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Radio wave (2.4ghz) travel in water?

M

MegaVovaN

Diamond Member
http://www.digi.com/products/wirele...es/zigbee-mesh-module/xbee-digimesh-2-4#specs

Dear ATHT,

I have two chips like in the link above [middle column, "pro"], one transmitting, and the other receiving with standard-size (maybe 3 inch long) antennas.

I want to submerge them in dirty water (of course seal so they won't get wet), couple feet apart. Would they still "talk" to each other? What is the formula to calculate how much signal is lost? I.E. if on land range is 100 ft as per spec, how much range, if any, would be in water? 2ft? 0.2 ft?

Thank you!
 
A couple feet apart in dirty water? No. My guess is that they will probably lose signal after a few inches. This will depend on the properties of the water but I recall calculating for sea water that you'd lose it after a few inches. What you need is a measure of the conductivity of the water and you can then use that to find the skin depth at your given frequency.
 
This is fun stuff. I'm envious that you have the time.

There is not a lot of data on this in fresh water. The general consensus is that signal loss is directly related to the conductivity of the water. So dirty water will have much greater attenuation that pure water. It is all relative & good luck with that in other words.

I'd suggest thinking about much lower ISM band ... 40 MHz? But since you already have these devices you are the only one in the position to experiment. Shouldn't be too hard.😵

You might want to include in your experimenting different antenna designs like small parabolas. The size of these will be influenced by dk. Keep in mind that the dielectric constant (dk) of water is 81. The sq. root of that (9) increases the electrical length of things like antennas & is not a bad thing. There is plenty of info on the net wrt antenna design & I think most should be scalable.
 
With water you want to go low frequency, really low, 30-40Hz is good. Think about instead of using RF to send your information use sound. A speaker inside a container and a mic with a filter for the specific frequency works great and can go several hundred miles underwater if needed, that is how submarines communicate even today with lasers and everything else available.
 
No, it won't likely work. The problem is much worse than conductivity. Conductivity is measured at VERY low frequencies - actually, it's supposed to be a DC measurement. The real issue is that at microwave frequencies, water has a VERY large Dielectric Loss Factor in the frequency range from about 0.5 to 50 GHz. That is, it absorbs the waves at those frequencies very well, causing substantially more rotation and vibration of the water molecules, which we perceive a heated water. That is why microwave ovens work. They heat water, fat and protein molecules by this mechanism. All this means that very little microwave radiation can penetrate water to any significant distance.
 
Some definitions ... loss is a relative term. Conductivity is the reciprocal of loss. Dielectric loss is a function of frequency as is dielectric constant. These values can be measured & material models developed to any frequency that test equipment can operate to (10s of GHz if not 100s) ... not limited to DC which is usually a useless value.

Yes, there is *a* peak of absorption in "... water, fat and protein molecule ..." & near 2.45 GHz, but the RF radiation does not stop at the surface & can penetrate from a few to several cm.

In other words the RF signal is attenuated at some rate dependent on the composition of the dielectric. It may be a very steep curve, but it is not a brick wall. I have designed a few antennas at these freqs. for use within the body as well on the body & have a feeling for the challenges.

I say all of this as the OP was not really specific. I suspect that he can't be because this sounded more like a feasibility experiment. OP did mention 0.2 ft or 2.4 in. A gap of 2.4 in could probably be tolerated with the right kind of antenna. At this frequency antenna gain can be substantial with modest complexity *IF* different antennas are acceptable.
 
Thanks for replies guys - I can assure you this is a very real project, and not a thought experiment.

I am having trouble finding a commercial product that would transmit & receive around 30-40 mhz. Can you guys please help?

AM short wave radio for short wave is about 2.3-26.1 mhz also (source=wikipedia)?

I am thinking of those radio controlled car transmitters that are what, 27 mhz?

I need a rather compact device, about 1 inch diameter tube few inches long.

Gap that signal should punch through is about 2 feet of dirty (conductive) water.

Thank you!
 
MegaVovaN said:
Thanks for replies guys - I can assure you this is a very real project, and not a thought experiment.

I am having trouble finding a commercial product that would transmit & receive around 30-40 mhz. Can you guys please help?
Click to expand...

Radios in those bands are usually custom designs. One place to research is dive shops and equipment sites. Divers use communication systems underwater that are wireless.
 
Is there a reason that you can't run a cable between the two? If you can connect them with a conducter then I would think it would be pretty simple to get the signal across a few feet.
 
Low frequencies do better in water. You could probably get away with making a small ELF transmitter at Part 15 regulations and using that.
 
We ran a radio controlled submarine in the fountains at the mall. It would maintain signal down to about 1.5 to 2 feet under water.
 
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