RF Loss underwater source of data

[Higgler]

Can anyone direct me to data on RF propagation loss underwater, specifically sea water. I found one reference and would like other sources. Frequency range of interest is 1-100 Mhz.
Thanks,
kch

 

[nbucska]

I don't think this band is usable underwater.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Higgler]

I only need to propagate about 6 inches at 100 mhz. I have a plot showing 50 dB loss at 100 mhz for 6 inches distance. Also 100 dB atten over 13 inches at 100 mhz.
In fresh water it's alot better propagation, I've measured previously 10 dB per inch loss at 2.54 ghz.

Hence I'm looking for corroborating data on the under 100 Mhz frequency band.
I know the salinity varies depending on which ocean you are in, so loss versus different salt content would be good.
kch

 

[biff44]

I have some empirical data. The receive antenna was 10 feet away. I measured the delta in path loss for a transmit antenna in the clear, and then with the transmit antenna below 9" water. The delta path loss vs frequency is (with the underwater part more lossy):
118 KHz 9.2 dB*
3 MHz 5.7 dB
12 MHz 2.8 dB
16 MHz 11.8 dB
49 MHz 33 dB

*--used a less efficient rod type antenna, is probably better than this

 

[biff44]

That is with fesh/tap water.

 

[Higgler]

Interesting data biff44,
the curve I obtained from the internet shows about 35 dB loss at 100 mhz and 20 inches distance for fresh water. Thanks for the info, it at least shows a correlation to the chart I have, even though they don't go to your longer distance. There's not much data out there.
kch

 

[biff44]

There is probably a bunch of military data out there for things like submarines, torpedos, hydrophones, etc. Don't know where to access it, though.

 

[nbucska]

There are many ways to skin a cat...

Why do you need to do it? Do you have to xfer info or power? what is the info content?



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[logbook]

Biff44,
the attenuation loss should be a monotonically increasing function of frequency, the loss increasing because the skin depth is decreasing. Your data does not reflect this change. I suspect the reason is that the water is detuning the antenna horribly and making a mess of the results. It would be necessary to set up the transmit antenna under water then change the position still under water, taking the difference in attenuation that way. This also eliminates the problem of locating the phase centre of a wideband antenna at different frequencies. Remember that the dielectric constant of water is huge (40 or 80 as I recall) so it will change the electrical length of the antenna tremendously.

 

[logbook]

I don’t think this is something that anyone would particularly bother measuring any more. I did find an obscure paper on it dated 1939 where the guys rowed out into Lake Michigan, hung an RF transmitter on a rope and lowered it into the water at various depths. The graphs confirm the validity of the exponential absorption law, although they are only done over the range of 26m free-space wavelength to 200m free space wavelength.

The signal is given by

V=Vo * exp(-a*d)

a standard exponential decay from an initial value.

d is the distance
a is 2*PI*sqrt(mu*sigma*f)

although they are using (obsolete) electromagnetic units.

I expect that you will find that particular frequencies will be attenuated more rapidly than would be expected from the formula due to molecular resonance. This will probably not happen until microwave frequencies though.

 

[biff44]

No, the antennas were not immersed in water, so they retained their approximate radiation characteristics. I can not go into too many details, but the water was placed, in a very wide 9" layer, over the antenna.

 

[Higgler]

Interestingly enough, the IEEE Transactions on Antennas and Propagation magazine Nov. 2004, page 2843 that I received this weekend in the mail has an article titled "Propagation of Electromagnetic Waves at MHz Frequencies Through Seawater". I consider that good timing as it was difficult to get data.
I have to digest the data before I summarize it.
kch

 

[bombrack]

Higgler-
Have could you summarize the data you found in the IEEE journal? I'm interested in this topic.