Why can't the electromagnetics waves travel through water?
what frequency electromagnet waves are you speaking of ?
Light is an electromagnetic wave. Can you see under water? Can you see through a glass of water? If so, then electromagnetic waves can travel through water.
If you are referring to RF EM-waves though the oceans, they do travel through but attenuate exponentially. There is less attenuation as you lower the frequency. Implication is that you can communicate under the ocean using ELF (extremely low frequencies)
See graph titled Attenuation of RF Passing Through Conductive Media, on this page.
I know that is used for submarine communication, but does anyone have any idea how that works? To receive EM signals, I thought your antena needs minimum dimensions of the wavelength divided by two. At 76 Hz the military uses, this involves a wavelength of several thousand kilometers. Somehow I doubt submarines have that long of a cable trailing behind them as receiver.
You need a resonant circuit with part or all of it exposed in such a way to radiate as well as pick up radiation. That is technically all an antenna needs to be.
Buoyant Cable Antennas have been used around the submarine service for quite some time. do a goggle search on the topic and you will find a wealth of info. Most BCA's are made for VLF/LF reception allowing them to be on the order of 1000' in length.
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