Development of Broadband Underwater Radio Communication for Application in Unmanned Underwater Vehicles

Journal of Marine Science and Engineering

Article Development of Broadband Underwater Radio Communication for Application in Unmanned Underwater Vehicles

Igor Smolyaninov 1,* , Quirino Balzano 2 and Dendy Young 1 1 Saltenna LLC, 1751 Pinnacle Drive, Suite 600, McLean, VA 22102-4903, USA; [email protected] 2 Electrical and Computer Engineering Department, University of Maryland, College Park, MD 20742, USA; [email protected] * Correspondence: [email protected]

Received: 31 March 2020; Accepted: 18 May 2020; Published: 23 May 2020

Abstract: This paper presents several novel designs of small form factor underwater radio antennas operating in the 2 MHz, 50 MHz and 2.4 GHz bands. These antennas efficiently excite surface electromagnetic waves (SEW) which propagate along the surface of seawater. The antenna operation is made possible due to implementation of an impedance matching enclosure, which is filled with de-ionized water. Enhanced coupling to surface electromagnetic waves is enabled by the enhancement of the electromagnetic field at the antenna apex. These features allow us to make antenna dimensions considerably smaller compared to typical free space designs. They also considerably improve coupling of electromagnetic energy to the surrounding seawater. Since SEW propagation length is considerably larger than the skin depth in seawater, this technique is useful for underwater broadband wireless communication. We conclude that the developed broadband underwater radio communication technique will be useful in networking of unmanned underwater vehicles.

Keywords: unmanned underwater vehicle; broadband radio communication; surface electromagnetic wave

1. Introduction Wide band communication remains the limiting bottleneck for command and control of unmanned underwater vehicles (UUV). Acoustic communication is bandwidth limited due to slow propagation speeds and is susceptible to high error rates due to multi-path effects. Conventional radio frequency (RF) signals may be used for wide bandwidth communications. However, they are severely limited in communication range due to rapid attenuation in seawater. Directional optical links are capable of providing bandwidth over 200 Mbps in seawater but, until now, have not been successfully integrated into operational UUVs due to the need for sophisticated pointing, acquisition and tracking. Moreover, water turbidity strongly affects performance of the optical links. As a result, the use of optical wireless communication for reliable UUV-to-UUV links remains highly challenging. Thus, underwater wide bandwidth wireless communication remains a critical technology gap that needs to be filled. Very recently we have reported a novel design of a SEW RF antenna, which operates in the 2.4 GHz band and which efficient launches surface electromagnetic waves along an interface between a conductor and a dielectric [1]. The antenna operation is based on the strong field enhancement at the antenna tip, which results in efficient excitation of surface electromagnetic waves propagating along nearby conductive surfaces. It was demonstrated that this antenna may be used to send broadband radio communication signals through such conductive enclosures as commercial Faraday cages. It was also hypothesized that a similar design could be used for broadband underwater communication. Indeed, a successful adaptation of the surface wave antenna design was reported in [2] which operates

J. Mar. Sci. Eng. 2020, 8, 370; doi:10.3390/jmse8050370 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 370 2 of 10 in the 50 MHz band and is able for launch SEWs along the seawater surface. In addition to the enhancement of electromagnetic ﬁeld near the antenna apex, the antenna design implements an impedance matching enclosure which uses de-ionized water [3]. This enclosure enables reduction of the antenna dimensions. It also improves coupling of electromagnetic energy to the surrounding seawater. Since the propagation length of SEW considerably exceeds the skin depth of radio waves at the same frequency, the surface wave technique may be used for underwater broadband wireless communication over long distances. In this article, we report further development of this concept. We will describe several designs of portable underwater radio antennas operating in the 2 MHz, 50 MHz and 2.4 GHz bands, which can be used for eﬃcient launching of SEWs along the seawater surface. In all cases, the developed surface wave underwater antennas are capable of broadband underwater wireless communication over distances which are much larger than the skin depth in seawater. We infer that the developed broadband underwater radio communication technique will be useful in communication among unmanned underwater vehicles.

