If we use our microwave oven for longer than about 30 seconds, our car鈥檚 alarm goes off. Why? The car is at least 20 metres away through two walls. The inside of the microwave is a little corroded and the car has a remote central locking/alarm system.
Certain car alarms, such as those fitted to recent Mazda 6, Toyota Rav4 and Mitsubishi Shogun models, transmit a continuous signal at 2.45 gigahertz at powers of up to 500 milliwatts. The microwaves are picked up by sensors inside the vehicle, which detect changes in intensity to signal the presence of intruders. Microwave ovens also operate at 2.45 GHz. While the power radiated within the oven is typically in the range 600 to 800 watts, the amount radiated outside the appliance will typically be less than a watt. When your oven is in operation, the microwaves reaching your car may be powerful enough to trigger the sensors inside it, which the alarm system interprets as a disturbance within the vehicle.
It is possible to set a car alarm so that the internal signal generator is disabled. You might also want to have your microwave oven serviced in case there is a serious leak of radiation. If your microwave has damaged shielding the radiated power could be higher than the values above.
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Joel Smith, Pateley Bridge, North Yorkshire, UK
It is odd that your microwave is leaking enough radiation to trigger the car鈥檚 alarm, considering the legal limit 鈥� in the US, at least 鈥� for leaked radiation from a microwave oven is 1 milliwatt per square centimetre at a distance of 5 centimetres. Perhaps your microwave has a serious leak, or you have an unusually sensitive car. You could try parking the car in front of a friend鈥檚 house and running their microwave oven to see what happens. If it appears to be solely your problem, consider getting the microwave replaced.
Alex Reinhart, Boerne, Texas, US