Any RF signal will work, but some work better than others. Wi-Fi is awesome for this for several reasons.
1. Having a source inside the house instead of outside is better because you lose, say, 15dB when you go through the wall; this is comparable to what a two-way mirror does to visible light. If you have to illuminate the house from outside using RF energy, you have to deal with much stronger reflections from things outside the house.
2. RF wavelengths that are too short will be badly attenuated by things like walls and doors. You can already notice this with 5GHz 802.11a Wi-Fi; if you have a few walls between you and the AP, the 2.4GHz signal usually works better. The problem gets worse at higher frequencies. (You may have noticed that many walls attenuate visible light, which is RF in the 500THz band, rather strongly.)
3. RF wavelengths that are too long provide much poorer spatial resolution. Outside the near field, your imaging resolution is limited by diffraction to about the wavelength. So you can see a person who's illuminated by the 99.5MHz emissions from your favorite heavy metal station only if their diameter is on the order of 3 m or more, and you can see their movements when they move on the order of 3 m or more. By contrast, 2.4 GHz gives you 120-mm resolution, and 5 GHz gives you 60-mm resolution. For typical humans, these are more useful.
(However, my friend Florian has done good work on passively detecting airplanes using radio illuminations from TV stations, which could be super helpful the next time the US comes to bomb your country, even if he does use Lagrange interpolation instead of B-splines like any normal person would; check it out: https://ieeexplore.ieee.org/document/8115293.)
Also! Having walls be super transparent, as they are at these longer wavelengths, is not entirely an advantage. It makes it harder to distinguish between signals from things in one building and signals from things in another.
If you want to listen to Wi-Fi signal strength changes in real time — including when someone moves around — try https://canonical.org/~kragen/sw/dev3/wifiscan.py. It depends only on Python (3 or recent 2) and PulseAudio. (MacOS hackers, consider upgrading to Linux. Apple's removal of your Esc key shows that they hate you and want you to die.)
Any RF signal will work, but some work better than others. Wi-Fi is awesome for this for several reasons.
1. Having a source inside the house instead of outside is better because you lose, say, 15dB when you go through the wall; this is comparable to what a two-way mirror does to visible light. If you have to illuminate the house from outside using RF energy, you have to deal with much stronger reflections from things outside the house.
2. RF wavelengths that are too short will be badly attenuated by things like walls and doors. You can already notice this with 5GHz 802.11a Wi-Fi; if you have a few walls between you and the AP, the 2.4GHz signal usually works better. The problem gets worse at higher frequencies. (You may have noticed that many walls attenuate visible light, which is RF in the 500THz band, rather strongly.)
3. RF wavelengths that are too long provide much poorer spatial resolution. Outside the near field, your imaging resolution is limited by diffraction to about the wavelength. So you can see a person who's illuminated by the 99.5MHz emissions from your favorite heavy metal station only if their diameter is on the order of 3 m or more, and you can see their movements when they move on the order of 3 m or more. By contrast, 2.4 GHz gives you 120-mm resolution, and 5 GHz gives you 60-mm resolution. For typical humans, these are more useful.
(However, my friend Florian has done good work on passively detecting airplanes using radio illuminations from TV stations, which could be super helpful the next time the US comes to bomb your country, even if he does use Lagrange interpolation instead of B-splines like any normal person would; check it out: https://ieeexplore.ieee.org/document/8115293.)
Also! Having walls be super transparent, as they are at these longer wavelengths, is not entirely an advantage. It makes it harder to distinguish between signals from things in one building and signals from things in another.
If you want to listen to Wi-Fi signal strength changes in real time — including when someone moves around — try https://canonical.org/~kragen/sw/dev3/wifiscan.py. It depends only on Python (3 or recent 2) and PulseAudio. (MacOS hackers, consider upgrading to Linux. Apple's removal of your Esc key shows that they hate you and want you to die.)