2. Methods Typically, it is impossible to establish RF communication through conductive media and enclosures, such as communication through seawater, metallic chambers, etc. Performance of conventional radio communication schemes in these geometries is limited by the very small skin depth δ of a conductive medium, which may be calculated as: s 1 δ = (1) πµ0σν where ν is the communication frequency and σ is the medium conductivity [4]. In the case of seawater (3.5% salinity, 5 S/m conductivity), the frequency dependent RF skin depth may be estimated by

270m δ (2) ≈ √ν

This effect severely limits the ability to use radio communication in seawater. For example, the skin depth of seawater at 50 MHz equals approximately 3.8 cm, so it is impractical to use conventional radio communication over useful distances. In addition, conventional RF signals cannot penetrate through small defects and openings in conductive barriers. For example, transmission of a conventional transverse electromagnetic wave through a subwavelength aperture was found by Bethe [5] to be equal to a 4 T , (3) ∝ λ where λ is the free space wavelength and a is the aperture size. It produces negligible transmission if a << λ. Therefore, conventional radio communication techniques are also impractical in situations where an enclosure is surrounded by conductive walls. On the other hand, it is well established that efficient coupling to surface electromagnetic modes which exist at conductor/dielectric interfaces [6] enables efficient signal transmission through continuous conductive barriers (including even metal layers) and through deeply subwavelength apertures in such barriers [7,8]. Following this approach, we have designed a 2.45-GHz SEW antenna [1], which can transmit video signals from inside a 90 dB isolation Faraday cage. We believe that this novel ability − may be utilized for remote examination of metal enclosures, as well as improving Wi-Fi connectivity in underground tunnels and buildings. Moreover, a similar surface electromagnetic wave-based approach J. Mar. Sci. Eng. 2020, 8, 370 3 of 10

may be used to implement broadband radio communication in seawater over long distances, which considerably exceed the skin depth of radio waves in seawater [2]. The operating principle of the SEW antenna is illustrated in Figure1a. The electric field of the SEW has a nonzero component in the longitudinal direction, which means that a good SEW antenna needs to be located near of a conductive surface, and it needs to produce a strong field enhancement J.at Mar. its Sci. tip, Eng. which 2020, will8, x FOR push PEER charges REVIEW along the conductive surface. When such an antenna is adapted 3 of 11 for surface wave-based underwater communication, it is encapsulated in an impedance matching inenclosure, seawater whichover long is filled distances, with which de-ionized considerably water, as exceed illustrated the skin in Figuredepth 1ofb. radio This waves enclosure in seawater enables [2].reduction of the antenna dimensions by approximately factor of 9 compared to the dimensions of similarThe antenna operating in freeprinciple space. of The the enclosureSEW antenna also improvesis illustrated coupling in Figure of electromagnetic 1a. The electric energy field of to the the SEWsurrounding has a nonzero seawater, component since compared in the longitudinal to the air/seawater direction, interface, which means an interface that a good between SEW de-ionized antenna needswater to and be seawaterlocated near is much of a conductive better impedance-matched. surface, and it needs In addition, to produce it also a strong reduces field the enhancement ohmic losses atwhich its tip, would which arise will due push to charges the immersion along the of theconductive antenna surface. in seawater. When Examples such an ofantenna such antenna is adapted designs for surfaceoptimized wave-based for operation underwater in the 50 MHzcommunication, and 2 MHz bandsit is encapsulated are presented in in an Figure impedance2. matching enclosure,A tuning which procedure is filled of with the surfacede-ionized wave water, antenna as isillustrated illustrated in in Figure3 1b.. It illustratesThis enclosure measurements enables reduction of the antenna dimensions by approximately factor of 9 compared to the dimensions of of S11 of the 2.45 GHz helical SEW antennas as a function of distance to a large planar conductive similarsurface. antenna As illustrated in free space. in Figure The 3enclosureb, depending also im onproves the location coupling of theof electromagnetic tapping point, energy the radiative to the surroundingbehavior of seawater, the antenna since may compared be optimized to the forair/seaw eitherater surface interface, wave an radiation interface or between radiation de-ionized into free waterspace. and The seawater antenna is tuning much was better also impedance-matche checked by maximizingd. In addition, the received it also video reduces signal the outside ohmic a losses closed whichFaraday would cage, arise as described due to the in detail immersion in [1]. Aof comprehensivethe antenna in description seawater. Examples of antenna of geometry such antenna shown designsin Figure optimized1a and its for fabrication, operation tuning in the 50 and MHz testing and may 2 MHz be found bands in are [1 ,presented2]. in Figure 2.

