electromagnetic wave propagation into fresh water

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Journal of Electromagnetic Analysis and Applications , 2011, 3, 261-266doi:10.4236/jemaa.2011.37042 Published Online July 2011 (http://www.SciRP.org/journal/jemaa)

Copyright 2011 SciRes. JEMAA

261

Electromagnetic Wave Propagation into Fresh

WaterShan Jiang, Stavros Georgakopoulos

Department of Electrical and Computer Engineering, Florida International University, Miami, USA.Email: [email protected], [email protected]

Received April 20 th, 2011; revised May 21 st, 2011; accepted June 2 nd, 2011.

ABSTRACT

Wireless communications from air to fresh water are studied here. Our analysis relies on plane wave propagation mod-els. Specifically , the transmission loss and propagation loss of RF waves penetrating into fresh water are calculated forvarious propagation depths. Even though RF wireless communications are not well suited for seawater due to its highattenuation , our paper illustrates that RF communications from air to fresh water are possible. Finally , this work de-rives the optimum frequencies , which provide minimum attenuation and maximum propagation depth , for RF commu-nications from air to fresh water.

Keywords : Wireless Communications , Propagation , Fresh Water , Optimum Frequency

1. Introduction

Underwater communications have attracted significantinterest in recent years since they have a wide range ofapplications including coastline protection, underwater

environmental observation for exploration, off-shoreoil/gas field monitoring, oceanographic data collection,autonomous underwater vehicles (AUVs) and remotelyoperated vehicles (ROVs), etc., [1]. Reliable data moni-toring and transmission in shallow fresh water (e.g., riv-ers and lakes) have also received growing interest sincethey can provide information that is crucial for the localeconomies as well as the environment. Traditionally,underwater communications have been done throughacoustic and optical systems that have certain advantagesand disadvantages. For example, acoustic communica-tions are widely used in underwater environment but

their performance in shallow waters is severely affected by multipath propagation. Also, the channel latencycaused by the low propagation velocity in water is an-other limitation of acoustic communications. On the con-trary, laser based optical systems have significantlyhigher propagation speed than underwater acousticwaves. However, strong backscattering caused by sus-

pended particles in water always limits the application ofoptical systems to very short distances.

Electromagnetic (EM) waves in the RF range can also be used for underwater wireless communication systems.

The velocity of EM waves in water is more than 4 ordersfaster than acoustic waves so the channel latency isgreatly reduced. In addition, EM waves are less sensitivethan acoustic waves to reflection and refraction effects inshallow water. Moreover, suspended particles have verylittle impact on EM waves. Few underwater communica-tion systems based on EM waves have been proposed

before [2,3].The primary limitation of EM wave propagation in

water is the high attenuation due to the conductivity ofwater. For example, it has been shown in [4] that con-ventional RF propagation works poorly in seawater dueto the losses caused by the high conductivity of seawater(typically, 4 S/m). However, fresh water has a typicalconductivity of only 0.01 S/m, which is 400 times lessthan the typical conductivity of seawater. Therefore, EMwave propagation can be more efficient in fresh water

than in seawater.This work focuses on the analysis of electromagnetic

waves penetrating from air to fresh water. The followingtwo types of losses are analyzed: 1) the transmission lossthat is due to reflection at the air-water interface, and 2)the propagation loss inside the water due to its material

properties. The electromagnetic properties of fresh waterare modeled using the Debye model. Also, the effects ofthe incidence angle as well as the polarization are exam-ined and an optimum frequency range is identified for RFair-to-water communications to minimize the power loss.


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Electromagnetic Wave Propagation into Fresh Water 263

1//

0 1

2 coscos cos

i

i t

T

(10)

1

1 0

2 coscos cos

i

i t

T

(11)

where i is the angle of incidence and t is the angle oftransmission, given by Snells law of refraction [6] as

2cos 1 sint i r .The transmission loss for oblique incidence can be

written as follows using (6) and (10)-(11):

2 0/ / 10 / /

1*10log Ret T

(12)

2 010

1*10log Ret T

(13)

The propagation loss for oblique incidence is writtenas:

2 /cos/ / 1010 log e

t d p p

(14)

Therefore, the total loss of the oblique incidence isgiven by

/ / / / / /total t p (15)

total t p (16)

and it depends on the complex permittivity of water aswell as the angle of incidence.

