Investigation on Radio Wave Propagation in Shallow Seawater: Simulations and Measurements Eugenio Jimenez, Gara Quintana, Santiago Zazo, Marina Perez Eduardo Quevedo Pablo Mena, Pablo Dorta and ETSI Telecomunicacion, Oceanic Platform of the Canary Ivan Perez-Alvarez Universidad Politecnica de Madrid, Islands (PLOCAN), Institute for Technological Development Madrid 28040, Spain. Telde 35200, Spain. and Innovation in Communications (IDeTIC), e-mail: [email protected] e-mail: [email protected] Universidad de Las Palmas de Gran Canaria, e-mail: [email protected] Las Palmas, 35017, Spain. e-mail: [email protected] Abstract—The authors present full wave simulations and receiver. In our case both transmitting and receiving antennas experimental results of propagation of electromagnetic waves in will be placed on the seabed as it is illustrated in Fig I. shallow seawaters. Transmitter and receiver antennas are ten- Full wave analysis of propagating EM waves in two-layers turns loops placed on the seabed. Some propagation frameworks are presented and simulated. Finally, simulation results are geometries was firstly carried out by A. Sommerfeld at the compared with experimental ones. begining of the XX century. Later, his work was extended to Index Terms—Conducting medium; underwater loop anten- multilayer geometries and in [1], [2] and [3] full wave analysis nas; EM wave propagation; shallow seawaters. of geometries with two, three and more layers and applications are well summarized. I. INTRODUCTION ε µ Electromagnetic propagation through sea water is very dif- AIR 0 0k 0 ferent from propagation through air because of water’s high permittivity and electrical conductivity. Plane wave attenuation is higher through water, and increases rapidly with frequency. h d With a relative permittivity of about ǫr=80, water has the highest permittivity of any material and this has a significant Tx ε σ µ k Rx impact on the angle of refraction at the air/water interface. SEA 1 1 10 Conductivity of seawater is typically around 5S/m, while nom- inally fresh water conductivity is quite variable but typically in ε σ µ SEABED 2 20 k 2 the mS/m range. Relative permeability is approximately µr=1 so there is little direct effect on the magnetic field component Fig. 1. Geometric configuration of testbed. but conduction leads to strong attenuation of electromagnetic propagating waves. Another important consideration is the effect of the air-to- II. SIMULATION FRAMEWORKS water interface. Propagation losses and the refraction angle Along this communication, four different frameworks are are such that an electromagnetic signal can cross the air- going to be simulated. to-water boundary and appears to radiate from an antenna directly placed in the air above the transmitter. This effect • Attenuation between two horizontal loops placed in free arXiv:1604.03694v1 [physics.ao-ph] 13 Apr 2016 aids communication from a submerged station to land and space. between shallow submerged stations without the need for • Attenuation between two horizontal loops inmersed in sea surface repeater buoys. The air path can be a key advantage. water. For example, if two divers are 1km apart at 2m below the • Attenuation between two horizontal loops placed on surface, attenuation will be significantly less than anticipated seabed to sea water interface without air layer; two layers from the 1km through-water loss. problem. A similar effect is seen at the seabed, where its conductivity • Attenuation between two horizontal loops placed on is lower than the water one. The seabed is an alternative low- seabed to sea water interface with an air layer over sea loss, low-noise, communications path if both transmitter and water; three layers problem. receiver are placed on the seabed. In all cases, transmitter and receiver antennas are of the In many deployments a single propagation path will be same kind: a 22cm. radius ten turns loop antenna made of dominant depending on the placement of transmitter and copper and isolated using a 1mm. teflon like coating. Sea water σωµ is modelled as a dielectric with permittivity ǫr=81 and conduc- conductivity (attenuation constant α = 2 Neper/m). This tivity σ=4.5 S/m. Seabed, fine sand, is modelled as a dielectric is true for frequencies over 100 kHzp but not so true for with permittivity ǫr=3.5 and conductivity σ=1 S/m [4]. Height frequencies between 10 kHz and 100 kHz. For low frequencies of sea water layer is set to h=4 m. and distances or for very low frequencies, attenuation de- Simulations are carried