Consumers have shown that they are eager to be rid of cabling and wires connecting them to devices situated or worn near or on the body. From athletic monitoring devices to headphones, and computer mice to cellphone hands-free kits, manufacturers have met a demand for connectivity without wires through the application of short-range RF technology.

Figure 1: There is a correlation between attenuation and operating frequency in underwater RF transmissions.

When meant for “in air” conditions, the operation of RF devices is well understood. But there are underwater activities that could equally benefit from such wireless technology. SCUBA divers, for example, need to monitor equipment such as air tanks; underwater sensor arrays would benefit from being connected wirelessly. Can the short-range RF technology used in air today operate as successfully under water?

This article describes the results of tests conducted in several underwater locations, using a low-frequency (LF) radio device. The aim was to discover more about the behavior of commercial off-the-shelf (COTS) RF components underwater. Aspects considered in conducting the tests included:

  • What is the range limitation?
  • Which frequencies are best suited for underwater use?
  • Are there other frequency considerations?
  • Which modulation modes are well suited to underwater use?
  • Are there data rate limitations beyond normal bandwidth constraints?
  • Does the end user need to take account of regulatory requirements?

Behavior of RF Energy in Water

RF has well defined propagation characteristics in air, and each frequency range has certain beneficial and detrimental attributes. In water, these attributes no longer apply. Propagation in water, unlike air, does not depend on ionospheric conditions; water has its own set of rules. In particular, RF energy is greatly attenuated in water, so the effective range is reduced.

The conductivity of water and frequency are the two factors determining how well RF energy will propagate. The following equation shows how attenuation changes with conductivity and frequency. α = 0.0173 √(fσ)i

where

α = attenuation in dB/meter

f = frequency in Hertz

σ = conductivity in siemens/meter

This shows that attenuation increases as frequency rises. This suggests that underwater applications will support lower-frequency transmissions and a reduced expectation for effective range compared to the equivalent RF system in air. This work sought to discover whether an effective range of 1m was possible for a small RF device intended for a body-area network or similar application.

Tests were carried out in water sources in Utah. The Great Salt Lake acts as a worst-case example: after the Dead Sea, it is the saltiest known body of water in the world. High saline content causes greater RF attenuation. Homestead Crater is also unusual because it is fed by a hot spring with high mineral content, which also affects attenuation.

Scientific publications on fresh water indicate that its conductivity is in the range 50-100mS/m. (1mho = 1 siemens.) In the test, fresh water was evaluated at 80mS/m. Publications on sea water state that the conductivity is 4-5S/m. A value of 4S/m at selected frequencies is assumed for the data in Figure 1.

Figure 2: Demo kits placed in dry bags.

The Great Salt Lake is an extreme case and used here as a data point, and not as a practical environment for underwater RF communications. The conductivity of Great Salt Lake water obtained from the Utah Division of Water Quality ranges from 146mS/cm to 200mS/cm. This translates to 14.6S/m to 20S/m. The average value of 17.3S/m was selected for this example. In all underwater environments, there is a correlation between frequency and attenuation (see Figure 1). But the problem of attenuation is particularly acute at high frequencies in salty water. Attenuation is substantially less in fresh water than in sea water. It can be concluded that a higher usable frequency is possible in fresh water.

There might be other operating frequency considerations to take into account. In recent years, amateur radio operators have begun using the VLF and LF bands. Numerous experimental radio stations around the world now make very low-power broadcasts at frequencies from around 150 kHz up to several MHz. It is unlikely that such stations will interfere due to the limited air-to-water RF transmission at these frequencies, but this phenomenon must be considered in any full-scale implementation.

A further consideration relates to regulatory requirements. Regulatory organizations such as the Federal Communications Commission (FCC) have certification requirements. A review of local governmental agencies should be conducted before starting an end product design.

Transmitting RF Signals Under Water

Underwater data transmission has mainly been implemented through acoustic transducers, since water is a benign medium for sound waves. There are many examples of acoustic modes, most notably SONAR. There are also many RF modes available to the engineer for underwater data transmission. For the purposes of the tests for body area networking applications, Amplitude Shift Keying (ASK) modulation was used for the RF signal because it is easily implemented by a microcontroller with little firmware. ASK represents the digital information — a 1 or 0 — by varying the amplitude of the carrier.

The data to be transmitted was in the form of infrequent small packets, so a slow data rate was adequate. Higher data rates can be realized at higher frequencies, but at these frequencies, the designer will have to battle with the challenges of achieving adequate range at a viable output power. Other than in fresh water, satisfactory operation might be impossible.

