I know basic electromagnetic theory very well and understand why this works but I'm not sure about one thing regarding usage of coaxial (shielded) cable vs. unshielded (normal conductor) cable when using in underwater data transmission.
Shortly: What is advantage (if at all) of using shielded cable underwater vs. unshielded?
This is for further info and results of my pondering:
In underwater environment is not much if at all EMI that could interfere with the signal in unshielded cable so I think that is ok. Next thing is that does coaxial carry signal (attenuation issue) a lot better than just a unshielded cable? For example does unshielded cable radiate and therefore lose signal power into water more than coaxial that has braid cover? Does that braid help to reflect and "store" signal better in cable?
What's more here is some antenna knowledge also needed because no physical contact will be used.
About my use-case: In practice I'm planning to try littlebit same setup as in the link (http://www.outdoorsmenforum.ca/showthread.php?t=168539). I will have 10 meter cabling and just pondering if I could use underwater unshielded and change it some before air environment to shielded coaxial cable. Signal that is "assisted" through water in cable is Wifi so it's little over 2,4 GHz.
Best Answer
You need to use shielded cable for a 2.4GHz signal, regardless of whether it is underwater or not. Co-axial cable will contain the signal's EM field within the cable, making it very efficient at transmitting with minimal losses. It also has a consistent impedance, and this is important because where you have impedance changes you will get reflections that will degrade signal quality (this is of particular concern at your connectors).
If you don't use a co-axial cable then significant amounts of the signal will propagate into the space around the cables. If water encloses these spaces then this will absorb some of that energy, reducing the signal that makes it to the end of the cable.