out using a commercial MoM creases with frequency in both cases: free space and sea water, solver: FEKO. This tool supports the features needed for and it seems to be independent of the electrical properties of this analysis: planar Green functions for multilayered media, the medium. In this case a magnetostatic approach can explain dielectric coated wires and special basis functions for low this behaviour. frequency analysis. For antennas in free space and frequencies over 100 kHz, Simulations are carried out at five different distances (d=2, atenuattion for each frequency increases 18 dB when doubling 3, 4, 5 and 6 meters) with frequency sweeps from 10 kHz to distance. It is a typical near field dependence (eg 1/R3). 1 MHz. Electrically small loop antennas work as vertical magnetic dipoles. The electromagnetic fields generated by this source [1] III. SIMULATION RESULTS are (cylindrical coordinates): A. Antennas inmersed in homogeneous medium 1 ρz jk2 3k 3j In these simulations, antennas are radiating into two homo- = − − jkr Bρ 4 2 2 3 e geneous mediums: free space and sea water. Results of those πω r r r r 1 2 2 2 3 3 simulations are shown in Fig 2 and in Fig 3. jk k j z jk k j jkr Bz = − − − − − − e 4πω r r2 r3 r2 r r2 r3 Attenuation between two ten-turns loops. 1 1 Free space vs. sea water (σ=4.5 ε =81) ρ jk jkr r Eφ = − − e 4π r r r2 -70 -80 These equations clearly show the aforementioned be- haviours: mainly magnetic field for low frequencies and dis- -90 tances and 1/R3 near field dependence for low distances. -100 B. Antennas inmersed in layered medium -110 d=2m (free space) dB -120 d=2m (sea water) Now we are going to compare the results from simulation d=3m (free space) of antennas in sea water with simulations of antennas placed -130 d=3m (sea water) d=4m (free space) on seabed. Both layers, sea water and seabed, are semi-infinite -140 d=4m (sea water) d=5m (free space) so it is a two layer geometry. -150 d=5m (sea water) d=6m (free space) Results of those simulations are shown in Fig 4 and in Fig 5. -160 d=6m (sea water) 100 200 300 400 500 600 700 800 900 1000 Freq [kHz] Attenuation between two ten-turns loops placed on seabed σ ε σ ε Sea water: =4.5 r=81. Seabed (sand): =1.0 r=3.5 Fig. 2. Free space vs. sea water. Full sweep. -70 d=2m (sea water) d=2m (sea water - seabed) -80 d=3m (sea water) d=3m (sea water - seabed) Attenuation between two ten-turns loops. -90 d=4m (sea water) σ ε Free space vs. sea water ( =4.5 r=81) d=4m (sea water - seabed) -100 d=5m (sea water) d=5m (sea water - seabed) -70 -110 d=6m (sea water) dB d=6m (sea water - seabed) -120 -80 -130 -140 -90 d=2m (free space) -150 dB d=2m (sea water) -100 d=3m (free space) -160 d=3m (sea water) 100 200 300 400 500 600 700 800 900 1000 d=4m (free space) -110 d=4m (sea water) Freq [kHz] d=5m (free space) d=5m (sea water) d=6m (free space) Fig. 4. Sea water vs. two layer. Full sweep. -120 d=6m (sea water) 10 20 30 40 50 60 70 80 90 100 As it can be seen, the effect of seabed layer is to decrease Freq [kHz] the attenuation at all frequencies. This means that a surface Fig. 3. Free space vs. sea water. Low frequency sweep. wave, lateral wave, has been launched and energy is mainly travelling on the sea-seabed interface. Once again, for low fre- As it is expected for antennas inmersed in sea water, atten- quencies and distances or for very low frequencies, attenuation uation grows exponentially with frequency due to sea water seems to be independent from the medium. Attenuation between two ten-turns loops placed on seabed σ ε σ ε Sea water: =4.5 r=81. Seabed (sand): =1.0 r=3.5 Optical fiber Optical fiber Air -70 d=2m (sea water) d=2m (sea water - seabed) Signal Spectrum d=3m (sea water) generator analyzer d=3m (sea water - seabed) -80 d=4m (sea water) d=4m (sea water - seabed) 10Base−T 10Base−T to fiber to fiber d=5m (sea water) Sea -90 d=5m (sea water - seabed) d=6m (sea water) Control Control dB d=6m (sea water - seabed) board board -100 Loop antennas Batteries Batteries Seabed -110 Fig. 6. Experimental testbed setup. -120 10 20 30 40 50 60 70 80 90 100 Freq [kHz] pressure and temperature using the fiber link. The generator is controlled from an external computer using Keysight VEE Fig. 5. Sea water vs. two layer. Low frequency sweep. software. B. Receiver and antenna Full expressions for the fields generated by a vertical dipole The receiver is built using a handheld Keysight 9340B placed on the interface between two mediums can be found spectrum analyzer, a Beaglebone Black board, a 10Base-T in [1] and in [5].