0
\$\begingroup\$

I am building an underwater drone and I connect to it via wifi when it is at the surface. The wifi signal often gets blocked by water waves though. I don't really want to have another penetration of the pressure housing and wondered what would happen if I took 50cm of RG58 coaxial cable, stripped 5cm of shielding from either end, attached one end to the housing close to the wifi transmitter and had the other end poking up out of the water. The shielding of the housing cable end would be exposed to the water for grounding.

Would there be any useful wifi transmission/receiving signal at the above water cable end? The connection runs Remote Desktop so I don't need a particularly large bandwidth, just something to make the wifi connection slightly more reliable.

\$\endgroup\$
  • \$\begingroup\$ Your description may work for you but I'm struggling to understand what you are proposing. \$\endgroup\$ – Andy aka Jan 19 '18 at 10:52
  • 2
    \$\begingroup\$ What you are proposing sounds somewhat to a 50 ohm line feeding a monopole on a groundplane. Will it radiate? Yes, everything radiates. Will it be enough? I can't say without knowing what kind of range you are dealing with, what kind of speed you want, etc. perhaps it is easier to just terminate your RG58 with something like a SMA connector, and connect a proper 50-ohm terminated antenna targeted for the wifi band to that end? \$\endgroup\$ – Joren Vaes Jan 19 '18 at 10:55
  • \$\begingroup\$ Thanks for the answer. I'm looking for about 10m, 200kbps. I'm basically trying to shift the signal from a partially submerged antenna (to which I don't have an electrical connection) by 50cm so it is always above water. \$\endgroup\$ – Toby Jackson Jan 19 '18 at 11:07
4
\$\begingroup\$

A better idea would be to get a real WIFI antenna, small enough so it can be sealed, and use that. There's no chance that a stripped piece of wire will perform better than an antenna specifically tuned to 2.4 GHz. Something like this:

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Enclosing a 'real' wifi antenna could result in a shift in performance, however it is likely to still outperform the "crude" monopole antenna suggested by the original author. \$\endgroup\$ – Joren Vaes Jan 19 '18 at 12:04
  • \$\begingroup\$ @JorenVaes Such small PCB antennas often come encapsulated in a piece of heat-shrink tube, and are then placed inside the plastic case of the device in which the antenna is used. So a reasonably thin enclosure shouldn't affect performance that much. \$\endgroup\$ – Dmitry Grigoryev Jan 19 '18 at 12:26
  • \$\begingroup\$ Good idea. I can dip it in epoxy I think. \$\endgroup\$ – Toby Jackson Jan 19 '18 at 12:47
  • \$\begingroup\$ @TobyJackson I'm sure you can dip it in epoxy but it's likely you'll render it useless due to changing the dielectric permittivity and at best you'll retune off by a few MHz or even tens of MHz. \$\endgroup\$ – Andy aka Jan 19 '18 at 13:35
  • 1
    \$\begingroup\$ @Andyaka I would be surprised if a <1mm layer of epoxy had such a drastic effect as to render the antenna useless. Do you think it will be that bad? \$\endgroup\$ – Dmitry Grigoryev Jan 19 '18 at 14:08
3
\$\begingroup\$

took 50cm of RG58 coaxial cable, stripped 5cm of shielding from either end, attached one end to the housing close to the wifi transmitter and had the other end poking up out of the water.

That would allow water to get into the cable, rendering it useless. RG58 performance at 2.4GHz is not great even when dry.

And even when you could magically keep the water away from either cable end, this construction does not work because only a tiny amount of power actually couples into the cable - barely enough for a sensitive receiver but not enough to re-radiate it witout some ampilifier.

\$\endgroup\$
2
\$\begingroup\$

I have experimented with crude mono-pole antennas made out of coaxial cable by stripping off the shielding at both ends and letting lambda/4 piece of cable exposed and it worked quite well. This was over the air, I am not sure about underwater.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.