this post was submitted on 03 Jun 2024
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I think you mean that statement the other way around, and it’s not going to perfectly absorb even in that case unless you have a true faraday cage.
Buildings won't perfectly absorb signals but it will attenuate the signals that pass through them.
The gaps In a faraday cage determine the maximum wavelength of the electromagnetic signal that can pass through. AM signals have very long wavelengths, and are more likely to get distorted
Any metal in a building will act to distort and absorb signals, the more metal, like rebar in concrete, the less signal can get through.
Examples:
Consider the ocean, ionic water, very difficult to get radio signals because there are so many dense charge carriers to absorb the radio waves.
Consider the earth: sending radio signals through the center of the earth is difficult because of all the metal, electron carriers in the earth itself.
Consider wifi in a modern concrete and rebar office building, one or two rooms over and the signal gets absorbed quite effectively.
I think I understand better where you’re coming from. I have a variety of homemade low frequency antennas, typically on the order of 20 meter wavelength, and I observe lower frequencies clearly get better reception inside buildings. Transmission tends to be more variable because I have an increasingly large near field zone that’s effectively impossible to clear. Indeed, my real world experience has always been the opposite. Lower frequencies appear to get through better, provided you can actually talk out. I usually prefer to modulate the H field because it’s orthogonal to household noise sources and after some distance away doesn’t couple to metal.
I’m not sure why this is. Perhaps buildings are different enough from Faraday cages? Lower frequencies diffract and bend around objects much much more effectively than high frequencies.
I have some RF design engineer friends and I’ll ask them why my experience is different.
If you find your having better reception indoors then outdoors then the signal your looking for is being attenuated less then the noise/interference is being attenuated by the structure your housing your antenna in.
Low power noise is less likely to bounce around and come in the building from a different angle, but AM is famous for its ability to propagate.
Consider driving a car into a parking structure with the radio on, listening to AM radio. You can hear the signal getting weaker, not stronger. Until you get to the roof. If you go into the basement, you only hear local noise