How does a through-wall radar work and what frequencies are most effective for building penetration?

A through-wall radar (TWR) works by transmitting electromagnetic signals through building walls and detecting the reflections from people, objects, and structural features inside the building. The radar exploits the partial transparency of common building materials (drywall, wood, brick, concrete block) to electromagnetic waves at certain frequencies. The most effective frequencies for building penetration are in the UHF and L-band range (0.3-3 GHz) because: lower frequencies have lower wall attenuation (wall attenuation is approximately proportional to the square of the frequency for most building materials), the wavelength is long enough that the wall acts as an electrically thin barrier (wall thickness less than lambda/4), and the frequency is high enough to provide useful range resolution. Typical wall attenuation: drywall (12 mm): less than 1 dB at 1 GHz, approximately 2-3 dB at 3 GHz. Wood frame wall with drywall both sides: 2-5 dB at 1 GHz. Single brick wall (100 mm): 5-10 dB at 1 GHz, 10-20 dB at 3 GHz. Concrete block (200 mm): 10-20 dB at 1 GHz, 20-40 dB at 3 GHz. Reinforced concrete (200 mm): 20-40 dB at 1 GHz (the rebar acts as a partial shield). Metal wall (steel, aluminum): impenetrable at all frequencies. The TWR must: transmit sufficient power to overcome the two-way wall attenuation (signal goes through the wall twice, once on transmit and once on the return path), detect weak reflections from human targets (RCS approximately 0.5-2 m² at UHF), and separate the wall reflections (strong) from the target reflections (weaker, delayed).