# Underwater Electromagnetic Sensor Networks--Part I: Link Characterization +
## 2 Experimental Section
### Testbed  
#### 2.2.1 Measuring System  
It is important to notice that our testbed was designed to take advantage of reliable and well-known laboratory instruments, instead of customized devices, and the whole system was meant to adapt these instruments to work underwater. Hence, the measuring system consists of two sealed chambers built with large PVC pipes that contain several instruments: one is the transmitter and the other one is the receiver. Both chambers include a coaxial cable to connect antennas to the instruments inside. Figures 2 and 3 show the interior structure, circuitry and instruments used in the transmitter and receiver, respectively.  
A bottom-to-top description of the transmitter chamber is now detailed and images of the interior and exterior are labelled in Figure 2. A 12 V battery is placed at the bottom between two methacrylate disk-shaped sheets. The next section holds a waveform generator (Keysight 33220A, Keysight Technologies Inc., Santa Rosa, CA, USA) controlled by Ethernet, which is needed to transmit signals. All instruments and battery are surrounded by foam rubber to avoid movement inside the container. On the disk above, temperature and air pressure sensors are monitored by a BeagleBone Black board running a Linux distribution and connected to the Ethernet switch. This device is also connected to the Fiber-UTP media converter, so that instruments can be controlled through a long fiber link with no coupling problems between transmitter and receiver cables. On top, there is a power inverter that provides AC current to the signal generator.  
Both chambers are formed by a PVC pipe of 30 cm interior diameter ended with two caps and a dresser coupling in the middle. Each chamber weighs approximately 80 kg when full and the exterior measures of the cylinder are 105 cm \(\times\) 48 cm.  
Receiver instruments in Figure 3 (front and lateral view) have an analogous layout. A similar battery is also located at the bottom. The section above was designed to hold a hand-held spectrum analyser (Agilent N9340B, Agilent Technologies Co Ltd., Santa Clara, CA, USA), but in current measurements this model is not used anymore due to its minimum frequency limitations and sensitivity. This layer has the same elements as in that of the transmitter. The subsequent disk contains the currently used spectrum analyser, the Aaronia GmbH (Gewerbegebiet Aaronia AG, Strickscheid, Germany) (1 Hz to 30 MHz), which comprises a 35 dB pre-amplifier and an Intel NUC (Intel Co., Santa Clara, CA, USA) PC. The Aaronia spectrum analyser requires a USB connection to a PC. For this reason, an Intel NUC was mounted inside the chamber running Windows 7 (Microsoft Co., Redmond, WA, USA)  
Figure 1: Measurement area in Taliarte’s harbour.  
and Aaronia software. Finally, in order to supply different voltage requirements for the devices (5 and 12 V) DC voltage converters were implemented in a board with a heat-sink.  
The last version of the receptacles includes four cables that pass through the pipe by cable glands and have their own plastic covering. When the fitting was screwed down, sealing grommets around the cable provided a watertight seal. These four cables provide:  
* Access to the battery for charging;
* ON/OFF switch;
* Fiber cable link to control equipment from the pier;
* Coaxial for antenna connection.  
The first two are not used while submerged, thus water sealed terminations were built with screwed small pipes and tapped unions. In addition, chambers were pressurized to provide appropriate water sealing and avoid water getting inside. Therefore, air valves were built on the sides of both capsules.  
Figure 3: Receiver chamber (front and lateral view of the inside).  
Figure 2: Transmitter chamber (the inside on the **left** side and the outside on the **right** side).