(a) (b)

FigureFigure 1. 1. OperationOperation of of the the SEW SEW antenna antenna near near a aconducto conductorr/dielectric/dielectric interface interface where where the the antenna antenna is is placedplaced either either on on the the dielectric dielectric (air) (air) side side of of the the inte interface,rface, or or on on the the conductor conductor (seawater) (seawater) side side near near the the interface:interface: ( (aa)) SchematicSchematic geometrygeometry of of a 2.4a GHz2.4 GHz surface surface wave wave antenna antenna design baseddesign on based helical on monopole helical monopoleshorted to shorted its feed lineto its outer feed conductor. line outer The conductor. tip of the The antenna tip isof shownthe antenna near a flatis shown conductive near surfacea flat conductivewhere it excites surface an omnidirectionalwhere it excites surface an omni electromagneticdirectional wave.surface The electromagnetic electromagnetic wave. field ofThe the electromagneticsurface electromagnetic field of the wave surfac ise partially electromagnetic longitudinal, wave which is partially means longitudinal, that an effi cientwhich surface means wavethat anantenna efficient needs surface a strong wave field antenna enhancement needs a at strong its apex, field which enhancement “pushes” charges at its apex, along which the metal “pushes” surface; charges(b) Principle along of the operation metal ofsurface; a similar (b underwater) Principle surfaceof operation electromagnetic of a similar wave underwater RF transmitter. surface electromagnetic wave RF transmitter.

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J. Mar. Sci. Eng. 2020, 8, 370 4 of 10 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 4 of 11

(a) (b)

Figure 2. (a) SEW underwater antennas attached to Yaesu VX-8 radios operated at 50 MHz. The impedance-matching enclosures are filled with de-ionized water; (b) Assembled surface wave underwater antenna operating in the 2 MHz band attached to a Yaesu FT-857 radio operated at 5 W output power. The impedance-matching enclosure (seen in the bottom section of the assembly) is filled with de-ionized water.

A tuning procedure of the surface wave antenna is illustrated in Figure 3. It illustrates (a) (b) measurements of S11 of the 2.45 GHz helical SEW antennas as a function of distance to a large planar conductiveFigure 2. surface.(a()a SEW) SEW Asunderwater underwater illustrated antennas antennasin Figure attached attached3b, depending to Yaesu to Yaesu VX-8 on VX-8 the radios location radios operated operated of the at 50tapping at MHz. 50 MHz. Thepoint, the radiativeTheimpedance-matching impedance-matching behavior of the enclosures antenna enclosures aremay arefilled be ﬁlled optimizewith with de-ionized de-ionizedd for either water; water; surface (b (b) )Assembled Assembled wave radiation surface surface or wave radiation intounderwater free space. antenna The antenna operating operating tuning in in the the was 2 2 MHz MHzalso checkeband band a attachedttachedd by maximizing to to a a Yaesu Yaesu FT-857 the received radio operated video signal at 5 W outside a closedoutput Faraday power.power. Thecage,The impedance-matching impedance-matc as described inhing detail enclosure enclosure in [1]. (seen A(seen comprehensive in thein the bottom bottom section description section of the of assembly)the of antennaassembly) is ﬁlled geometry is shownwithfilled in de-ionizedwith Figure de-ionized 1a water. and water. its fabrication, tuning and testing may be found in [1,2].

A tuning procedure of the surface wave antenna is illustrated in Figure 3. It illustrates measurements of S11 of the 2.45 GHz helical SEW antennas as a function of distance to a large planar conductive surface. As illustrated in Figure 3b, depending on the location of the tapping point, the radiative behavior of the antenna may be optimized for either surface wave radiation or radiation into free space. The antenna tuning was also checked by maximizing the received video signal outside a closed Faraday cage, as described in detail in [1]. A comprehensive description of antenna geometry shown in Figure 1a and its fabrication, tuning and testing may be found in [1,2].

(a) (b)

of the antenna; (b) Tuning of the fabricated helical antennas resonant at 2.45 GHz via measurements of

S11 as a function of distance from the large copper plane. The tuning parameter is the tapping point of a feeding coaxial line. The red, green and blue curves correspond to diﬀerent positions of the tapping point on the same antenna. Behavior of a conventional dipole antenna is presented for a comparison.

(a) (b)

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Figure 3. (a) Measurements of S11 of the fabricated helical antenna resonant at 2.45 GHz near a large copper plane. The micro-positioning stage is located below the copper plane. The inset shows a photo of the antenna; (b) Tuning of the fabricated helical antennas resonant at 2.45 GHz via measurements

of S11 as a function of distance from the large copper plane. The tuning parameter is the tapping point of a feeding coaxial line. The red, green and blue curves correspond to different positions of the J. Mar. Sci. Eng. 2020, 8, 370 5 of 10 tapping point on the same antenna. Behavior of a conventional dipole antenna is presented for a comparison. 3. Results 3. Results 3.1. Broadband Transmission through Faraday Cage 3.1. Broadband Transmission through Faraday Cage The performance of the designed SEW antenna in the 2.4 GHz band has been tested by verifying The performance of the designed SEW antenna in the 2.4 GHz band has been tested by verifying Wi-Fi video signal transmission through a 90 dB isolation Faraday cage, as illustrated in Figure4. Wi-Fi video signal transmission through a −−90 dB isolation Faraday cage, as illustrated in Figure 4. The video signal was generated inside a locked Faraday cage and transmitted through free space live. The video signal was generated inside a locked Faraday cage and transmitted through free space live. There was no cabling or connecting ground between the transmitter and receiver. The video signal There was no cabling or connecting ground between the transmitter and receiver. The video signal received outside the enclosure by a similar antenna at a distance on the order of 10 to 100 cm was received outside the enclosure by a similar antenna at a distance on the order of 10 to 100 cm was displayeddisplayed on on a livea live TV TV monitor. monitor.

