Single wire tether


#1

A buddy of mine is an avid fisherman and has a boat loaded with all of the latest fishing gadgets. One of his toys is the “Sub Troll 900” which attaches to his down rigger line and sends back subsurface speed and temperature data. This device has a radio transmitter to send the data but uses the down rigger line (a steel cable) as the transmitter antenna. The receiver antenna is a small coil that wraps around the down rigger cable and goes to the display unit. So in other words, the unit gets over the radio opacity of water but using a really long antenna that extends to the surface where the data is then received by the receiver antenna which is only a millimeter from the transmitter antenna - this also has the benefit of allowing the transmitter to use very low power to save batteries. So my idea is, what if you use a single wire as a wifi antenna from the ROV that extends to the surface? This would be even simpler than the current two wire design and would probably have higher bandwidth. Plus you could use a steel strand cable (such as my buddy’s down rigger cable) which is really strong in case you need to “reel in” the ROV due to a failure.
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#2

I’ve wondered if you could use a single wire with insulation, and use RF frequencies from both ends to communicate(like XBEE).


#3

Great idea! We used this method on the early prototypes and it was pretty good for controlling the ROV, but we started to have a lot of problems when we tried to get live video back.

It'd be interesting to hear more if you tried it with a different type of insulated wire.


#4

David,
I’m unsure what’s up with the website, but I cannot seem to reply to a reply using IceWeasel 10.0.9 Or Chrome Version 22.0.1229.94.

That said, what kind of RF networking were you using? Additionally, how much bandwidth does the video stream take? We’ve got ours down to sub 200kb/s. I have some XBEE pro radios on order and I’ll see if they have got enough bandwidth for our h.264 video format.


#5

Looks like the XBEE Pro has a bandwidth of 250kb/s which, with overhead and error correction, will make things very tight but possible. It also has the benefit of being pretty high power (63mW). I still believe that running wifi over the cable will work better and provide higher bandwidth. Even old 802.11b can provide 1mb/s at it's lowest data rate and a transmitter power of 100mw. Plus, many of us have tons of this hardware sitting in our basements collecting dust.

Also, I know they make shethed downrigger cables (to protect the steel cable inside from the corrosive salt water). What's the difference between that and a coaxial cable (with the water acting as the shielding)? Even if the sheath cracks or leaks, the signal should still travel the path of least resistance (which is the metal of the cable) and make it to the surface with enough power to be useable. Of course the hard part will be impedance matching which will be critical to achieve high bandwidth, but I don't see this as impossible. Unfortunately, I am not an RF engineer so I can't be very helpful on that problem but I do know enough about RF (N2SSP) to know it's possible.


#6

Victor and Richard- I really like the direction you guys are thinking. As David mentioned, we've done something kind of like this where we used a wire attached to the antenna of an RC car receiver to sort of duct RF down through the water at 72MHz. I call this method "RF Snorkeling" and have looked into it a bit to figure out what else it can do. Unfortunately, it turns out that for the same reason WiFi antennas have to have short feed-lines, RF Snorkeling becomes significantly less effective at high frequencies: higher frequencies experience greater losses in wire. Systems like this will work pretty well for low bandwidth data that can travel over low frequencies (such as data for trolling sensors and RC controllers) but it really falls apart for anything requiring higher bandwidth (and therefore higher frequencies). For this reason, I was never able to effectively send the signal from my GHz range analog video transmitter through a wire to its receiver, and I didn't pursue trying to RF Snorkel WiFi between the ROV and the surface. (See my tether decision tree here)

That being said, if either of you find a way to make this work- please let us know! I just thought I'd give you a heads up on my experiences.

Good luck!

Eric


#7

The XBEE-pro series 2 I picked up for testing were only able to supply about 20kbit/S, which is a quite small for our video stream(~110kbits/S). Does anyone have a pair of XBEE series 1 modules to test? I'm wondering of the overhead of the mesh-network added in series 2 is part of the problem.


#8

I've just disovered this forum/website today, and it has intreged me. I am an RF engineer by profession and a scuba diver as well. I'm afraid the attenuation at wifi frequencies, 2.4 or 5.8 Ghz would be hoplessly lost over a cable length of 100m. Lower freqs like 72 mHz might just barely make it, but not enough bandwidth for video. Although expensive, at least for the connectors, fiber optic is the way to go. You could easily run a couple of 1000 feet of fiber and suffer almost no signal loss. LAN and Video over fiber would be perfect, but would cost more than a simple copper pair of wire. I wonder how deep the housing is "rated" for if you have a longer tether ???

Mark Jenkins


#9

I'm going to add to this old thread, hopefully someone likes this idea. We would all like a fiber cable that could carry power. Fiber or copper alone aren't that expensive because manufacturers tool for it and make billions of feet. There is combo copper+fiber available with steel jacketing for strength. The problem is that it is very expensive because it is usually a custom order.

If there are enough people who would like to do a combination fiber+copper cable for super deep dives, maybe the community could work together to put together a large enough order to get the price down?


#10

Many of the very early ROVs used an RC control unit and coaxial cable with a multiplexers for power ,control and data transfer so It works . custom cable design is Very expensive in the size range you would need being so small in Diameter OUCH the off the shelf is far to heavy and no less expensive Gulp.