I am trying to figure out a few things related to a microwave steam generator I am working on.

1. If a microwave magnetron's waveguide is aimed sideways through a glass window into a vessel of water, will the microwaves pass through the window and be absorbed by the water, or will some be reflected from the glass/water transition boundary back towards the magnetron? If reflection occurs, how serious would it be?

2. If a regular 2.45 Mhz oven magnetron's metal antenna were put through a hole and sealed in the side of a water vessel so that it was completely submerged, would the microwaves be transmitted into the water by the antenna and absorbed, or would they be reflected back to the magnetron by the metal/water transition boundary?

3. What is the penetration depth of 2.45Mhz microwaves into water? How does this depth vary with temperature of the water? What angle do microwaves need to hit the surface of water to be absorbed and not reflected?

Please don't try to shoot my project down, it is just for fun anyway. I just need some info on the questions above. Help is always appreciated.

Thanks!

1: What you need is the "reflection coefficient at normal incidence" which you can look up under that name on the internet. This depends on the dielectric constant of glass, which is a function of frequency, so you will need to find this number somewhere. My hunch is that since microwaves are relatively low frequency, and glass is not inherently very polarizable, that nearly all the microwaves will pass through the glass.

2: Since the microwaves would have to eventually get from the glass to the water anyway, you would have nearly the same total amount of reflection if you put the antenna directly in the water as if you passed the waves through glass first. It is possible to choose the thickness of the glass so that the reflections at both surfaces exactly cancel and you get 100% transmission to the water. In this sense, it is better to use the glass than the direct contact with water. On the other hand, it is also possible to eliminate the reflection at the antenna/water interface by changing the impedance of the antenna to match the intrinsic impedance of water at microwave frequencies. You can do this by adding a "matching stub" which in the case of a microwave antenna would look like a small metal box growing out of the side of the main microwave channel near the end, like a short branch sticking out of the side of a tree. This box forms a resonant cavity in which standing waves of microwaves form, and if it is correctly sized and positioned, it can change the apparent impedance of the antenna to anything you like. Most antennas come tuned to the intrinsic impedance of the vacuum, which is 377 ohms

3: Normal incidence is best for absorption. The penetration depth depends on the imaginary part of the dielectric constant, which you can find on the internet. It will change with temperature, but not very much. It is more sensitive to frequency, which you are not changing anyway. As a rough estimate, kitchen microwaves have a penetration depth of 2 or 3 cm. Less than that, and they would burn the outside of the food, more than that, and not enough absorption would occur as the waves passed through normal sized food items.

The reason I know about this stuff is that a couple of years ago I had an idea of using tuned electric and magnetic fields at microwave frequencies to exert force on water, making basically a pump with no moving parts. I did some research and found out a lot about the polarizability of water, etc. Then I did some calculations and found that you needed EXTREMELY high electric field strengths to get even modest sized forces using this method. I also found out that this exact idea had also been worked out before by ... Albert Einstein. Since you don't see these water impellers all over the place powering stealth submarines today, I assume he couldn't make it practical either. I gave up on the project at that point, but I still have the data. I got it all from the internet, but some of the sites may not be there any more.
This site has a lot of stuff about properties of water.

I have a bunch of other downloaded information about microwaves and water, but I don't know any longer where I got it. Send me a private message with your email address and I'll forward them to you.

Hope this helps!

--Stuart Anderson

Now that is what call good info! Thanks for the response! I am also looking at the electrode boiler style of creating steam, though I am thinking of using two flat electrodes facing each other instead of a perforated bucket & rod electrodes.

Keep the info coming :-D

Hi, I've a question about magnetrons.

The nub or button that emits the microwaves; can I use a contact wire
and draw the energy off to power a load? Everything I read about
microwaves talks about wave-guides, and yet i know that nub is
tapped into a segment of seething electrons deep inside the magnetron.
Seems there should be some current there, that a person should be
able to draw off. Yes I know microwaves are hazardous, requiring
proper shielding, etc.