> Interestingly, objects outside the straight line between antennas can still cause interference! For best signal quality, the Fresnel zone between the antennas should be clear of obstructions. But perfection isn't achievable in practice, so RF equipment like Wi-Fi uses techniques like error-correcting codes so that it can still work without a perfectly clear Fresnel zone.
I wonder if other waves like pressure/audio waves also have a similar effect.
My mind was blown when I saw the 4F experiment, where a lens transforms an image into the Fourier domain. I'm not sure if it's related to the Fresnel zone (I think it is not or only very vaguely), but it's pretty amazing:
A widened beam of collimated light (i.e. parallel beams) is sent through e.g. a slide with some image printed on it. Using a lens placed one focal length away, it is focused down to a point (one focal length from the lens). One more focal length from that point, the beam will have reached its original width again, and another lens makes it parallel again, projecting it onto a screen placed one focal length from the second lens:
| () . () |
image lens point lens screen
This will behave exactly as expected at first glance. The image will be visible on the screen (upside down IIRC) and if you hold a piece of paper into the point, you'll just see a single bright dot. However, what's actually present (due to diffraction) is the Fourier transform of the image! If you put an iris around the point, the image on the screen becomes blurry because you just filtered the high frequencies! And what's even more impressive, if you remove the center of the point (e.g. by inserting a glass slide with a small black circle in the middle), you'll get only the high frequencies, and the image on the screen will be the edges of the original image.
Yes they do. That’s what an echo is, sound waves bouncing off an obstacle between two points. That obstacle doesn’t need to be within the direct line of sight, just within the dispersion area of the outgoing sound wave.
At the far end you’ll hear (although in reality, your brain will almost certainly cover this up for you) distortion caused by the sound wave defracting off the obstacle and interfering with the primary wavefront. Hence the reason why people put so much effort into design concert halls, and adding sound dampening treatments to recording studios. Obstacles will distort sound, but energy absorbing obstacles will distort less.
> At the far end you’ll hear (although in reality, your brain will almost certainly cover this up for you) distortion caused by the sound wave defracting off the obstacle and interfering with the primary wavefront.
I wonder how much polarization affects things; I was once told that terrestrial FM Radio is transmitted with vertical polarization to reduce interference from tall objects between you and the transmitter.
Terrestrial TV (some of which used bands that overlap FM radio) uses horizontal polarization.
> Interestingly, objects outside the straight line between antennas can still cause interference! For best signal quality, the Fresnel zone between the antennas should be clear of obstructions. But perfection isn't achievable in practice, so RF equipment like Wi-Fi uses techniques like error-correcting codes so that it can still work without a perfectly clear Fresnel zone.
I wonder if other waves like pressure/audio waves also have a similar effect.
[1] - https://en.wikipedia.org/wiki/Fresnel_zone
(Side note, is this story old? 802.11n isn't particularly new enough to upgrade to.)