I’m looking for if anyone has discovered an ethernet tether that is neutrally buoyant and compatible with wet link connectors. I teach a class that designs and build ROVs, and I want to replace our current DIY tether with something a little more professional. My current fleet of tethers is cat5e with poly cord and is potted into the bulkheads. I have to redo a few of them and want to upgrade to Wetlink for a number of reasons. At our distance, I would like to avoid interface boards so staying native ethernet is important. Any recommendations?
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Hi @BillyA, welcome to the forum 
This isn’t a matter of “discovery” so much as contacting a tether manufacturer and getting them to manufacture it for you. “Normal” cables aren’t manufactured to be neutrally buoyant, and tethers generally don’t follow strict Category N specifications because those specifications were designed by the telecommunications industry for cables that mostly aren’t intended to move during operation, and have transmission requirements that may not apply when the cable is submerged in a conductive fluid.
As an example, Wikipedia describes the maximum length for 10/100/1000BASE-T Cat 6 cable as:
That description just doesn’t make sense in the context of a tether for a vehicle that can move vertically, and also seems impractical because solid wires are quite inflexible and would add significant amounts of resistance to the tether bending and moving around.
An ideal tether for communications has good signal characteristics (e.g. high bandwidth and low signal noise), while also having minimal physical effect on the vehicle that’s dragging it (e.g. slim, low mass, neutrally buoyant, flexible), but understandably several of those factors compete against each other, and different trade-offs have different benefits depending on the application. The cost, abrasion-resistance, strength, and visibility can also be important for practical use.
That’s not a product that exists (yet) - I presume you’re talking about our WetLink Penetrators? If so I’d recommend looking through our How to Choose a WetLink Penetrator guide to see what kinds of cable constructions can be used, as well as a selection of cables we’ve verified to work well with particular sizes of the WetLink Penetrator range
Note that if your tether needs to use an RJ-45 plug to connect directly to the Raspberry Pi (rather than, for example, a JST-GH connector connected to our Ethernet Switch) then you’ll need to either add that plug termination yourself after sliding on the WetLink Penetrator components, or get the cable manufacturer to do so for you (which they may not be prepared to do).
Hi Elliot,
Yes, I meant the Wetlink Penetrators. I hoped there were some tests on the Fathom tethers over short distances. The 2-28MHz range for Homeplug AV is very different in signal attenuation to the 100Mhz of 1000Base-T. That being said Cat 6 is 250Mhz and rated for 10GBase-T. I’ve done a lot of attenuation testing in my past life as a network engineer (still remember the headaches), but I’m just looking for some anecdotal information. Measuring insertion loss is very important to do while submerged as the fluid has a big impact on it depending on the amount of shielding and cable construction. Normal Cat 6 will not get 1000Base-T with 90m of it in the water, even less if it is stranded.
I can clarify the horizontal cabling jargon so anyone else reading this in the future won’t get the wrong idea. That is the definition of horizontal cabling is radiating out from a distribution closet (think racks of switches and patch panels). It’s not about the physical orientation of the cable but rather the purpose. There is also backbone or vertical cabling, which connects distribution centers and patches used to perform interconnects.
SeaView Systems sells a fiber upgrade kit and tether. The most expensive part of the kit is the penetrator but if you wanted to save money, you could use a WetLink Penetrator and splice the connectors onto the fiber yourself.