Radar cross-section (RCS) in one number
An aircraft's RCS is the area of a perfect metal sphere that would return the same energy. A B-52 has an RCS of about 100 m². An F-15 is around 10 m². An F-35 is reportedly 0.005 m². The B-2 is rumoured below 0.0001 m². Radar detection range scales with the fourth root of RCS — cutting RCS by a factor of 10,000 only cuts detection range by a factor of 10.
Where the returns come from
Flat surfaces perpendicular to the radar give huge returns — like a mirror to a flashlight. Edges, gaps, right angles between surfaces (corner reflectors), and protruding shapes all contribute. The F-117 was made of flat panels because in 1975 nobody could simulate curved-surface RCS yet. The B-2 used curves once computing caught up. Both work; they're different decades of the same physics.
Radar-absorbent material (RAM)
RAM is a thin coating that converts incoming radio energy to heat. It works best at the frequency it was tuned for. F-22 and F-35 surfaces use a different RAM than the B-2. Maintenance is expensive — sweat, rain, exhaust and bird strikes all degrade RAM. Stealth aircraft spend more time in dehumidified hangars than in the air.
Why low-frequency radar still sees stealth
Stealth is optimised for X-band (around 10 GHz) — the band used by most fighter radars. Drop to VHF (30-300 MHz) and the wavelength becomes comparable to aircraft features, which destroys the RCS reduction. Russian and Chinese OTH and VHF early-warning radars are designed exactly for this. They can detect that a stealth aircraft exists, but they can't track it precisely enough for a missile.