- Sun Feb 09, 2014 8:59 pm
#168264
I know that RF in free space has odd special cases to the usual frequency dependent inverse square law (which is why we can hear Voyger's signal from outside the solar system)... e.g., at about 20GHz, the wavelength versus rain or fog water droplet size causes high losses in a narrow band of frequencies. But this isn't in a pool or sea!
Navy for years used VLF (very low freq) transmissions from giant shore antennas fed by huge amounts of power, and digtal coding at just a few bits per second. They had other, er, non-disclosed schemes too, as did the Russians, during the cold war era. http://en.wikipedia.org/wiki/SOSUS
Moggity wrote:Hi stevech,I know a lot about the laws of physics but not so for water. I do know that fresh and salt water differ greatly. And the military uses acoustics or ultraviolet for 100's of meters or more. Many years ago I worked on one underwater acoustic data com project - it used a very long spreading code and the receiver operated at a large negative signal to noise ratio due to the post-detection correlation gain.
Thanks for your reply! I'll have to look at the performance figures for the radios I used and see if I can figure out what sort of improvement I'll get.
Perhaps you can correct me here, but this is my understanding which, if this is true then I'll be in good shape. The attenuation of the signal is going to be some sort of a cubic function vs the power of the transmitter. So if i was to increase the power output of the transmitter by a factor of 9, I would get 3 times the penetration into the water.
Is this correct? If it is then having something like 9" of submergance would be more than enough for my needs and I'll be quite pleased.
Cheers
I know that RF in free space has odd special cases to the usual frequency dependent inverse square law (which is why we can hear Voyger's signal from outside the solar system)... e.g., at about 20GHz, the wavelength versus rain or fog water droplet size causes high losses in a narrow band of frequencies. But this isn't in a pool or sea!
Navy for years used VLF (very low freq) transmissions from giant shore antennas fed by huge amounts of power, and digtal coding at just a few bits per second. They had other, er, non-disclosed schemes too, as did the Russians, during the cold war era. http://en.wikipedia.org/wiki/SOSUS