(a) (b)

FigureFigure 4. 4.(a ()a SEW) SEW antenna antenna maintains maintains transmissiontransmission of video signal signal from from a a locked locked −9090 dB dB Faraday Faraday cage cage − (JRE(JRE Test, Test, model model 0709). 0709). (b ()b Conventional) Conventional dipole dipole antenna antenna cannot cannot transmit transmit thethe videovideo signalsignal whenwhen used in thein same the same experimental experimental conﬁguration. configuration.

TheThe surface surface wave wave mediated mediated mechanism mechanism of 2.4 GHz video video signal signal transmission transmission has has been been verified verified byby measurements measurements of of the the transmitted transmitted signalsignal near the Faraday cage cage as as a afunction function of of distance distance from from the the outsideoutside wall wall of of the the cage cage [1 ].[1]. The The surface surface wave wave character character of of the the transmitted transmitted signal signal waswas confirmedconfirmed byby an exponentialan exponential decay decay of the of transmitted the transmitted signal signal outside outside of the of cage. the cage. However, However, the signal the signal received received farther awayfarther from away the cagefrom was the cage a conventional was a conventional TEM signal, TEM which signal, originated which originated due to the due transmitted to the transmitted SEW field SEW field reaching the cage corners and scattering into the conventional TEM fields. reaching the cage corners and scattering into the conventional TEM fields.

3.2.3.2. Broadband Broadband Underwater Underwater RF RF Communication Communication Experiments in in Laboratory Laboratory Settings Settings It is obvious that the experiments with a Faraday cage depicted in Figure 4 above are It is obvious that the experiments with a Faraday cage depicted in Figure4 above are topologically topologically equivalent to the experiments with the same 2.4 GHz SEW antennas performed in a equivalent to the experiments with the same 2.4 GHz SEW antennas performed in a seawater aquarium, seawater aquarium, which are depicted in Figure 5. In these experiments, the seawater surrounding which are depicted in Figure5. In these experiments, the seawater surrounding the Wi-Fi video the Wi-Fi video transmitter (which is enclosed in a watertight plastic case) plays the role of a Faraday transmitter (which is enclosed in a watertight plastic case) plays the role of a Faraday cage. cage. Note that the skin depth of seawater at 2.4 GHz is 3 mm, while the thickness of seawater layer around the watertight plastic case was at least 15 cm on each side. Note also that water turbidity did not aﬀect video signal transmission, as illustrated in Figure5b. Thus, similar to transmission through a Faraday cage, the SEW antenna clearly demonstrates increased capacity for Wi-Fi transmission through seawater. This increased capacity may be understood based on the theoretical values for SEW propagation length Lr along the seawater-air interface, and the penetration depth Lz of SEW ﬁeld into the seawater [2]. Based on the detailed theoretical consideration in [6,9], they are given by the following expressions: λ0 Lz , (4) ≈ 4π √ε00 J. Mar. Sci. Eng. 2020, 8, 370 6 of 10

and λ0ε00 Lr , (5) ≈ π respectively [4], where λ0 is the free space wavelength, and ε” is the imaginary part of the dielectric constant of saltwater. For example, at 50 MHz the theoretical SEW propagation distance is quite large (Lr = 60 m), while the communication depth Lz may reach several meters assuming Tx operation down to 90 dB relative signal level. These distances appear to be much larger than the 3.8 cm skin depth of − seawater at 50 MHz. These observations are illustrated in Figure6, which demonstrates the radio ﬁeld distribution near the seawater surface, which is produced by a point source of radio waves located near the air/seawater interface (these simulations were performed using the RF module of COMSOL Multiphysics). At some distance from a source (which is much longer than the bulk skin depth of seawater) the RF ﬁeld is dominated by the SEW contribution, which enables radio communication from point A to point B. This communication would be impossible in the absence of the surface wave. J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 6 of 11

(a) (b)

