# Underwater Electromagnetic Sensor Networks--Part I: Link Characterization +
## 1 Introduction
On the contrary, in [8], it is stated the need of using low frequency carriers, up to 20 KHz, due to the dramatic rise in attenuation as frequency increases. In this case, with vertical dipoles, distances of up to 20 m were reached in real environment experiments, and these results were compared to theoretical ones. These antennas were also employed in [9], at 100 KHz and 14 MHz, in order to validate theoretical solutions with experimental measurements in near and intermediate field conditions. According to [PERSON] and [PERSON] [11], a parallel dipole antenna contained within a water-filled PVC barrel could excite water molecules to emit dipole radiation in seawater. All of these works can give the reader a view of the variety of the approaches in this field and how difficult it is to design an experiment based on their different opinions. Electromagnetic underwater communications are definitely not a mature technology and require further research to start the development of a reliable body of knowledge. Different researchers try different approaches. Therefore, there is an urgent need to characterize the underwater channel before attempting the development of underwater sensor networks. A productive way of doing so is to carry out measurements and simulations. Measurements would validate simulation tools and this would make possible conducting additional simulations reducing the need of going to field sites. Reliable simulation tools are extremely useful as several scenarios can be easily tested. Due to this lack of consolidation of underwater communication in the scientific field and the great potential of radiofrequency communication in shallow waters, in 2014, a project entitled UNDERWater radiocommunications for Optimized monitoring using multiReLay Devices (UNDERWORLD) was proposed. Its aim was and is to re-evaluate the possibility of using EM communications in underwater wireless sensor networks, and the first results can be found in [12; 13]. Nodes for this network should require low power to extend battery life, should not be too bulky and predicted distance between nodes is around 15-30 m. This network will be useful for monitoring shallow waters from the seabed. For instance, where air RF communications are not practical. The rest of the paper is organized as follows. Firstly, experimental measurements are explained in detail, starting with the description of the measurement site. Experimental testbed, which is divided into two parts, is extremely important in order to achieve reliable results and that is why it is fully described. This includes the measuring system, as well as monitoring and control of the testbed. Another important aspect of the experiments, as it has been discussed previously, is the set of antennas employed and which one shows better performance. Afterwards, setup procedures will be explained, not just for launching equipments, but for measuring. Secondly, a great variety of results is presented. The most remarkable aspect of this work is that real marine measurements were performed, with several kinds of antennas and a reliable testbed, and, at the same time, simulations were carried out in the laboratory, achieving high agreement between both methods. Thus, simulation setup parameters and the design of the testbed were also verified. Even though sea water EM properties have been known for decades and several practical and theoretical works have been published, there is no consensus in literature on how to characterize the underwater channel. Therefore, hard work is still needed. A comparison between two types of underwater models for simulations is made in order to decide which one is more appropriate. The performance of dipole and loop antennas is discussed with the support of simulations and real measurements. A characterization of the underwater link is achieved with the latter antennas for frequencies ranging from 10 KHz to 1 MHz, since agreement between simulations and measurements has been obtained. This agreement enables the validation of simulation tools and setup parameters.