Microwave oven electric field mapping

[ymstandard]

Hello,


I am trying to map the mode structures present in a standard microwave oven cavity. I am planning on using wet thermal paper and cobalt-chloride weather indicator paper to get an idea where hot spots are, but I still need a way to identify the electric field orientation.

My current idea is to use a neon bulb with leads on either end, and mount this on a positionable stand within the microwave, moving and rotating until I pickup the electric field orientation. The only problem is I have yet to find neon bulbs sold with double ended leads.

The other idea would be to use a regular neon bulb and bend the leads 180 degrees to each other, but I am not sure how well that would work.

If anyone has any ideas or experience in this area I would appreciate it!

Thanks.

 

[Comcokid]

I would make a guess that a neon bulb with no leads (or even no electrodes) would light at the peak electric field points in a microwave oven. Small gas-flled tubes are used in old vacuum tube radar sets as a T/R switch, (along with a small radioactive source to keep the gas weakly ionized and to make it switch faster).

A neon bulb brought close to the high voltage wire on a CRT will light. A flourescent bulb hung on a amateur radio antenna will light as well (with only 100 watts transmitter power).

Actually I believe the cavity modes of a microwave oven have been well studied, and I think I saw some simulation software companies posting colorful 3D simulations on their web sites as sales demonstrations. A google search may have the info you want.

 

[ymstandard]

I found an old neon bulb and tried it out. It had regular wire leads and indeed the leads/electrodes I realize don't make much difference, the gas will glow whenever the bulb is in a peak.

I am aware that there is modeling software for this sort of thing, however that won't help me. We are trying to do an experiment and we need to know with some certainty where the nodes are so we know how to arrange our set up. Any simulation would thus have to be verified experimentally, so we mind as well skip it.

Locating the nodes isn't such a problem, that can be done with thermal paper or some neon bulbs. My problem is checking the field orientation. The method that works at the moment is a piece of pencil graphite. You stick it in the microwave and you know its parallel to the field when it starts making a loud buzzing noise and spits flames and sparks.

I am looking for a slightly less dramatic test. I was thinking of putting the pencil lead in a test tube with an inert gas, but I don't if that will help. I am not so experienced with all this RF stuff...


Thanks for any suggestions!

 

[Higgler]

I'm curious why you want to find out such details? The only way to really find accurate results is to calculate it using expensive computer codes. It runs fast after you set it up. Everything changes when you place objects in the oven though.

I'd use an infrared camera and tiny (0.1" to 0.5" diam.) heatable objects, you could see the effective field when you opened the door after a bit of heating. A good looking 2D view would be to add dots of material (simple black paint) onto a plastic microwave safe board (maybe even rigid paper) and let the heat dots warm up for a bit (some may melt and run, so non-running heating items may be better), the infrared camera view would show you a good 2D view of heating. Move the board around to different orientations to see the full 3D picture.

kch

 

[logbook]

I think the field strength you are looking at is so high that a physical detector, such as your graphite rod, is much more suitable than any sort of electrical detector. Clearly you need a visible indication and apparently the thermal camera idea is unsuitable. It seems as if a slightly less conductive rod is what is needed. Pencils come in various types according to hardness (H) and blackness (B). You are probably using HB pencil graphite. You could try 3H and or 3B (the series is something like 3H, 2H, 1H, HB, 1B, 2B, 3B). The 3H may be less conductive and therefore less pyrotechnic.

You don’t say how long the rod is, but if you make it shorter that should reduce the pyrotechnics as well.

Putting the graphite rod in an inert atmosphere probably won’t give any visible indication.

 

[biff44]

You wouldn't by any chance be working on this for a company in downtown boston, would you?

 

[ymstandard]

Thanks for the tips.

Here's my progress so far:

I put the graphite pencil lead in a ceramic standoff and put it inside a small glass bottle filled with nitrogen. My goal was to reduce the arcing and get just a glow, like a mini light bulb. Well when I put the thing in the microwave, it got super bright (with no sparks that I could see) and the bottle subsequently exploded.

I guess the nitrogen got super hot really fast and expanded more than my little bottle could handle.

I think my next effort will be similar to logbook's suggestion. I plan on drawing a pencil line on a non flammabe/heat resistant surface (e.g. ceramic). This should give some visible sparking/arcing, but hopefully minimized enough that it can be used as a safe and reliable test method.

