Testing 2.4GHz Wi-Fi underwater and between boundaries

In air our WiFi system provides stable radio link at distances 500 meters (tests conducted at 0-2 meters height above the water surface), which is good for AUV and many ASV applications. Distance is much longer if antennas elevated above the water surface and water does not attenuate the signal.

This spring 2021, we complete extensive tests in real environment of our recently made “Subsea (submersible 6000 m) compact 2.4GHz Wi-Fi radio transceiver” (https://www.apsubsea.com/?page_id=824).

Major question we asked ourselves – is it possible by using standard 3-5 dBi omnidirectional antennas organize Ethernet communication in the way:

2-way subsea Wi-Fi band radio communication when antennas submersed;
2-way Wi-Fi band radio communication when one of the antennae submersed and another – above the water;
What would be maximum distance of signal propagation and bitrate, latency in both scenarios;
What is the application for that and how/where it can be helpful?

AP Subsea Inc. is giving answers, thank you for staying tuned.

So, in short – the answer is YES! Bidirectional radio datalink underwater and as well as between air-water boundaries is not just possible, it is fast, low latency, secure. To be honest, we watched live Full HD video stream using our system! So, we are super excited.

Now, details.

Theory
For many people it sounds obvious (or even more – like an axiom) that there is no radio comms in water and between boundaries at such high frequencies. Also, that there is no practical use for that. By doing theoretical study we found that there is neither many sources of information, no details about radio signal propagation thru/ (or in) water. One problem is that water is not a uniform substance: turbidity, temperature and salinity may play significant role in RF signal propagation. Another “feature” that fresh and seawater have different radio transparency properties. Also due to significant RF signal attenuation and conductivity properties of water – much, much higher than air, almost all the radio signal energy used to interact with water “snow”, particles, water molecules. This effect we can watch by warming water in the microwave oven.

To compare water and air as media we found that these two parameters in the equation are playing main role:

relative dielectric permittivity (dielectric constant)
electrical conductance

See parameters in table below for 2.4 GHz signal

Parameters	Fresh water	Sea water	Air
Relative dielectric permittivity (dielectric constant)	60	80	~1
Electrical conductivity, S/m	1	4	3 to 8 * 10^-15

If lower dielectric constant and smaller electrical conductivity – then further radio wave can travel. These limitations are true for high frequency (short wavelength) radio signals. For low frequency and long wavelength radio signals this is less critical. More important becomes to provide enough transmit power and tune sensitive receiver to detect signal. However, for long wavelength signals, i.e. 10kHz (~30 km wavelength) classic E-antennas dimensions would be at least 7.5 km long (1/4 of wavelength). The only option to minimize antenna (but sacrifice signal transmission distance) is to account on magnetic component of the RF and use magnetic loop type of antenna.

Testing
By doing tests we have confirmed that our system, which is equipped by common use 3-5 dBi omnidirectional antennas, can establish solid, low latency, reliable wireless radio connection at frequency 2.4 GHz.
We complete tests in freshwater lakes and seawaters around beautiful Vancouver metro area, British Columbia, Canada.
Table below demonstrates what we achieved during the tests

——	Link distance, inches	Datalink speed, Mbit/s	Latency, ms
Antennas in water	4-6	50-100	1-2
One of antenna in air another in water	30-40	10-100	1-2

So, why to use 2.4 GHz radio datalink underwater if distance is that short?

First – at short distance – Pretty good baud rate: 50-100 Mbit/s

Second – security – Modern ciphering protocols are used and no data leak due to nature of environment

Third – integration – Ease to add number of payload onto highly integrated vehicles with limited amount of “Guest ports”, i.e. setup access point with antennas basket

Forth – standard interfaces – Well known interface Ethernet (POE) and TCP/IP protocol are used

Fifth – signal latency – It is low enough and compatible to provide payload NTP clock synchronization.

Sixth – speed in water – 2.4 GHz radio wave propagation in water is faster then acoustic.

In conclusion we would like to say thank you everyone who have shown interest to what we do and waited for the interesting results.

Specification is on the page: https://www.apsubsea.com/?page_id=824