FigureFigure 5. 5.(a ()a Similar) Similar to to experiments experiments depicteddepicted in Figure Figure 4,4, a 2.4 GHz GHz su surfacerface wave wave antenna antenna transmits transmits videovideo signal signal from from inside inside an an aquarium aquarium ﬁlled filled with with seawater; seawater; ( b(b)) The The video video transmissiontransmission isis notnot aaffectedﬀected by J. Mar.waterby Sci. water turbidity.Eng. 2020turbidity., 8, x FOR PEER REVIEW 7 of 11

Note that the skin depth of seawater at 2.4 GHz is 3 mm, while the thickness of seawater layer around the watertight plastic case was at least 15 cm on each side. Note also that water turbidity did not affect video signal transmission, as illustrated in Figure 5b. Thus, similar to transmission through a Faraday cage, the SEW antenna clearly demonstrates increased capacity for Wi-Fi transmission through seawater. This increased capacity may be understood based on the theoretical values for SEW propagation length Lr along the seawater-air interface, and the penetration depth Lz of SEW field into the seawater [2]. Based on the detailed theoretical consideration in [6,9], they are given by the following expressions: λ L ≈ 0 z π ε (4) 4 " , and λ ε" L ≈ 0 r π (5) ,

respectively [4], where λ0 is the free space wavelength, and ε” is the imaginary part of the dielectric constant of saltwater. For example, at 50 MHz the theoretical SEW propagation distance is quite large (Lr = 60 m), while the communication depth Lz may reach several meters assuming Tx operation down to −Figure90Figure dB 6.relative 6.Numerical Numerical signal simulations simulations level. These ofof radiodistancesradio ﬁeldfield appear distribution to be near nearmuch the the largerair/seawater air/seawater than theinterface, interface, 3.8 cm which skin which isdepth is of seawaterproducedproduced at by by50 a aMHz. point point sourceThesesource observations located near near the theare sea seaillu su surface.stratedrface. At in large At Figure large distances 6, distances whic fromh demonstrates froma SEW a antenna, SEW antenna, the the radio fieldthe fielddistribution ﬁeld is is dominated dominated near bythe by the theseawater SEW SEW contribution. contribution. surface, which Th Theseese issimulations simulationsproduced wereby were a performed point performed source using using of COMSOL radio COMSOL waves locatedMultiphysicsMultiphysics near the solver. air/seawatersolver. interface (these simulations were performed using the RF module of COMSOL Multiphysics). At some distance from a source (which is much longer than the bulk skin depthWe of should seawater) also notethe RF that field waviness is dominated of the seawater–air by the SEW interface contribution, may further which promote enables coupling radio communicationof the electromagnetic from point energy A tointo point the B.SEW This modes. comm Suunicationch an increased would be coupling impossible is well in the established absence ofin thethe surfaceclosely relatedwave. field of plasmonics [6], and it was observed in our model experiments performed in a seawater aquarium at 2.4 GHz (see Figure 7). These simple experiments indicate that an agitated sea state may not necessarily present a problem for SEW-based broadband underwater RF communication.

(a) (b)

Figure 7. (a) The Wi-Fi underwater video transmitter is moved beyond the free space communication range in a seawater aquarium with still water surface; (b) The video link is re-established when the seawater surface is agitated.

Directionality of SEW beams, which is also well known in plasmonics [6,10] may further improve performance of underwater RF communication links, since it may to some extent alleviate deterioration of link performance due to high propagation losses in seawater. We were able to demonstrate directional excitation and propagation of SEW waves in model experiments performed in a freshwater aquarium in laboratory settings, as illustrated in Figure 8.

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Figure 6. Numerical simulations of radio field distribution near the air/seawater interface, which is produced by a point source located near the sea surface. At large distances from a SEW antenna, the field is dominated by the SEW contribution. These simulations were performed using COMSOL J. Mar.Multiphysics Sci. Eng. 2020, solver.8, 370 7 of 10

We should also note that waviness of the seawater–air interface may further promote coupling of theWe electromagnetic should also note energy that wavinessinto the SEW of the modes. seawater–air Such an interface increased may coupling further is promote well established coupling ofin the electromagneticclosely related field energy of plasmonics into the SEW [6], modes. and it Suchwas observed an increased in our coupling model isexperiments well established performed in the closelyin a seawater related aquarium ﬁeld of plasmonics at 2.4 GHz[ (see6], and Figure it was 7). observedThese simple in our experiments model experiments indicate that performed an agitated in a seawatersea state aquariummay not atnecessarily 2.4 GHz (see present Figure 7a). prob Theselem simple for experimentsSEW-based indicatebroadband that underwater an agitated seaRF statecommunication. may not necessarily present a problem for SEW-based broadband underwater RF communication.

