efficiency/bandwidth is governed by the Chu-Harrington limit [80]. For low-frequency applications, the requirement for miniaturization typically necessitates a severe sacrifice in efficiency. As introduced in Section [Magnetoelectric], ME antennas offer a compelling alternative by exploiting mechanical (acoustic) resonance. This mechanism allows the antenna to resonate at much lower frequencies than conventional electrical antennas of comparable dimensions. However, the radiation efficiency of ME antennas is often compromised by weak electromechanical coupling, with reported coupling coefficients frequently below 5% [72,81].

While surveys indicate that the absolute radiation efficiency of ME antennas can be lower than 10⁻⁷ [82], suggesting they cannot universally replace conventional antennas, they offer distinct advantages in size-constrained, low-frequency applications where they approach the theoretical performance bounds defined by the Chu-Harrington limit [83]. Furthermore, ME antennas are particularly advantageous when integrated as planar structures near a conductive ground plane (e.g., an AUV hull). At electrically small separation distances (< 1 MHz), conventional electric antennas suffer from gain degradation due to destructive interference from image currents, mainly due to the dipole being shorted. In contrast, ME antennas behave as magnetic dipoles, inducing parallel image currents that result in constructive interference and gain enhancement [84]. Therefore, to derive accurate performance benchmarks, ME antennas must be compared against equivalent magnetic antennas (such as loops). As detailed in Table 3, the ME antenna consistently outperforms the conventional loop antenna by at least 15 dB in these scenarios.

However, the fabrication and operational deployment of ME antennas present significant challenges. The realization of high-performance ME devices is hindered by critical material synthesis and processing constraints. A primary difficulty lies in engineering a rigid, high-quality interface between disparate thin films—typically magnetostrictive metals (e.g., FeGaB or Metglas) and piezoelectric ceramics (e.g., AlN or PZT). Since the device relies fundamentally on efficient strain transfer, any interface defects, lattice mismatches, or poor adhesion will severely degrade the electromechanical coupling coefficient [91]. Furthermore, process compatibility poses a significant hurdle; the high thermal budgets required to optimize the crystallinity of the piezoelectric layer often induce deleterious effects such as oxidation, residual stress, or phase degradation in the magnetic layer, particularly when specific magnetic anisotropies must be preserved [92]. Additionally, for polymer-based nanocomposites (e.g., magnetic nanoparticles embedded in PVDF), preventing particle agglomeration during the curing process is critical to ensuring uniform dispersion and minimizing dielectric losses. Finally, fabricating these devices at the operational NEMS/MEMS scale necessitates complex release steps to define suspended vibrating structures, a process often plagued by mechanical fragility and low yield [93,94].

In conclusion, despite their potential, current ME antennas face barriers to practical deployment, primarily due to insufficient gain and radiation efficiency. To address these limitations, researchers have proposed the implementation of densely packed arrays of ME elements [91]. Unlike conventional electromagnetic antenna arrays, which typically require half-wavelength (λ/2) spacing to avoid mutual coupling, ME arrays—operating on acoustic resonance principles—benefit from a significantly more compact footprint. Studies on three-element ME arrays indicate that this configuration increases the effective number of magnetic dipoles, resulting in a threefold enhancement of the induced output voltage [95]. Consequently, establishing similar array-based architectures specifically optimized for the underwater acoustic-magnetic environment is a critical direction for future research. A comparison table containing the benefits and drawbacks of each studied antenna type is presented in Table 4.

2.4. Comment:
The authors may also consider broadening the related work discussion to include recent system-level studies on underwater wireless networks and multi-AUV networks, which complement antenna-level design considerations. Including such perspectives would help connect antenna design choices with higher-layer system performance and practical deployment scenarios.

Authors’ Reply:

Thank you for this thoughtful suggestion. While we fully agree with the importance of system-level perspectives, the limited time available for revision prevented the development of a dedicated new section on underwater network architectures. Nevertheless, we have incorporated additional discussion and selected references related to system-level considerations within the Discussion and Requirements sections. We regret that more extensive treatment was not feasible at this stage.

Revised text (263-281).

Beyond the physical optimization of individual radiating elements, the design of underwater antennas must be intrinsically linked to the broader system-level requirements of UWSNs and the Internet of Underwater Things (IoUT) [28,29]. Recent system-level studies emphasize that the severe bandwidth limitations and high attenuation of the underwater channel necessitate a cross-layer design approach, where the physical layer attributes are constrained by higher-layer performance metrics such as network throughput, latency, and routing reliability. Consequently, the antenna cannot be treated as an isolated component; its integration must account for the dynamic link quality and energy constraints inherent to battery-operated underwater nodes [30].

