hello
i want to design an antenna for under water communication
which substrate i have to use ??
It depends on what frequency you want to transmit.
Water is opaque to almost all EM waves: exceptions are light, and ultra low frequencies (
to add to the above comment:
At 1 MHz the power falls off about 35 dB per metre in sea water. In fresh water it is better but even then it is about 1/2 dB per metre at 10 MHz. This is as well as the effect of the inverse square law.
At extremely low frequency the transmit antenna has to be tens or hundreds of kilometres long or it is very (very, very, very) inefficient, e.g. use megawatts to transmit watts.
The receive antenna can be small and has extremely low efficiency but this doesn't matter because (I think) the noise level is so high in this band that even an inefficient antenna can pick it up, so if the signal is bigger than the noise (which it has to be to be detectable), then an inefficient antenna can pick it up. Also the effective aperture (for receiving power), or effective height (for picking up voltage), before taking efficiency into account, is enormous.
Have a look at these refs.
https://en.m.wikipedia.org/wiki/Extremely_low_frequency
https://en.m.wikipedia.org/wiki/Communication_with_submarines
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