Sucessful Underwater Video - Transmitting Video

[ramius-ii]

Hi All]http://www.subcommittee.com/forum/icon_smile.gif[/img]
For a long time I read posts on what and, more importantly what was not possible when attempting to send video from an underwater transmitter. So, today I decided to see what could be done and what results could be expected. Using a borrowed color bar generator and a transmitter on 1.265 GHz with a 2 watt output, I got a solid, reliable picture while the transmitter, etc. was under 3 feet of water in my jacuzzi! At first, I thought one the antenna went below the surface, all signal would be gone. Starting with the receiver about 5 feet away, I still had signal! The I grabbed the television, extension cord, etc. and walked over to the house. Set the TV on a lawn chair, ran back to the jacuzzi, lowered the stuff into the water again, ran back and there were these great beautiful color bars, all the right colors, in the right places, just like in the studio. So I am now smiling from ear to ear as this was done with a zero gain antennas at the transmitter and receiver. Once an higher gain antenna is used, the depth and distance should be greater!

Best to All, Ed

[interpol]

If you live in the

If you live in the US, I hope you have a license for that.

And it will be less in pond water, your jacuzzi water is a lot cleaner and has less conductivity than other water.

[ramius-ii]

Hi Interpol]http://www.subcommittee.com/forum/icon_smile.gif
Yes, I

Hi Interpol]http://www.subcommittee.com/forum/icon_smile.gif[/img]
Yes, I do have a license, in fact, I have several! 1.265 was chose by the local frequency coordinator as it is in the center of the amature radio band and is used for "simplex" (one way) communications. Therefore, since the signal does not interfere or conflict with what's know as repeater networks (you come in on one frequency and out on another), no one really cares.
Since neither the transmitter or the receiver have an antenna that provides any gain (ERP), once gain is introduced, water conductivity (less than salt water), should not present much of a problem.
Most installations I've seen use a small transmitter on the back of a camera. Such cameras do not have any kind of "ground plane" to direct, reflect, or amplify the signal. Therefore, what you have is the water attempting to be the ground plane, which is un-tuned and "confines" the signal.
If on your television set you use a single wire for an antenna you will get some signal. Once you add that second wire (like with "rabbit ears") you now get directionality. Add more elements and now you get gain!
While the conductivity of the water is a factor, any solid metal ground plane over rides this factor and reduces the waters influence to just being a capacitor attempting to detune the antenna. When the driven element of the transmitting antenna is given more mass, the antenna becomes more "broadband" (handling a wider frequency range). Detuning the antenna via the capacitance effect of water still allows for a greater power transfer.
Think of it this way, if you have an antenna that is cut for 1.3 GHz. and you add capacitance to the antenna, you lower it's resonant frequency. To drop .1 GHz or 100 MHz, takes a bit of capacitance. If you start with a broadband antenna (one that will work from 1.2-1.4 GHz) then varying amounts of capacitance as the result of different chemicals in the water, still allows a reasonable amount of signal to be sent out rather than being reflected back to the transmitter (SWR).
What was important was to know for sure if it was possible to get a quality signal after hearing both the "yes, it can be done" and no, it can't be done".

Best, Ed

[interpol]

Actually.....your biggest loss in signal

Actually.....your biggest loss in signal strength is the change in medium from water to air. Its a bigger loss over more conductive water, because it is a bigger change in medium.

I believe its a 26-28dB loss from air-to-water or water-to-air transmission. And that's not including the loss of transmitting through the water itself.

[ramius-ii]

Hi Interpol]http://www.subcommittee.com/forum/icon_smile.gif
You are

Hi Interpol]http://www.subcommittee.com/forum/icon_smile.gif[/img]
You are correct about loss considerations, at the same time, with an antenna providing 3 DB of gain, this gives an ERP (Effective Radiated Power) of 4 watts. With a loss of 27 DB, the signal is still at a .0078 watts (7.8 mw), which is more than enough for a good signal which only requires 1-2 mw.
I'm not sure where the 26-28 db of loss number comes from, it would seem it would be a function of frequency, depth and the angle of the signal leaving the water. I'll see if at some point I can measure signal strenght. I do know you can not get much of a signal directly over the antenna, you have to be to side slightly.
By the way, my sincere thanks for the info. It's great to keep the brain cells active!

Best, Ed

[interpol]

Yeah, let me know how

Yeah, let me know how it goes. I am in the process of obtaining a license so I can use the 1.2 range myself. I already have all my equipment soldered up and ready to go, and have tested it all without my RF amp, and it all works good. I just haven't hooked up the amp yet because I don't want the FCC screaming at me for shooting 5W into the air without a license.

Another thing you might want to look into, go to www.cyber-flyer.com

He has a unique set-up where he uses two receivers, and has a switch that automatically pulls the best signal. That would be a good option for you, you could put one receivers antenna just above the waterline, and the other receiver's antenna below the waterline, and see how that works

Just a though.

[scott t]

Just skimmed these real quick.

Just skimmed these real quick. What about a water/air relaying station device which recieves the video from the water and retransmitts on something else in the air.
Anchor this device half in and half out of the water to relay
your needed signals.

[ramius-ii]

It’s a wonderful thought and

It’s a wonderful thought and adding in Scott’s idea, one could receive on 1.2 GHz and re-transmit on say 2.4 GHz! A basic repeater as long as 2.53 GHz isn’t used.

The system you mentioned with the two receivers is a version of what’s called a “diversity” system used by Vega Wireless and Modulation Science. The difference is, they looks at the signal strength at the antenna pre-amp and uses that to select the antenna. We use this system for “talent” on the stages and in the helicopter.

I think you and I live fairly close to each other, I’m in Lomita by the Torrance airport and the frequency coordinator knows I’m testing so you are welcome to come up and test whenever you would like. If you are short on equipment, I can always borrow things from work for the weekend.

If you’d like to see the transmitter I’m using, it’s at www.hamtv.com.

Best, Ed