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Old Jul 29, 2012, 02:41 AM
m_g
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RC transmission through water using tether

Hi all,
I have recently started an ROV project that uses an RC system (tx and rx) to control the thrusters. The design suggests that using the video feed tether (umbilical) or an antenna "extension" will act as an antenna and allow the transmission to occur through water.

I have done a lot of reading and asked on other forums, but there is no real consensus on:
A. Whether this will work at all,
B. whether this will work in saltwater, and
C. Whether this will work at any significant depth ( eg 30 metres)

Has anybody on this forum tried this method?

If I was going to extend the antenna, what sort of wire would be best to use?
Or, if I should maintain a direct connection, is there instructions for making a dsc type cable (my hobbyking tx has the output for a dsc connection, but I don't think the actual cable is commercially available.)

I could just go and try it, but wiring all the components into the rov is almost permannent (potting cables etc), so I would prefer not to have to undo anything.

Any help much appreciated.
Once I have some progress to show, I'm looking forward to posting some pics.

Regards,

Mg
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Old Jul 29, 2012, 09:01 PM
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This post gives one option and I think there are some more details in the thread it's in.

In answer to you questions. I'm no expert as just getting into this as well, but I think the answers are:
A. Yes
B. Yes
C. Yes, although not sure what the range limit would be. The post above mentions 100m.

Hope that helps.
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Old Jul 30, 2012, 06:30 AM
m_g
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Great thanks! I have pm'ed the original poster for his advice. Many thanks for the tip. I'll be sure to post my results.
Cheers,
Mg
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Old Jul 31, 2012, 11:55 AM
no wings any more, just dust!
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Quote:
Originally Posted by which.wabbit View Post
This post gives one option and I think there are some more details in the thread it's in.

In answer to you questions. I'm no expert as just getting into this as well, but I think the answers are:
A. Yes
B. Yes
C. Yes, although not sure what the range limit would be. The post above mentions 100m.

Hope that helps.
I think thats how they do it full size and you are talking km's not m's in those cases
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Old Aug 01, 2012, 04:20 AM
Tygum Lagoon Terror
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That circuit I posted works well, its designed to allow a composite video feed to be sent from the ROV back up to the surface.

From memory if you just want to send RC down the line all that is required is to follow the RX-Tx line and components, the DC filter caps can also be eliminated. IE only need a 1K ohm, 10 K ohm and ,001 Micro Farad capacitor in series. the shield goes to battery negative at both ends.

Nick
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Old Aug 01, 2012, 05:22 AM
m_g
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Ok cool thanks. Without using your circuit nick, what would be the best way to lengthen the antenna? Is it as easy as attaching some wire to the rx antenna and running that to the surface? What wire would you recommend?
Cheers, mg
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Old Aug 01, 2012, 06:01 AM
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I (and im by no means an expert) think the best wire to use is very thin Co-ax. I have a quantity of RG - 179 from memory that is 3 mm diameter.

The resistors and caps are to attenuate (weaken) the signal as the signal does not need to pass through air but through a wire.

Nick
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Old Aug 01, 2012, 04:15 PM
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Hi
I did this many years ago...
This is the cable I used. http://www.maplin.co.uk/individually...cart-cable-292

What I did was :- remove the existing aerial wire from the receiver.
Connect the small red cable to the receiver.(the one with the shield)
On the surface I had a length of piano wire as the aerial.
Some say you can put it into the transmitter aerial via a resister (no idea on what value )
The video signal was sent up via a video balun to the surface.
The spare wires were used for leak detection.
I bought 100 meters of this cable and the Max I dived to was 78 meters in the English channel.

Regards Antony
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Old Aug 02, 2012, 01:30 AM
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Hi
Wow, I've never thought of this before for control and video transmission.
This is basically the way DSL/ADSL work right?
Will this system also work over DC power lines for the ROV?
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Old Aug 02, 2012, 10:06 PM
m_g
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thanks for all the replies, some great options. I would really like to convert my tether to have one line carrying RC signal AND video, but I fear that is beyond my abilities.

So, if I just wanted to send and receive RC signals (not video) could I get away with just running some wire from the surface into the ROV housing where the RX lives? (ie does RC signal need a certain type of wire to be trasnmitted?)
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Old Aug 03, 2012, 02:53 AM
Tygum Lagoon Terror
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Quote:
Originally Posted by m_g View Post
thanks for all the replies, some great options. I would really like to convert my tether to have one line carrying RC signal AND video, but I fear that is beyond my abilities.

So, if I just wanted to send and receive RC signals (not video) could I get away with just running some wire from the surface into the ROV housing where the RX lives? (ie does RC signal need a certain type of wire to be trasnmitted?)
Read my last post again

Cheers
Nick

If anyone wants some I could pull off 100 metres or so of the RG179 Coax for a nominal cost plus postage, say 20c per meter
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Old Aug 04, 2012, 01:43 AM
m_g
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Quote:
Originally Posted by nick_75au View Post
Read my last post again

Cheers
Nick

If anyone wants some I could pull off 100 metres or so of the RG179 Coax for a nominal cost plus postage, say 20c per meter
Ok, cool thanks. I wasn't sure what you meant by this:


The resistors and caps are to attenuate (weaken) the signal as the signal does not need to pass through air but through a wire.

Are these not necessary if Im just transmitting RC signal? Sorry, I'm a novice with this stuff. Cheers, mg
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Old Aug 05, 2012, 03:23 AM
Tygum Lagoon Terror
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Radio waves have a much easier time going through wire than air.

If you don't use the resistors and capacitor the receiver sees too much signal, overloading its circuitry, it micht wont damage anything but the control may be unpredictable.

Nick
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Old Aug 07, 2012, 11:56 AM
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Déjà vu

Hey MG.... just happened to find this post looking for information on OSD or FPV video overlays... are the same MG as the "Homebuilt ROV forums"??

bikerbones1968
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Old Oct 11, 2012, 03:27 PM
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I'm currently having problems with using cat5 cable as an antenna attached to a Futaba 2.4 ghz receiver inside the ROV. I was thinking of using the above solution of attenuating the signal as suggested above, but I'm not exactly Einstein when it comes to electronics. Would I need the same value of resistors/capacitor using cat5?
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