Effects of Water on RF Circuits?

[mrkenneth]

Hopefully this is the best forum for this post... Anyway, here goes:

What are the effects of water on radiofrequency circuits in the range of 500 MHz to 3000 MHz? I have been thinking about immersing an entire circuit consisting of a planar antenna and some RF circuits into some water. The water will probably be from the tap, or if needed, I can also use distilled water. With an el cheapo multimeter, the water registers resistivity higher than the range of the multimeter. What about the impedance?

The reason I am doing this is because I would like to absorb/attenuate RF interference from one side of the planar antenna. One side of the antenna would be surrounded by air, while the other side of the antenna (where the RF circuits are) is covered with water. Something like the diagram below:

Are there any problems with the idea?

Thanks in advance!

---------------------------------------------------------
Operation Radiation:

http://www.mrkenneth.com

 

[IRstuff]

Are you going to overcoat the board?

Otherwise, it'll probably fizzle and grind to a screeching halt, regardless of what the antenna properties might be.

TTFN

 

[pennpoint]

How about using dielectric fluids such as polyalphaolephen (PAO) or 3M brand FC-77 these can be cooled also.
The 3M stuff leaves zero residue as opposed to the oily PAO.
These fluids are Not as cheap as water by the way.

Best Regards
pennpoint

 

[loftust]

Obviously low frequencies travel further through water than higher frequencies. Refresh my memory, how do planar arrays act? Are they omni directional or what?

 

[itsmoked]

mrkenneth; You will have a nonlinear attenuation. Remember that a microwave oven heats by oscillating polar molecules. They are tuned to water. I think the are around 2000MHz which means around there you would get major attenuation and less elsewhere. Does sound like fun tho.

You should consider water in thin bags that your board can lay on. Avoids the whole water on active circuits headache.

 

[pennpoint]

Chwck out this article:

http://www.electronics-cooling.com/Resources/EC_Articles/MAY96/may96_04.htm

 

[BrianR]

The resistance and loss factor of ionic warer (tap water) is quite poor and the dielectric constant is in the region of 80 so unless your design can tolerate every component being shunted with a variable lossy low impedance it aint going to work. Conformal coatings will not help much due to capacitive effects.

For interest, military sono-buoys are floating radio transmitters with a whip antenna protruding above water. All the electronics are underwater in a shielded waterproof housing. Submarines receive radio signals at shallow depth at 14-60KHz or at greater depths <100Hz.

 

[mrkenneth]

Thank you for all of the quick replies!

I do not think I can use the expensive chemicals mentioned by pennpoint. This is just a personal project with limited resources. ;-)

loftust, the planar antenna that I am using should have a radiation pattern similar to that of a dipole antenna.

I do not need attenuation at specific frequencies. I would just like it to attenuate everything that is not DC, so linearity is not a concern.

pennpoint, thank you for the link to the link to the direct immersion article! Unfortunately, it only mentions this about water: "Water [...] is generally unsuitable for direct immersion cooling on account of its chemical characteristics."

BrianR, do you mean that water has a low impedance, and would shunt AC signals? Is there something like a impedance versus frequency chart?

I do not think capacitive effects will be a large issue; the circuits are very simple and almost all low-power DC.

As for the sono-buoys that you mentioned, I assume no water ever comes into contact with any electrical circuits?

---------------------------------------------------------
Operation Radiation:

http://www.mrkenneth.com

 

[BrianR]

Sonobuoy electronics lives in a waterproof metal box with the water on the outside. Why can't you do that too. Your circuitry might be mostly low power DC but somewhere that converts to 500 MHz to 3000 MHz and capacitive effects are an issue there. I used the term impedance which is the AC resistance but use conductivity instead, eg pure de-ionised/ distilled water conductivity is 0 (insulator with a dielectric constant of about 80) while fresh water has about 10uS/cm and salty lake water up to 160,000 uS/cm (near short circuit).

You have to isolate your circuitry from these effects which appear as variable capacitors shunted by variable resistors across and between everything. The same applies to your antenna - keep it dry and always above water if you want any consistancy in performance.

For water properties, go to:

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/waterphys.html