This interdependence is particularly critical in the deployment of multi-AUV networks and swarm robotics. Practical scenarios include cooperative sensing, formation control, and survey missions, all of which require robust and resilient inter-vehicle communication that becomes especially important due to the continuous motion of ROVs and AUVs [30]. In such dynamic network topologies, antenna spatial coverage becomes a decisive factor for overall network stability. For example, while high-gain directional antennas are well suited for static, point-to-point data offloading to a surface gateway, they often fail to maintain the continuous connectivity required for mobile swarms because of frequent misalignment and beamforming errors. In contrast, recent research on AUV swarms advocates the use of compact, omnidirectional radiators to mitigate the “hidden terminal” problem and ensure reliable telemetry exchange regardless of vehicle orientation [31].

2.5. Comment:
The manuscript requires improvement in language quality and clarity. Grammatical errors, inconsistent terminology, and some redundant explanations should be addressed to improve readability. Strengthening the conclusion to better synthesize the main findings and outline concrete future research directions is also recommended.

Authors’ Reply:

We apologize for these issues and appreciate the reviewer’s careful reading. The manuscript has been thoroughly proofread to improve language quality, consistency, and clarity. In addition, the conclusion has been revised to better synthesize the main findings and articulate concrete future research directions.

2.6. Comment:
Furthermore, a more explicit discussion of open challenges and research gaps would enhance the contribution of the review. For example, issues related to scalability, integration of antenna systems with mobile AUV swarms, trade-offs between electromagnetic and optical underwater communication technologies, and practical deployment constraints in real marine environments deserve deeper analysis. Highlighting these aspects would make the paper more forward-looking and valuable to both academic and industrial readers.

Authors’ Reply:

We appreciate this important suggestion. In response, we have expanded the Discussion–Challenges–Future Aspects section to explicitly address open challenges and research gaps, including scalability in multi-AUV swarms, technology integration, modality trade-offs, and real-world deployment constraints. We believe these additions significantly strengthen the forward-looking perspective of the review.

Revised text (992-1040).

On Future Challenges

While the optimization of isolated antenna elements is essential, the transition from laboratory prototypes to deployment in realistic marine environments introduces an additional layer of challenges that remains largely underexplored. A critical research gap concerns the scalability of antenna systems for mobile AUV swarms. As underwater networks evolve from point-to-point links to dense, multi-agent swarms, the inherently limited bandwidth of low-frequency antennas—particularly high-Q resonant structures such as magnetic induction coils or magnetoelectric sensors—emerges as a fundamental bottleneck. This narrow operational bandwidth constrains achievable data rates and spectral efficiency, thereby complicating interference mitigation and medium-access control (MAC) protocols when multiple vehicles attempt simultaneous communication. Consequently, future antenna architectures must explore frequency-agile or reconfigurable designs that enable dynamic spectrum utilization, thereby supporting the scalability and robustness required for large-scale swarm deployments. It is important to note that such systems will typically involve very closely spaced nodes.

The practical deployment constraints of real marine environments pose severe threats to long-term antenna performance, which are often overlooked in simulation-based studies. Beyond well-known attenuation mechanisms, underwater antennas are subject to biofouling, which alters local dielectric boundary conditions and can cause significant impedance detuning—a critical issue for narrowband resonant antennas. Additionally, extreme hydrostatic pressure at depth can mechanically compress antenna substrates, modifying their effective permittivity and shifting the resonant frequency. Future research must therefore focus on pressure-neutral materials, encapsulation strategies, and active compensation circuits capable of dynamically tuning antenna parameters to counteract environmental detuning and corrosion, thereby ensuring consistent performance during extended missions in harsh oceanic conditions.

Furthermore, the realization of effective underwater communication and sensing networks necessitates a holistic system-level design that integrates multiple complementary technologies—namely RF, optical, and acoustic modalities—to compensate for the inherent limitations of each. In this context, RF systems are not intended to replace optical or acoustic links, but rather to operate synergistically as complementary channels, providing enhanced quality of service when other modalities degrade or fail. For example, while optical systems can support very high data rates (on the order of Gbps) over short distances in clear water, they experience catastrophic link degradation in turbid environments where RF waves remain comparatively robust. Conversely, acoustic systems enable long-range underwater propagation with relatively low attenuation but suffer from limited data rates and severe multipath effects, particularly near the seabed in cluttered environments. In such scenarios, RF-based communication and sensing systems can be activated to provide higher data rates and more resilient links. Moreover, in sensitive military and security applications, RF communication offers inherent advantages due to the strong attenuation of electromagnetic waves in seawater, which significantly reduces the risk of interception by adversaries. Despite these advantages, a substantial research gap remains in the development of compact hybrid “triple-mode” systems that seamlessly integrate RF or magnetic radiators with optical and acoustic transceivers. A key challenge for antenna designers lies in exploiting the severely constrained volume of an AUV to maximize RF radiation efficiency without compromising hydrodynamic performance or the functionality of co-located subsystems, thereby enabling robust fallback mechanisms that balance the high throughput of optical links and the long-range capability of acoustic channels with the reliability and security of electromagnetic communication.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded to the comments satisfactorily; therefore, I recommend the publication of this paper.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have well addressed all the comments. No further comments from my side.