3. Results

The transmission loss as well as propagation loss for a plane wave propagating from air to water is analyzed inthe frequency range of 23 kHz to 1 GHz for normal inci-dence. This frequency range includes frequencies fromthe VLF (used in submarine communications) to theUHF band. Various propagation depths are consideredsince the propagation loss increases as the depth in-creases. In addition, the transmission loss remains thesame for all propagation depths.

Figure 2 illustrates that the transmission loss de-creases dramatically as the frequency increases from 23kHz to 10 MHz, and then remains almost constant forfrequencies higher than 10 MHz.

The propagation loss (see Figure 3 ) exhibits an oppo-site trend when compared to the trend of the transmissionloss. Specifically, Figure 3(a) shows that it increasesslowly for frequencies up to 100 MHz and then increasesdramatically for higher frequencies (especially for fre-quencies around 1 GHz).

Furthermore, the transmission loss and propagationloss are added together to obtain the total power loss for

104

105

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108

109

4

6

8

10

12

14

16

Frequency (Hz)

T r a n s m

i s s

i o n

L o s s

( d B )

Figure 2. Transmission loss for normal incidence.

104

105

106

107

108

109

0.1

1

10

100

300

Frequency (Hz)

P r o p a g a

t i o n

L o s s

( d B )

d=0.5 md=1 md=2 md=5 m

(a)

104

105

106

107

108

109

1

10

100

1000

5000

Frequency (Hz)

P r o p a g a

t i o n

L o s s

( d B )

d=10 md=20 md=50 md=100 m

(b)

Figure 3. Propagation loss for normal incidence at differentpropagation depths, d : (a) shallow and (b) deep.



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Electromagnetic Wave Propagation into Fresh Water264

the air-to-water propagation (see Figure 1 ) and are plot-ted in Figure 4 . As expected, due to the reverse varia-tions of the transmission and propagation losses, an op-timum frequency range exists for shallow propagationdepths. In this optimum frequency range, there is sig-nificantly smaller power loss. For example, in Figure4(a) , the total loss in 3 - 100 MHz frequency range for adepth of 1 m is 10 dB to 45 dB smaller than the loss atthe lowest and highest frequencies of our frequencyrange. Therefore, according to Figure 4(a) there is arange of optimum frequencies that exhibit minimumlosses when a wave propagates from air to shallow freshwater depths. This range of frequencies can improve RFcommunications with underwater vehicles, or devices.Potential applications that can benefit from operating inthe optimum frequency range include the following: 1)

104

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109

1

10

20

100

300

Frequency (Hz)

T o

t a l L o s s

( d B )

d=0.5 md=1 md=2 md=5 m

(a)

104

105

106

107

108

109

10

100

1000

5000

Frequency (Hz)

T o t a

l L o s s

( d B )

d=10 md=20 md=50 md=100 m

(b)

Figure 4. Total attenuation for normal incidence at differ-ent propagation depths, d : (a) shallow and (b) deep.

communications with underwater robots [7], 2) RFID based shore erosion detection [8], 3) water quality moni-toring of bodies of fresh water (e.g., temperature, pH,etc.), such as, artificial lakes, swimming pools, and watertanks, using wireless underwater sensors.

Figure 4(b) also illustrates that the total loss mono-tonically increases when the propagation depth is largerthan 10 m, thereby providing no optimum frequencyrange. This happens because the transmission loss is thesame for different propagation depths whereas the

propagation loss increases as the propagation depth be-comes larger. Therefore, as the depth increases it reachesa value for which the propagation loss becomes largerthan the transmission loss. Therefore, the optimum fre-quency range exists only for small propagation depths(less than 5 m).

Following similar steps to the normal incidence case

of water half-space, the total attenuation for oblique in-cidence is calculated across incident angles from 0 to 89degrees for both parallel and perpendicular polarizationcases. Figure 5 illustrates that normal incidence has theleast power loss for fixed frequency and propagationdepth, since the total attenuation increases along with theincident angle for both polarization cases. For example,for parallel polarization, the total loss increases slowlyfor angles up to 80 degrees, but increases dramaticallyfor larger incident angles.

Also, the average total loss across all incident anglesranging from 0 to 89 degrees is calculated for different

propagation depths. Figure 6 plots the average total lossversus frequency for both parallel and perpendicular po-larization cases. It can be concluded that the average totalloss for an oblique incident plane wave is larger than thenormal incidence case, but with similar trend. For exam-

ple, for a propagation depth of 1 m, the average total loss

0 10 20 30 40 50 60 70 80 905

10

15

20

25

30

35

40

45

Incident Angle (Degrees)

T o

t a l L o s s

( d B )

Parallel polarizationPerpendicular polarization

Figure 5. Total attenuation for oblique incidence when f =100 MHz, d = 1 m.



