Electromagnetic field Sensor

Discussion in 'General Electronics Chat' started by funnygirl, Aug 17, 2011.

1. funnygirl Thread Starter New Member

Aug 17, 2011
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for a college project, we need to measure electromagnetic field within microwave oven. Is there any Sensor used to measure it directly or indirectly?

Thanks.

2. THE_RB AAC Fanatic!

Feb 11, 2008
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I have a commercially made microwave oven tester, it uses a small tuned circuit attached to a moving coil voltmeter. It's a standard design as used in RF field measuring equipment, so you might want to look there.

3. #12 Expert

Nov 30, 2010
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I've been thinking about this for hours because the standard measuring probe I had was for leakage measurements. If you put that "within microwave oven" it would fry.

I have always put a measured quantity of water into the microwave oven, timed a heating event, measured the temperature difference, and calculated the energy. Great news, it works!

I don't know if it will work for you.

4. THE_RB AAC Fanatic!

Feb 11, 2008
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I have 2 types of commercial internal probes, the simplest is a LED, diode and small leads in a tiny plastic sleeve thing. If I knew where they were I would take a photo.

When I repaired MWOs I used your method, coffee cup 2/3 full of tapwater. 30 seconds on high. You get pretty good at judging the wattage of the MWO.

5. t_n_k AAC Fanatic!

Mar 6, 2009
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I wonder if one could easily arrive at a credible answer.

Placing the cup of water (RF load) in the oven presumably concentrates the RF energy at (in) the water volume.

An unloaded empty oven (not usually recommended) would probably have a different field distribution with a complex standing wave profile.

Could one assume the cup of water is notionally free space? I doubt it. Nor could one validly assume the power flux density is constant throughout the water.

So one couldn't use the free space power (flux) density relationship Pd=E^2/Zo - where Pd is the power density in W/m^2, E is the field strength in V/m & Zo=377Ω.

I guess one could insert small probes in the water and bring the demodulated RF signal (DC equivalent) out somehow. You still have to calibrate the probe. It all sounds rather unsafe to me. Not the sort of thing I would recommend as a student project.

6. SgtWookie Expert

Jul 17, 2007
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If you operate a microwave oven without a suitable load placed within it, you will damage the tube.

Placing metallic objects in a microwave oven usually results in a rather spectacular lightning display inside the oven, along with damaging it.

Anything you use to measure the field must be able to be placed entirely within the oven; as you cannot have wire leads entering/exiting via the door gasket, due to risk of radiation of microwave energy.

As has already been recommended, measuring the temperature differential over time of a known amount of water is a very effective method of determining the power output.

I'll add that using a thin-walled Pyrex beaker supported approximately in the center of the oven will result in the best measurement, as the thin-walled glass will minimally impact the measurement. Not all ceramics are equal; some can absorb a considerable amount of RF and radiate it as heat.

To support the beaker in the center of the oven, you might use another beaker of appropriate height with a piece of cardboard between it and the one containing water.

7. #12 Expert

Nov 30, 2010
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What I'm seeing here is that indirect methods are what we know about. Even this patent uses an indirect method. However, it's interesting, so I attached it. Besides, it has some cool math (if you're a nerd).

I would like to mention that the hot water method is fairly accurate. Retail microwave ovens are rated in watts applied to the contents, and my measurements have shown they deliver what the label says, so the method must be good.

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8. debe Well-Known Member

Sep 21, 2010
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Water heat method of power measurement of magnetron.

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