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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.