(a) (b)

Figure 7. ((aa)) The The Wi-Fi Wi-Fi underwater underwater video transmitter is moved beyond the free space communication range in a seawater aquarium with still water surface; (b) The video link is re-establishedre-established when the seawater surface is agitated.

Directionality ofof SEW SEW beams, beams, which which is also is also well well known known in plasmonics in plasmonics [6,10] may [6,10] further may improvefurther performanceimprove performance of underwater of underwater RF communication RF communication links, since links, it may since to some it may extent to alleviatesome extent deterioration alleviate ofdeterioration link performance of link due performance to high propagation due to losseshigh propagation in seawater. Welosses were in able seawater. to demonstrate We were directional able to excitationdemonstrate and directional propagation excitation of SEW and waves propagatio in modeln experiments of SEW waves performed in model in experiments a freshwater performed aquarium inJ. laboratoryaMar. freshwater Sci. Eng. settings,2020 aquarium, 8, x FOR as illustrated PEERin laboratory REVIEW in Figure settings,8. as illustrated in Figure 8. 8 of 11

(a) (b)

FigureFigure 8. 8.( a(a)) 400 400 MHz MHz SEW SEW ﬁeld field of of an an antenna antenna array array submerged submerged into into a a fresh-water fresh-water aquarium aquarium is is probed probed byby a a distant distant dipole dipole receiver;receiver; ((bb)) AntennaAntenna arrayarray field ﬁeld measured in in the the transverse transverse direction direction at at 12 12 and and 24 24cm cm from from the the array. array. The The inset inset shows shows numerical numerical mode modelingling of SEW of directional SEW directional beaming beaming from the from antenna the antennaarray. array.

InIn these these experiments, experiments, we we have have used used an an array array of of four four dipole dipole antennas antennas spaced spaced at at 5 5 cm cm distances, distances, whichwhich resonate resonate at at 2.4 2.4 GHz GHz in in air. air. After After the the array array was was submerged submerged into into a a freshwater freshwater aquarium, aquarium, its its resonantresonant frequency frequency shifted shifted to 400to 400 MHz. MHz. The antennaThe antenna array array field was field probed was probed by a distant by a dipole distant receiver dipole identicalreceiver toidentical individual to individual antennas inantennas the array, in asthe illustrated array, as inillustrated Figure8a. in It Figure was verified 8a. It was that verified there was that nothere relevant was no coupling relevant between coupling the between feed lines the feed above lin water.es above The water. antenna The antenna array field array measured field measured in the transversein the transverse direction direction at two distancesat two distances from the from array the isarray plotted is plotted in Figure in Figure8b, which 8b, which also shows also shows our numericalour numerical modeling modeling of SEW of beaming.SEW beaming.

3.3. Field Testing of Broadband Underwater RF Communication The field performance of the developed SEW antennas has been tested at an underwater testing facility near Panama City, Florida (average water salinity 3.0%) [2]. The SEW antennas were tested in the 50 MHz band. The seawater testing environment was sufficiently large, so that no significant boundary effects were present. These tests were conducted using separate battery-operated transmitting (TX) and receiving (RX) antenna and radio systems, which were enclosed in watertight containers shown in Figure 2a. The underwater radio systems were operated by divers as illustrated in Figure 9a. The divers verified their respective depth and distance from each other using fixed markers made of buoys and ropes. The signal propagation data were read by the divers from the LED indicator and the S-meter of the Yaesu radios. These data were reported by the divers to the test personnel, which was located on a nearby vessel. The measured averaged measured link probability data are plotted in Figure 9b. The skin depth at 50 MHz in seawater (3.8 cm) is shown near the bottom left corner of the plot for comparison. These results clearly demonstrate that the novel underwater SEW antennas described above enable radio communication over range/depth combinations which go far beyond the known skin depth of seawater. Note that the relatively large variations of the link probability observed in our experiments may be explained by the variations in seawater salinity during the experiments and changes in the sea state (the seawater ripples), bubbles and biological objects, which scatter the surface electromagnetic waves.

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3.3. Field Testing of Broadband Underwater RF Communication The field performance of the developed SEW antennas has been tested at an underwater testing facility near Panama City, Florida (average water salinity 3.0%) [2]. The SEW antennas were tested in the 50 MHz band. The seawater testing environment was sufficiently large, so that no significant boundary effects were present. These tests were conducted using separate battery-operated transmitting (TX) and receiving (RX) antenna and radio systems, which were enclosed in watertight containers shown in Figure2a. The underwater radio systems were operated by divers as illustrated in Figure9a. The divers verified their respective depth and distance from each other using fixed markers made of buoys and ropes. The signal propagation data were read by the divers from the LED indicator and the S-meter of the Yaesu radios. These data were reported by the divers to the test personnel, which was located on a nearby vessel. The measured averaged measured link probability data are plotted in Figure9b. The skin depth at 50 MHz in seawater (3.8 cm) is shown near the bottom left corner of the plot for comparison. These results clearly demonstrate that the novel underwater SEW antennas described above enable radio communication over range/depth combinations which go far beyond the known skin depth of seawater. Note that the relatively large variations of the link probability observed in our experiments may be explained by the variations in seawater salinity during the experiments and changes in the sea state (the seawater ripples), bubbles and biological objects, which scatter the surface electromagnetic waves. J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 9 of 11