As for biff44 question, I am not working on this for any company in boston.


All the best

 

[biff44]

Good for you. I would have had a warning for you.

 

[logbook]

>Well when I put the thing in the microwave, it got super bright (with no sparks that I could see) and the bottle subsequently exploded.

Thanks for keeping us posted on this. It is nice when the discussion flow is two way

 

[biff44]

The only problem with using a neon bulb is that if it bursts, then you might end up creating "ball" lightning, which is wiked dangerous! Do not touch it, and be careful when opening the oven door!

I was thinking that you could make a simple quarterwave probe at the end of 3 feet of semirigid cable, and put it thru a small hole in the door. You might be able to rig a microwave choke joint at the door, so there would not be too much RF leakage. On the outside, you would put a pad and power meter. You could move the antenna around a good amount, and record the Efield at every point.

 

[ymstandard]

Hello again,


Ok, I tried my ceramic idea and it didn't quite work as well as I hoped. There was hardly any sparking, it might have glowed occasionally, but not reliably enough to use it as a test.

It did however get quite hot so there might be some potential if I could figure out how to visualize it. I was thinking about maybe taping it to piece of thermal paper.

I retried my nitrogen bottle test, this time with a vent hole in the cap. For some reason it did spark and shoot flames. Possibly because this bottle was much smaller than the previous and the n2 leaked out too early. I am running out of little bottles...

Biff44's suggestion regarding the quarter wave probe is interesting, I'll have to do more research on that. I am not sure though I want to drill holes the oven quite yet. Also, won't a long piece of wire in the chamber disturb the e-fields?


Thanks again!

 

[logbook]

Sorry biff, but I think your idea sucks, big time. Not only is it exceedingly dangerous, it is going to mess with the fields totally.

A thermal indicator, perhaps like those paint on liquid crystal paints sounds interesting, subject to experiment. Clearly the paint may get heated by the field directly, but it is definitely worth a go.

Would the alcohol in an alcohol based thermometer (not Mercury!) get heated directly by the field? I'm not sure, but that would be interesting as well.

 

[Higgler]

Marshmallows work too,
sounds silly, but partially fill the bottom of your oven with them, you get a 3d contour of the fields based on how much they heat up and expand. It's cheap, works slightly and they don't explode.

Maybe some other heatable material (food or rubber) could show better results.

Did you ever say why you are doing this?

kch
PS:I've used the paints before that melt at specific temperatures on radomes having 1500 watts through them. They work quite well, I think Tempalac was the name though I can't find that brand.
Only could find this product though

http://www.tiptemp.com/assets/17/paints.pdf

 

[ymstandard]

First of all, thanks to all of those who have offered their thoughts and advice. I thank you all!

For the sake of those posting, I would like to clarify what I am looking for.

I need to know two things: 1) The em-field distribution, specifically the location of "hot spots" 2) The electric field orientation in a given hot spot.

I have a methodolgy for number 1 that other research groups have used succesfully. Wet thermal paper, or cobalt chloride paper placed in the microwave oven will give a nice visual record of the em-field distribution. I have already gotten some nice results with the cobalt-chloride paper.

Number 2 is where I have had difficulty and why I originally came to this fine forum. I need a quick reliable method of checking the orientation of the e-field, something that I can't necessarily get from the various paper indications.

A method which works is a piece of graphite pencil lead. When parallel to the field it puts on a fireworks show, spitting sparks and flames, and when perpendicular does not do as much. I have been seeking a similarly simple method, just without the pyrotechnics.

I hope this clarifies what I am looking for. There have been many suggestions for mapping the field e.g., marshmallows , thermal paint, aluminum foils pieces, etc etc. These are all good suggestions, but I already have a tried and true methodology for getting a map of the field, what I need now is the orientation.

To give everybody an update on my progress:

I tried the graphite pencil lead again, only this time I put little balls of aluminum foil on either end. Lo and behold there was no arcing, just a glow!!

I'll have to try it a few more times to see if it will prove reliable (I keep breaking my pencil leads and the office supply cabinet is running low on mechanical pencils ).

If any one is interested I might be able to post some pictures of the em-field map. (Assuming you can do that on this forum?)


All the best.

 

[logbook]

>If any one is interested I might be able to post some pictures of the em-field map.
I'm very interested. You can upload pictures to your own website and link to them.

 

[ymstandard]

Sorry, I don't have my own website...