# Society of Robots - Robot Forum

## General Misc => Misc => Topic started by: Admin on May 01, 2006, 11:46:27 AM

Title: RC Frequency in Water
Post by: Admin on May 01, 2006, 11:46:27 AM
I want to operate an underwater vehicle in a big YMCA size swimming pool using your everyday RC transmitter and reciever. I know water blocks radio waves to some extent, but not sure how much. Has anyone tried this? How many feet of water do you think the RC signal would pass through before it is too weak?  ???

Note that I am referring to chlorine filled water, not salty sea water. I understand that radio waves pass differently through each, just not sure how much or why . . .  :-\
Title: Re: RC Frequency in Water
Post by: Botanic on May 23, 2006, 05:34:04 PM
Well this is quite complicated. But mostly it depends on what frequency you transmit. You would want a very low frequency for the signals in water.
Then offcourse it depends on the power that you use to transmit, the antenna and what kind of water it is.

The attenuation for radio signals is different for different frequencies.
Some examples:
The attenuation gets big in rain for frequencies over 10 GHz.
At around 1800 MHz vegetation (trees) can be a problem.
In cities the signal travels by bouncing from walls and other objects. High frequencies (1800 MHz) reflect better than low frequencies (450 MHz).

http://www.qsl.net/vk5br/UwaterComms.htm
Title: Re: RC Frequency in Water
Post by: Admin on May 24, 2006, 10:26:36 AM
Ok so I plan to use a cheap-n-simple RC hobby transmitter:
http://www.servocity.com/html/6-ch_laser_6_fm_systems.html

Operation is at 75mhz (ground freq).

I have used that same transmitter at 72mhz through 10 stories of a concrete building and transmission still worked with no noticable loss.

If my signal can pass through 30ft of swimming pool water with no more than say 25% loss it should be fine.

So that link doesnt work . . .
Title: Re: RC Frequency in Water
Post by: Botanic on May 25, 2006, 03:48:06 PM
That link explains the relation with frequency and attenuation quite well. It is very educational if you are interrested in these things.

75 mHz is not so very high, but it is not so very high either. You just have to test it if you want to be certain I guess.

The fact that it transmits through 10 stories of concrete shows that it transmits with pretty good power. But then again wavelengths in water and concrete does not behave the same way.
Title: Re: RC Frequency in Water
Post by: Ben on May 25, 2006, 10:40:01 PM
I know I've played with a cheap remote controlled sub that would work down to 8ft in my pool, and it operated at 27MHz.  The toy was cheap, so I don't believe it was using a very strong transmitter.
Title: Re: RC Frequency in Water
Post by: Afroman on May 29, 2006, 06:07:45 PM
Pika
Unless I'm mistaken that's a 1W 75Mhz transmitter - 30ft of water should be a piece of cake. I'd be amazed if 100ft even did anything. If you were using a 2.4Ghz radio though, you would get pretty much 100% absorption by the water so stay away from that band.
Title: Re: RC Frequency in Water
Post by: zamboniman60 on June 24, 2006, 12:02:32 PM
well, for submarines I know they use VLF or ULF, around 120hz I think. Unfortunately, with such a low frequency data transmission is quite slow -- you should be fine with the lowest frequency hobby transmitter you can find.