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Re: magnetrons as diodes

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 09:54 PM 1/4/2007, you wrote:
Original poster: Robert Amaya <dimon20042004@xxxxxxxxx>


When microwaves hit a metal surface, they bounce off and in the process, induce a voltage in that metal. An example of this is when you put a small strip of aluminum foil in the microwave, it will blow like an overloaded fuse as a result of the induced voltage. Using that knowledge, a thicker piece (so it will not short out) of the aluminum foil, can be placed in front of the magnetrons output to be monitored for any voltage fluctuation across the aluminum that would be caused from microwaves hitting it. Im not shure on how much voltage or amperage this will induce in the aluminum foil but I hope this helps.

Except that the induced voltage will be at 2.45 GHz. You can make perfectly good sparks at that frequency, but the run of the mill voltmeter isn't going to detect it.

If you're worried, go find one of those microwave oven leakage detectors. The professional calibrated ones (e.g. from Narda) are pretty expensive, but there are very inexpensive ones for consumer use (Radio Shack used to sell them). It's basically a small dipole, a suitable diodes and a voltmeter. The safety level is 2 mW/cm2, so the consumer units are quasi calibrated in that general area. (The pro field strength meter has a lot more dynamic range AND is calibrated to a traceable standard)

you could build your own field strength probe, but calibration is a pain.

Just to get anyone interested in the ballpark: 2.45 GHz is 12.2 cm. The effective aperture of a 6 cm long dipole is about 1/8th square wavelength, or roughly 18 square centimeters. So a dipole will intercept about 40 mW at the ANSI exposure limit. Into 72 ohms (the impedance of the dipole), this works out to about 1.6-2.7 volts.

A smaller probe intercepts less power. Make it 1 cm, and you intercept 1/36th the power (about 1 mW) and the voltage is correspondingly smaller (a bit less than a volt).

A variety of diodes will work. What you want is a zero bias planar schottky or an old germanium (1n34, for instance). But, almost anything will work (1n914, 1n5711, etc.)