Sunday, July 17, 2016

Underwater wifi for photography

A couple of weekends ago I spent some time canoeing and took along a submersible Wifi enabled camera.  The Wifi link works great for mounting the camera on the frame of my car and watching the suspension and tire sidewall flex while the control arms articulate.  Cool stuff.  I mean REALLY cool stuff! You can calculate shock damping and tire stiffness... AMAZING.  Back to the underwater stuff-

I knew there should be some attenuation of signal when dipping the cam just below the surface of the river, but have to admit I was surprised at NO wifi connection at all even though the cam was just a few inches down.

So what the heck happened?

Wifi is a group of channel frequencies (802.11 somethings)  they range from 2.4 GHz to as high as 60 GHz.  Most devices use the 2.4 or in the US, 3.6 or 5 GHz bands.  I'll not get into the center frequencies and stuff like that because it's immaterial.  The challenge is attenuation of the low power transmission signal from ambient air to something submersed in a conductive media like water.

There are two losses to consider in this situation: The refraction of the signal at the air / water interface and the attenuation of the signal in the water!

Interesting you say... me too!  This ability of the high frequency Wifi signal to get through is greatly influenced by how much bounces off of the surface of the water and the distance the wave must then travel in the water to get to my camera.  Of course we could also play around with frequency and power, but the cell phone I have and the camera I have define my willingness to design antennas for lower frequencies and additional amplifiers!

So river water IE "Fresh" or "Salt" water is quite conductive, unlike air which is a nice insulator.

Fresh water conductivity is around 0.05 S/m whereas Salt water is around 5 S/m

My cell uses either 2.4 or 5 GHz depending on when I am in the world.  So on the river my phone was using 5 GHz at power of 100 milli-Watts.  I think that's the maximum allowable for a mobile device in the US for Wifi - I think...

 The reflective attenuation is

 Thank -you C. A. Balanis, “Advanced Engineering Electromagnetics,” John Wiley & Sons,   New York, 1989. 
and the  propagation loss is:

Thank-you G. Benelli, A. Pozzebon and G. Ragseo, “An RFID Based System for the Underwater Tracking of Pebbles on Artificial Coarse Beaches,” IEEE 2009 Third International Conference on Sensor Technologies and Applications, Athens, 18-23 June 2009, pp.294-200.

Where: α is the attenuation in dB of our Wifi signal via reflection or water transmission.  
Hmmm, looks like a giga-anything frequency amounts to about a 4.5 dB loss.  So my 100 mW Wifi signal is only 36% of what it is in air!  There's the problem.  The reflected wave is a huge loss component and the transmission in water doesn't help so much either!  I'm not putting my phone under water, so I need an alternative solution.
So how about this?  We make d  be  as short as possible and we run a shielded conductor from case cover to case cover to try to beat the reflected wave loss.  Yup, preparing a water sealed and shielded cable to run from the back of my cell phone to the surface of the camera's Wifi antenna with a little duct tape should improve things immensely.  Looks like what a lot of folks have already tried and posted on-line, lets see how this turned out...



I just came back from a camping - kayaking trip and uploaded my ION movies to iPhoto where they promptly either deleted or disappeared!  Hmmmm, will re-post equations - which seem to have been deleted in the draft of this post too... 

In a nutshell, and more later... the taped waterproof shielded cable from my ION to my iPhone only allowed about 7 or 8 cm of  depth before loss of signal.  Better than no wifi when submerged, but still not very practical.     

More in a bit!

In this clip you can hear the amazing transition between air and water on the mic.  Nearby jetskis barely audible via air are clearly heard.  I suppose you could do an fft on the sound signal and back out the impeller count and look at the whole signature to identify the brand of jetski... Neat stuff.  The Wifi signal allowed me to command the camera to a depth of 7 or 8 cm.... an improvement over going dark when you hit the water, but still not what I'm looking for.  Next step - move the Wifi transmitter out of the caseJust a 20 or 30 cm cable on a float with the xmitter above water.  THAT should work...

All for now, hope you all had a great weekend! 


No comments: