Frequency-domain signatures
A chipless tag is a set of resonant structures — dipoles, slots, or reflectors — each tuned to scatter strongly at a specific frequency. When illuminated by a broadband radar pulse, the reflected spectrum has peaks and nulls at those frequencies. The pattern is the ID. A tag with 10 resonators gives 2^10 = 1,024 unique codes. 20 resonators give a million.
Materials and fabrication
Because there is no chip, chipless tags can be printed with conductive ink on paper or plastic. They survive temperatures, flexing and moisture that would destroy electronics. A tag printed on a cardboard box costs fractions of a cent. The reader is a swept-frequency or impulse radar that interrogates the tag and decodes the spectral response.
Applications beyond supply chain
Chipless tags are being tested for anti-counterfeiting (luxury goods, pharmaceuticals), structural health monitoring (cracks change the tag's signature), and even as disposable environmental sensors (humidity swells the substrate, shifting resonance). Every application treats the tag as a radar target with a designed RCS spectrum.
Limits
Range is short — metres, not tens of metres — because the tag doesn't amplify. Multiple tags in the same beam create spectral overlap. And the reader must be calibrated against environmental clutter. But for item-level tracking where cost is everything, chipless RFID is a compelling alternative to barcodes.