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I read that the Faraday cage on a microwave door prevents the microwave radiation from escaping the microwave because the wavelength of the microwaves is "too large to fit" through the holes in the door, but this has only made me more confused.

Why, then, do radio waves still permeate through buildings and other objects like the microwave door, even though they have massive wavelengths in comparison?

To test this I placed my phone inside the microwave and called it. The phone call was received through the microwave, causing my phone to ring, so I know the radio waves can permeate the microwave door, even though the faraday cage supposedly stops microwaves with smaller wavelengths from passing through.

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A microwave oven isn't a Faraday cage: there's a gap all around the door, narrow but much longer than a wavelength. Microwaves can easily leak through such a gap.

However, the edge of the microwave door is what's called a "choke flange". It contains a resonant slot, tuned to reflect waves at 2.45 GHz, the frequency the oven uses. So, around that frequency, leakage around the door is suppressed. At other frequencies the gap is much less reflective.

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  • $\begingroup$ 2.45GHz -> $\lambda= 12 $ cm. I do not see a gap that large around my microwave door. $\endgroup$
    – mike stone
    Commented Jan 23, 2023 at 0:32
  • $\begingroup$ @mikestone It's a narrow slot, only 1/4 of a wavelength long. 1/2 a wavelength round trip, so the reflected wave from the end cancels the incident wave. The door of a microwave oven is typically ~4 cm thick to accommodate the slot. $\endgroup$
    – John Doty
    Commented Jan 23, 2023 at 0:46
  • $\begingroup$ I have difficulty visualizing this. Do you have drawing or figure? $\endgroup$
    – mike stone
    Commented Jan 23, 2023 at 1:21
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No real Faraday cage can stop any radiation perfectly in either direction. There is always some small intensity inside due to radiation from outside, and vice versa, the value depending on thickness of walls and geometry of holes. Microwave with small enough holes and good enough alignment of closed door produces low enough radiation outside that it is not a big concern. Mobile phone can work with very weak intensity of cell tower signal, so your experiment shows that even though microwave shields cell tower radiation, the decreased intensity inside is still strong enough to activate the phone.

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How do you know that the door is sealed in an RF sense? Have you ever tested how much is actually leaking out of the oven and not just through the front window but all around, say, with a spectrum analyzer? Specification is one thing reality is another.

Anyhow, almost anything below 40GHz would easily penetrate a wood frame house and even a steel reinforced concrete building has no problem except with wavelengths longer than the separation between the iron bars. There can be difficulty also with all the wiring but again unless the concrete is really wet it should let pass much of the RF through.

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