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Electromagnetic Wave Propagation into Fresh Water


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104

105

106

107

108

109

5

10

100

300

20

Frequency (Hz)

T o

t a l L o s s

( P a r a l l e

l )

d=0.5 md=1 md=2 md= 5 m

10

410

510

610

710

810

910

100

300

20

Frequency (Hz)

T o

t a l L o s s

( P e r p e n

d i c u

l a r )

d=0.5 md=1 md=2 md= 5 m

(a) (b)

Figure 6. Average total loss across all incidence angles from 0 to 89 degrees at different depths for: (a) parallel polarizationand (b) perpendicular polarization.

for perpendicular polarization (see Figure 6(b) ) in the 3MHz to 100 MHz frequency range is about 5 dB morethan the total loss for normal incidence (see Figure 4(a) ).From the results of Figure 6 for the oblique incidencecase we observe that there is also an optimum frequencyrange similar to the one we identified for the normal in-cidence case.

4. Conclusions

A plane wave model for the air-to-water interface has

been used to calculate the transmission loss, propagationloss as well as the total loss at various incident angles.The values of these losses depend on the electromagnetic

properties of water, frequency, incidence angle and the propagation depth. For such air-to-water communications,an optimum frequency range (3 - 100 MHz) was identi-fied when the plane wave propagates to depths less than5 m. In this optimum frequency range, the wave experi-ences significantly smaller losses than the losses at thelowest and highest frequencies of our analysis. Specifi-cally, this frequency range includes the bands of short-wave radio (3 - 30 MHz), VHF TV (54 - 72 MHz, 76 -

88 MHz), parts of FM (88 - 108 MHz) and US militaryVHF-FM (30 - 88 MHz). Therefore, various communica-tions systems can benefit from using the optimum opera-tion frequencies that we identified here. Also, wireless

power harvesting by wireless sensors can be significantlyenhanced if it is performed inside the 3 - 100 MHz range.

It should be also pointed out, that practical compactantenna designs can be developed in the optimum fre-quency range of 3 - 100 MHz for underwater devices dueto the large permittivity of water ( r = 81). For example,a half-wavelength loop antenna operating at 100 MHz

inside fresh water has a diameter of only 5.3 cm. There-fore, the optimum frequency range can be used in practi-cal underwater communication systems and it will pro-vide minimum losses for shallow propagation depths.

5. Acknowledgements

This work was supported by the Dissertation Year Fel-lowship provided by Florida International UniversitysGraduate School.

REFERENCES[1] L. Liu, S. Zhou and J. Cui, Prospects and Problems of

Wireless Communication for Underwater Sensor Net-works, Wiley WCMC Special Issue on Underwater Sen-sor Networks (Invited), 2008.

[2] J. H. Goh, A. Shaw, A. I. Al-Shanmmaa, UnderwaterWireless Communication System, Journal of Physics ,Conference Series 178, 2009.

[3] A. I. Al-Shammaa, A. Shaw and S. Saman, Propagationof Electromagnetic Waves at MHz Frequencies throughSeawater, IEEE Transactions on Antennas and Propa-tion, Vol. 52, No. 11, November 2004, pp. 2843-2849.

[4] S. Bogie, Conduction and Magnetic Signaling in theSea, Radio Electronic Engineering , Vol. 42, No. 10,1972, pp. 447-452. doi:10.1049/ree.1972.0076

[5] J. B. Hasted, Aqueous Dielectrics, Chapman and Hall, New York, 1973.

[6] C. A. Balanis, Advanced Engineering Electromagnetics,John Wiley & Sons, New York, 1989.

[7] T. Shaneyfelt, M. A. Joordens, K. Nagothu and M. Jam-shidi, RF Communication between Surface and Under-water Robotic Swarms, Automation Congress , Hawaii, 9December 2008.

[8] G. Benelli, A. Pozzebon and G. Ragseo, An RFID Based
http://dx.doi.org/10.1049/ree.1972.0076http://dx.doi.org/10.1049/ree.1972.0076http://dx.doi.org/10.1049/ree.1972.0076


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System for the Underwater Tracking of Pebbles on Artifi-cial Coarse Beaches, IEEE 2009 Third International

Conference on Sensor Technologies and Applications ,Athens, 18-23 June 2009, pp.294-200.


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