My guess is that you need to have the kind of wavelength that meaningfully interacts with human features and body composition (i.e. something that scatters and

My guess is that you need to have the kind of wavelength that meaningfully interacts with human features and body composition (i.e. something that scatters and reflects off of you, instead of being absorbed). It's not specific to the WiFi spectrum though, there are plenty of frequency ranges that meet these requirements.