802.11 (b/g/n) operates on the 2.4 GHz ISM band. This is conveniently the same band that your microwave oven operates on. This isn't a coincidence, both operate in the 2.4 Ghz ISM band, because it can be freely used at low power without licence nearly anywhere in the world. Many other RF technologies including Bluetooth, walkie talkies, baby monitors, etc. also use the same band.

Most microwave ovens tend to be very well shielded and will not emit enough radiation1 to interfere with wireless communications. It is possible that your unit has a damaged shield. You could look into replacing it. That said, the amount of radiation a microwave has to "leak" for it to interfere with your wireless is tiny - 0.01% of it's output, roughly the same amount as your router or laptop, and far less than a typical mobile phone.

A better thing to do would be to upgrade your wireless networking equipment and devices to be 5 GHz compatible (used with 802.11 a/n/ac). This is the other major band WiFi networks can operate in, is capable of providing increased performance, and should not suffer interference from microwave ovens. You can tell if a device supports 5Ghz by checking if it lists "dual-band" capability, or supports 802.11a (e.g. "a/b/g/n") or 802.11ac. 5Ghz capable wireless equipment is becoming more widespread, but 2.4Ghz remains common in older and lower-end devices.

Addressing your different channels, microwave ovens (which should label the output frequency somewhere) should use ~2.450 GHz.

WiFi (b/g/n) channels typically range from 2.412 GHz to 2.472 GHz, with a bandwidth of 20 MHz and a 2 MHz band gap. If you pick a channel from the upper or lower end, and assuming your microwave oven is precise enough with its frequency, you could sidestep it entirely. This is, however, just a guess.



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1 I must point out that 2.4 GHz is far from ionising radiation, which is at least 2400000 GHz (the type that can harm human tissue and/or cause cancer). Even if the shield is faulty, it will not cause any harm. Any (very slight) damage would be caused by heating (and not directly by 'radiation'), which you most definitely will feel before any real damage. Also, just don't stand in front of it for hours a day. That always helps. Some countries also have regulations on the maximum energy allowed to be transmitted, ostensibly for human safety (there are other limits designed to reduce interference). Do note that such regulations tend to play far on the safe side, well below any level with concretely proven ill effects.