Microwaves Engineering

Why are the Microwaves used in through the wall detection radars, pass through walls (being of such frequency and energy) but still get reflected from Human targets ?

And specially in ultrawideband UWB imaging technologies,microwaves pass through walls and get reflected from furniture etc and the contents of the room to give somehow picture of the room and a pseudo see-through system is developed?

Through-wall detection radars cannot distinguish between objects or humans that are on the other side of the wall, if humans are not moving at all.
But humans at least breath or move slowly. Breathing causes minor variations in the body’s shape which can be determined by radar.

almost everything reflects microwave energy to some extent. By the magnitude of the reflection, and sometimes the phase angle of the reflection, you can differentiate different materials. Also, obviously, if something is moving it can be detected by various methods, not the least of which is doppler shift.

Thanks All for your answers.Anyone could tell what type of data conditioning is required before implementing classical radar motion detection techniques on data acquired from Agilent Network Analyser?
The problem is that 1) detection techniques are available for time domain data whereas from analyzer s-parameters are available for frequency sweep
2) for classical techniques we need time domain real reflected data ,but s parameters are ratio-ed numbers of reflected over incident, and niether do we have one single quantity?
3) How to tackle with phase amplitude data