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        Discussion RC transmission through water using tether

#1 m_g Jul 29, 2012 02:41 AM

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

#2 which.wabbit Jul 29, 2012 09:01 PM

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.

#3 m_g Jul 30, 2012 06:30 AM

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

#4 Ghost 2501 Jul 31, 2012 11:55 AM

Quote:

Originally Posted by which.wabbit (Post 22301345)
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

#5 nick_75au Aug 01, 2012 04:20 AM

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

#6 m_g Aug 01, 2012 05:22 AM

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

#7 nick_75au Aug 01, 2012 06:01 AM

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

#8 hargard Aug 01, 2012 04:15 PM

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

#9 tincanman Aug 02, 2012 01:30 AM

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?

#10 m_g Aug 02, 2012 10:06 PM

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?)

#11 nick_75au Aug 03, 2012 02:53 AM

Quote:

Originally Posted by m_g (Post 22341507)
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

#12 m_g Aug 04, 2012 01:43 AM

Quote:

Originally Posted by nick_75au (Post 22342987)
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

#13 nick_75au Aug 05, 2012 03:23 AM

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

#14 bikerbones1968 Aug 07, 2012 11:56 AM

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

#15 micros Oct 11, 2012 03:27 PM

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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