sea surface 0.0 1.000 0.9000 0.8000 0.7000 0.5 0.6000 0.5000 0.4000 0.3000 1.0 0.2000 0.1000 0 1.5 link probability Depth (m) Depth 2.0

2.5 sea floor 012345 skin depth at 50 MHz is 3.8 cm Distance (m) (shown to the scale)

(a) (b)

FigureFigure 9. 9.(a )( Photoa) Photo of the of underwater the underwater test range test used range in ourused experiments. in our experiments. The inset shows The experimentalinset shows conﬁguration;experimental (configuration;b) Contour plot (b of) Contour the link probabilityplot of the measuredlink probability in seawater measured as a functionin seawater of diver as a depthfunction and of distance diver depth between and distance the divers. between The sea the ﬂoor divers. was The located sea floor at 9 was m depth. located The at 9 skin m depth. depth The at 50skin MHz depth in seawater at 50 MHz is shownin seaw toater the is scale. shown to the scale.

TheThe underwater underwater experiments experiments depicteddepicted inin FigureFigure9 9were were conducted conducted without without any any external external cables, cables, inin order order to to exclude exclude any any possibility possibility of of a a spurious spurious crosstalk. crosstalk. All All the the transmitter transmitter and and receiver receiver radios radios and and antennasantennas werewere placedplaced underwaterunderwater asas shownshown inin thethe inset inset in in Figure Figure9 a.9a. In In the the absence absence of of a a network network analyzeranalyzer underwater, underwater, the the values values of of S 11S11and and S S2222 werewere notnot measuredmeasured duringduring thethe experimentsexperiments depicted depicted in in FigureFigure9 .9. However, However, these these values values were were measured measured in thein the lab lab in ain seawater a seawater tank, tank, as depicted as depicted in Figure in Figure 10. 10. While the observed SEW signal propagation at 50 MHz was considerably below the theoretically projected Lr = 60 m, we anticipate that further optimization of the SEW antenna will result in reaching the theoretical depth and distance limits, which are described by Equations (4) and (5). These performance limits are summarized in Table1 for the set of RF bands explored in this paper. Note that predictions given by Equation (5) may not be reliable at smaller frequencies due to the fact that it was derived for a planar conductor-dielectric interface.

Figure 10. S11 and S22 of the antennas used in the experiments depicted in Figure 9. These parameters were measured in the lab in a seawater tank. A de-ionized water antenna enclosure was implemented during these measurements.

While the observed SEW signal propagation at 50 MHz was considerably below the theoretically projected Lr = 60 m, we anticipate that further optimization of the SEW antenna will result in reaching the theoretical depth and distance limits, which are described by Equations (4) and (5). These performance limits are summarized in Table 1 for the set of RF bands explored in this paper. Note that predictions given by Equation (5) may not be reliable at smaller frequencies due to the fact that it was derived for a planar conductor-dielectric interface.

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sea surface 0.0 1.000 0.9000 0.8000 0.7000 0.5 0.6000 0.5000 0.4000 0.3000 1.0 0.2000 0.1000 0 1.5 link probability Depth (m) Depth 2.0

2.5 sea floor 012345 skin depth at 50 MHz is 3.8 cm Distance (m) (shown to the scale)

(a) (b)

Figure 9. (a) Photo of the underwater test range used in our experiments. The inset shows experimental configuration; (b) Contour plot of the link probability measured in seawater as a function of diver depth and distance between the divers. The sea floor was located at 9 m depth. The skin depth at 50 MHz in seawater is shown to the scale.

The underwater experiments depicted in Figure 9 were conducted without any external cables, in order to exclude any possibility of a spurious crosstalk. All the transmitter and receiver radios and antennas were placed underwater as shown in the inset in Figure 9a. In the absence of a network analyzer underwater, the values of S11 and S22 were not measured during the experiments depicted in Figure 9. However, these values were measured in the lab in a seawater tank, as depicted in Figure J. Mar. Sci. Eng. 2020, 8, 370 9 of 10 10.

