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I'm building a boat towed underwater platform. This underwater platform has a single board computer with a remote wifi antenna connection via a u.fl connector.

I'm experimenting around with a way to extend this remote antenna to 30-50ft to get it above the water line to broadcast. The extension would be sort of an antenna feedline over either coax or twisted pair to an above water antenna assembly (a simple plastic high gain antenna used commonly on routers). I've tried doing this with RG-178 coax and think I'm getting unacceptable line losses. I think that is 30db loss over 100ft at these frequencies.

Any ideas for how to accomplish this?

Balun/twisted pairs on both ends? LMR100, 195, or 200?

I've also thought about putting my underwater device on a twisted pair ethernet wired network but since this device is being towed with a tow cable, I'm trying to embed the antenna assembly into the hollow cable tow cable to simplify operation and to avoid having to do slip rings on the tow cable spool.

enter image description here

Update: Bathtub test was a success. Now getting:

  • 70db with 50ft of RG174 and simple dipole at end. Improving that by ~10-15db with hi-gain antenna. Low latency video transmitting across wire.
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    $\begingroup$ Over 50', anything less than LMR-400 will have at least 6db loss. I'm not sure you'd want to tow a cable that's 0.6" diameter, but you don't mention any physical limits. While outside the question, it seems to me it'd be way easier just send data to the surface and convert it to RF on the boat. $\endgroup$
    – Duston
    Commented Oct 21, 2023 at 1:56
  • $\begingroup$ 30dB of cable loss would probably actually not be a big deal if I understand what you're doing — wifi is used to dealing with 80dB or so of free space loss, and I imagine your laptop or whatever is going to be right next to the "remote antenna". $\endgroup$ Commented Oct 21, 2023 at 4:36
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    $\begingroup$ But RG-178 isn't 30dB/100' at 2.5GHz, it's more like 70dB/100'. $\endgroup$ Commented Oct 21, 2023 at 4:37
  • $\begingroup$ What data rates do you need to achieve? $\endgroup$ Commented Oct 23, 2023 at 3:58
  • $\begingroup$ @Duston, I thought about this. But, my hunch is that if I can get an antenna just above surface of water, it dramatically simplifies my reel/retrieval as I don't have to deal with interconnects/slip rings through a reel. $\endgroup$
    – leeprevost
    Commented Oct 24, 2023 at 14:01

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If you can incorporate a strong, thin coax into the tow cable, you could use it to connect directly from radio to radio.

Wifi radios expect up to 80 dB of path loss, even 100 dB at lower speeds (+20 dBm to -80 dBm is fine for 1 Mbps 802.11b). This would work through 50 metres of thin coax. Much more if you use a UHF or VHF radio system.

A Teflon cable like RG316 is quite tough and still nice and thin.

The slip ring in the spool can be much simpler if you can tolerate 6 or 10 dB of loss and don't need a DC connection. Something like two dipole antennas held against each other, one rotating and one fixed.

Keeping the water out will be difficult.
Some cables are filled with a low loss dielectric gel, so they don't fill up with water if the jacket is nicked, otherwise you could wrap it in a (non-glue) heat shrink, then inside a poly rope for more protection and strain relief.
SMA isn't waterproof, you'll need to solve that part. You could probably invent a metal bush with o-rings, glue the coax through the centre of the moving part.

Or if it's in fresh water, and you have some dB to spare, you might invent a through-the-hull non-contact coupler with a type of antenna inside, pressed against one outside. Imagine a circular PCB disk 30 mm diameter, with a central 10 mm patch, 3 mm gap and 14 mm annular ground ring, this might couple OK through a few mm of plastic.

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  • $\begingroup$ I will look into RG316. See my test result updates added above. can you elaborate on the idea for the slip ring? That's interesting and maybe I should revisit that idea. I can avoid DC for now. Plenty of battery capacity underwater. The water infiltration is something I prob haven't learned yet. This is salt water. not contact circular very interesting. $\endgroup$
    – leeprevost
    Commented Oct 24, 2023 at 14:13
  • $\begingroup$ Also, I could move away from Wifi if necessary and go to UHF/VHF radio. what did you have in mind? Also, I could probably live with 20m as I'm just trying to get the antenna above the surface of the water so it an propagate. Don't think I need to get it back to the boat. $\endgroup$
    – leeprevost
    Commented Oct 24, 2023 at 14:20
  • $\begingroup$ I'm now thinking you are talking about something like the FPV systems. I need to process the video underwater while also sending back for FPV. So, would need to digitize the camera feed. Bewildering selection on amazon and RC websites. Some "FCC legal" and others not. Thats probably important for me as this could be commerical product. $\endgroup$
    – leeprevost
    Commented Oct 24, 2023 at 15:30
  • $\begingroup$ The point of my answer is that you can tolerate a huge cable loss, if you connect directly between radios. For some class of problems this could be a cheap solution. If you want to take RF to the surface, then make a regular wireless connection, for wifi, you need to keep the loss < 10 dB, so choose an appropriate cable. I don't know about UHF data links, if you find one it will have less cable loss. Wifi is WiFi, it could be used to stream video. And you can't commercialise a product that's not "FCC Legal"... $\endgroup$
    – tomnexus
    Commented Oct 24, 2023 at 16:31
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Depending on the type of data and bandwidth/rate you are planning to pass to the boat. If it is only telemetry/alarming information then a narrowband off-the-shelf system such as LORA may suffice. Even though LORA operates in the 915MHz unlicensed band the bandwidth requirements are far less than WiFi and therefore a better link budget. Concerns also on exactly how to keep the antenna and antenna housing dry yet still functioning with low coupling loss between the antenna and the waterproof shroud? A low data rate telemetry application may point to a low frequency inductive loop coupler similar to the old loops used in churches for hard of hearing and operating at audio frequencies where multiple hearing aids couple to the loop.

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  • $\begingroup$ I may reduce scope to low bandwidth link. But for now, I’m stubbornly trying to get HD video for preview across the wire. Would love to find a coupling method to avoid mechanical complexities of a rotating slip ring at the boat. Meanwhile just getting Antenna above waterline and wifi from there. $\endgroup$
    – leeprevost
    Commented Nov 28, 2023 at 3:15

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