Feature: Communications
Figure 1: Illustration of the operating principle for the model submersible A dipole antenna is omnidirectional and sensitive in the
plane perpendicular to the antenna, and its radiation pattern resembling a donut. Note that the lower the frequency, the greater the range and antenna. At 13MHz, the total length of a dipole antenna is about 1.1m. Although there are many antenna types, a dipole is used because of its man ease of fabrication. In the horizontal plane, the greatest distance can be
bridged with a vertically-installed dipole antenna. In theory, 13MHz should provide enough range for this project with all communication under water. At 27MHz, the range is only sufficient if the signal travels partly through air, which for this applicaton is not allowed. There are however no radio regulations for sending and receiving electromagnetic waves of underwater origin, states telecom agency NL. A portable short-wave receiver can be used to ensure that
signals generated under water do not cross the water-air boundary – 27MHz radio waves that cross the water-air boundary propagate parallel to and just above the water’s surface.
RF transceiver and PCB design Because of the limited space, it is best to use a model integrated in an SMD, but I could not find an IC suitable for 13MHz; the closest I found was the AX5043 suitable for 27MHz. However, the speed at which the receiver locks onto the
transmitter is still a problem. I use the automatic pre-amble pattern match mechanism built into the radio transceiver. Nevertheless, prior to sending the pre-amble, it is necessary to switch on the carrier wave for approximately 4ms (without
Figure 2: Ki-Cad PCB
modulation), regardless of the length of the pre-amble. A pre- amble is data sent prior to the real message, aimed to wake up the receiver when it is in power-save mode and it synchronises the timing with the transmitter. This 4ms is unfortunate, because it is a real-time application and the manufacturer’s data sheet for the radio transceiver claims it to be faster. I designed a PCB around the AX5043 with KiCad; see Figure
2. Note that the PCB is not equipped with motor drivers, but comes with many interface options and a sensitive 3D accelerometer. By placing an extra component, this board can be used without detailed knowledge of the RF transceiver IC. The PCB contains an Atmel XMEGA microcontroller that’s
supported by many software development tools. The relatively low frequency of 27MHz is useful too, since designing a PCB for 2400MHz is a completely different matter. For use at 27MHz, a coil must be added to the radio transceiver, whose value varies from board to board and
www.electronicsworld.co.uk February 2022 35
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