Figure 10. S11 andand S S2222 ofof the the antennas antennas used used in in the the experiments experiments depicted depicted in Figure 99.. TheseThese parametersparameters were measured inin the lab in a seawater tank. A de-ionized water antenna enclosure was implemented during these measurements. Table 1. Summary of the theoretically predicted propagation range Lr and communication depth Lz While the observed SEW signal propagation at 50 MHz was considerably below the theoretically (@ 5W transmit power) at the RF bands explored in this paper. projected Lr = 60 m, we anticipate that further optimization of the SEW antenna will result in reaching the theoretical depth and distance2.4 GHz limits, Band which 50 MHzare described Band 2by MHz Equations Band (4) and (5). These performance limits are Lzsummarized0.054 in Table m 1 for the 0.7 set m of RF bands 3.5 explored m in this paper. Note that predictions given byLr Equation (5)3.8 may m not be reliable 60 m at smaller900 frequencies km 1 due to the fact that it was derived1 Theoretical for a planar numbers co givennductor-dielectric by Equation (5) may interface. not be reliable in this band due to Earth curvature.

4. Discussion and Conclusions The communication distance/depth combinations summarized in Table1 provide quite an optimistic outlook for potential applications of SEW antennas in underwater communication between divers and UUVs, since even larger communication depth may be achieved at lower frequencies. For example, it appears that Lz~15 m is achievable in the 0.1 MHz band at quite modest 5 W transmit power. Our experimental and theoretical results appear to be novel and important since, until very recently, the general belief was that broadband RF communication through seawater is impossible over any practical distance. We anticipate that further development of SEW antennas and further optimization of the antenna parameters will result in reaching the theoretical limits on underwater depth and communication distance, which are described by Equations (4) and (5). These developments will enable novel technology for wide bandwidth radio signal communication through seawater. We should also note that our SEW-based RF communication scheme should be able to breach the seawater barrier for UAV to UUV communication, since the surface wave EM field is present both above and below the seawater surface. This would enable, for example, a drone skimming the surface of the water to pick up signal transmitted from a UUV. The described antennas may also be made multi-spectral so that the communication bandwidth at a given distance/depth combination may be optimized under software control, shifting to larger bandwidths over shorter distances. Our communication scheme may also find applications in frogman to frogman communication, underwater object detection, UUV swarming and mesh networked UUVs, offshore oil platforms, etc. We also expect that our technology will enable considerable improvements in Wi-Fi connectivity in buildings and underground tunnels. Remote examination of metal and partially metal enclosures, such as shipping containers and metallic test chambers, should also become possible in the near future. J. Mar. Sci. Eng. 2020, 8, 370 10 of 10

Author Contributions: I.S., Q.B. and D.Y. contributed equally to this work. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Smolyaninov, I.I.; Balzano, Q.; Young, D. Surface wave-based radio communication through conductive enclosures. Prog. Electromagn. Res. M 2019, 85, 21–28. [CrossRef] 2. Smolyaninov, I.I.; Balzano, Q.; Davis, C.C.; Young, D. Surface wave-based underwater radio communication. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 2503–2507. [CrossRef] 3. Smolyaninov, I.I. Communication and Sensor Techniques for Underwater Radio Communication. U.S. Patent Application Publication # US 20180198536, 7 December 2018. 4. Cheng, D.K. Fundamentals of Engineering Electromagnetics; Pearson: London, UK, 1992; Chapter 8. 5. Bethe, H.A. Theory of diffraction by small holes. Phys. Rev. 1944, 66, 163–182. [CrossRef] 6. Zayats, A.V.; Smolyaninov, I.I.; Maradudin, A. Nano-optics of surface plasmon-polaritons. Phys. Rep. 2005, 408, 131–314. [CrossRef] 7. Ebbesen, T.W.; Lezec, H.J.; Ghaemi, H.F.; Thio, T.; Wolff, P.A. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998, 391, 667–669. [CrossRef] 8. Elliott, J.; Smolyaninov, I.I.; Zheludev, N.I.; Zayats, A.V. Polarization control of optical transmission of a periodic array of elliptical holes in a metal films. Opt. Lett. 2004, 29, 1414–1416. [CrossRef][PubMed] 9. Michalski, K.A.; Mosig, J.R. The Sommerfeld half-space problem revisited: From radio frequencies and Zenneck waves to visible light and Fano modes. J. Electromagn. Waves Appl. 2016, 30, 1–42. [CrossRef] 10. Smolyaninov, I.I.; Mazzoni, D.L.; Davis, C.C. Imaging of surface plasmon scattering by lithographically created individual surface defects. Phys. Rev. Lett. 1996, 77, 3877–3880. [CrossRef][PubMed]

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