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I am building a device that needs to be inside of a microwave oven but somehow needs to communicate with another device outside of the microwave.

How can I reflect microwaves so they don't come into contact with the electronics in the device, while still allowing the device to transmit and receive data?

The frequency of the microwaves in a microwave oven is pretty much the same frequency that is used in Bluetooth. I was thinking the only possible way of doing this is to have an antenna that is small enough that it can slip outside of a microwave oven through the door (but small enough so the door is still able to close properly).

Could I use some material to protect the electronics of the device from microwaves? Is there a way to have this device communicate with another device outside of the microwave oven without the microwaves interfering with it?

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    $\begingroup$ Why does it have to be Bluetooth? It would make more sense to carry the baseband signal outside the oven on fine wires, and then convert to Bluetooth (if necessary) on the outside. $\endgroup$
    – Dave Tweed
    Mar 13 '15 at 12:12
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Your best option for wireless communication in this situation is some kind of optical link.

Consumer microwaves typically operate at a frequency of 2.45 GHz and Bluetooth operates from 2.4 GHz to 2.485 GHz, so any shield you design for one will be equally effective for the other. You could design a Faraday cage which gives you enough suppression at 2.45 GHz but still allows enough power at higher frequencies (EHF or THF) to communicate. However, there aren't many devices which operate at those frequencies, and designing one yourself is not a simple feat. An antenna will likely suffer from the same problem. If it transmits at 2.4 GHz it will also receive at that frequency, and the load caused by this will likely also destroy your device.

Instead, you should build a Faraday cage around your device which suppresses 2.45 GHz but still allows light to pass through. A cage with 0.5 cm holes should give you more than enough suppression since the wavelength of 2.45 GHz is ~12 cm. Building an optical link may sound complicated, but the hardware is fairly cheap and you might be able to steal some of it from a television remote. The amateur radio community has some experience building DIY optical communications systems which you can draw on. You can obtain hardware for the encoding and decoding using either the PSK31 or RTTY protocols from the same set of devices they use for radio communications.

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With regard to protecting active electronics inside the oven, you have the choice of either absorbing the microwaves (e.g., water jacket) or reflecting the microwaves (any high-conductivity metal).

Any sort of reflector for microwaves inside an oven must be designed very carefully, because there's a good chance of creating unwanted "hot spots" and/or areas of high field strength (causing arcing).

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Is there a way to have this device communicate with another device outside of the microwave oven without the microwaves interfering with it?

Not with most local wireless communication protocols because they use the same frequency band as a microwave.

However on lower power settings the microwave will turn off periodically. That will be your chance to communicate.

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    $\begingroup$ You won't be able to communicate when the microwave powers down because it is still shielded by the standard shielding in the oven box. This will block the radio communications (at those frequencies) regardless of whether the magnetron is powered on. $\endgroup$
    – jhabbott
    Mar 17 '15 at 16:58

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