Feature: Communications
A portable short-wave receiver can be used to ensure that signals generated under water do not cross the water-air boundary
Building and evaluating an underwater RF communications link
By Rob Schuckman, Application and R&D engineer, Zilvertron, The Netherlands
H
ere is a project that evaluates the underwater sailing behaviour of a model submersible – remotely and in real time; see Figure 1. Model submersibles are about one to two
metres long, making on-board space rather limited. A built-in computer regulates the
submersible’s depth, with commands sent via a simplex radio remote control. Radio waves travel first through air, then water. By moving the computer from model to shore, we avoid
taking the submersible out of the water for a software change, leaving on board mainly actuators and sensors. While doing so the effect of changes made to the software can be observed instantaneously. Thus, two-way communication is required between shore and model, which can be sound, light or radio waves. I avoid using cable since it can limit the model’s movements, but also it can become entangled in the propeller. Hence, I opted for radio waves, which because of radio regulations are not allowed to travel through the air, forcing me to explore underwater radio communications in fresh water (the range is too limited in salt water). Due to the real-time nature, I thought of using full-duplex
communication. But, two antennas placed close to each other in a small space leads to problems, such as receiver clogging up, for example. The next best option is half-duplex, with the added advantage of occupying less space. The protocol used for testing consists of 22 bytes of which
six are overhead (two pre-amble, two sync and two CRC) and 16 are payload. The protocol frequency – the speed of sending data back and forth – is about 30Hz. An initial exploration of the payload required to control a model submersible shows a need for approximately 104 bits (which equals 13 bytes).
Design details The model’s dimensions help determine the frequency – bear in mind that the propagation speed of electromagnetic waves under water is slower than in air by a factor of about ten. This side effect comes in handy because it makes the antennas smaller or shorter by the same factor. Assuming a half wavelength dipole antenna, the frequency will be about 13MHz.
34 February 2022
www.electronicsworld.co.uk
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