Just as the propagation of radio waves in water is different from in-air transmission, so antenna design is different. As with attenuation, wavelength is affected by the conductivity of water and is expressed by the following formula:

λ = 1000 √{10/fσ)}iv

where

λ = wavelength, in meters

f = frequency, in Hertz

σ = conductivity in siemens/meter

From this equation, one can see that underwater wavelength is less than the wavelength in air at the same frequency. An additional consideration is that the antenna cannot be in direct contact with the water. A waterproof barrier such as a plastic coating is needed to allow the electromagnetic wave to be launched.

There are many underwater antenna configurations currently in use, from wires trailing behind a submarine, to directional loop antennas, to omnidirectional loop antennas. In this investigation, a simple transmitter antenna is used: the Grupo Premo KGEAWT680703B0332J. In the tests, the receiver system antenna selection was greatly simplified because the receiver IC offers automatic antenna tuning.

Operation of the Underwater RF Tests

Figure 3: Testing RF performance under water at Great Salt Lake.

The equipment used to test the operation of short-range RF transmissions under water was a demo kit that consisted of a 3-axis LF wakeup receiver board, a 125-kHz transmitter board, power supply, documentation, and software.

Three experiments were undertaken. An open-air test was performed to determine a baseline for the water tests. Two in-water tests and one shore-based event were also carried out. The open-air test yielded a transmission distance of 2.9 m (7.5 feet). This provided a base line for the performance of the system under water.

The first in-water test was a dive event at Homestead Crater in Midway, UT. This is a 19.8-m (65-foot) mineral spring-fed body of water inside a volcanic dome crater with a high average water temperature of 35 °C (95 °F). The receiver board and transmitter board were placed in separate waterproof dive bags (see Figure 2). Measurements were taken at 7.6 m (25 feet) and 15.24 m (50 feet), which allowed the test to determine whether depth affected the range.

The demo kit used is capable of transmitting a wakeup signal consisting of a 32-bit Manchester-coded ASK wakeup pattern with a data rate of 1.35 kbaud, repeating at intervals of one second. The transmitter is powered by two AA batteries, providing a relatively low power output. The wakeup receiver is programmed with this same Manchester code pattern. As this pattern is transmitted, the board listens for the signal. If a signal is received and the patterns match, it accepts the transmission.

The board can determine the received signal strength in the form of an RSSI value that is indicated with five LEDs on the receiver board; each bit is equivalent to 2 dB. This was the indicator used to gauge the distance of the transmission. Three other LEDs indicate on which axis the signal was received. The benefit of a 3-axis receiver is that no matter which orientation the transmitter and receiver are in, there is always a receiving antenna to capture RF energy.

There was no difference in the range of transmission at the two test depths. This distance was measured at approximately 1.85 m (6 feet).

The shore-based test took place at Great Salt Lake. This time, the test equipment was suspended below the surface of the water from two PVC poles (see Figure 3). The receiver was set at a fixed depth of approximately 6 cm (2.4 inches) below the surface. The second pole was used to lower the transmitter into the water and gradually extend the distance between the transmitter and the receiver until the RSSI indicator showed 0. Surprisingly, given the high saline content of the lake, the transmission distance was 1.5 m (5 feet).

Based on the results of the two tests, it seems reasonable to conclude that RF transmission under water over a short distance is possible using commercial off-the-shelf RF ICs available today. Given that the transmitted power of the test setup was limited by the small power output available from the batteries, it is also reasonable to conclude that a greater distance would be achievable with modified equipment.

Applications for Underwater RF

The low-power demo kits can transmit a wakeup signal that repeats at intervals of one second.

It is possible to envision a wide range of applications for underwater RF devices. In fact, hydrologists from the United States Geological Survey (USGS) encountered during the testing at Great Salt Lake described their requirement for submerged sensor arrays that do not require tethering to a surface buoy for data collection. Such a sensor array could be realized using the technology described in this article.

Other potential applications could include wireless monitoring equipment for SCUBA gear, and a simple “buddy call system” to allow a pair of divers to alert one another when visual cues are obscured by lack of eye contact or poor visibility. The 3-axis topology of the system could support the provision of indicators to point the receiver towards the transmitter. With a transmitter and receiver, both interfaced to simple microcontrollers, an effective underwater RF communications system can be implemented, with transmission range up to 1.8 m possible.

This article was written by Mark Hoferitza, Field Applications Engineer at austria microsystems AG, Unterpremstaetten, Austria. For more information, Click Here