An RFID reader's antenna footprint determines where RFID tags can be read in relation to the reader's antenna. Antenna footprints are precise descriptions of read ranges, which are only approximate. The position of the antenna, not the reader, is the important factor.
Shape
An antenna footprint is roughly shaped like a three-dimensional ellipsoid projecting from the RFID reader antenna. In the real world, the ellipsoid is distorted by interference patterns of radio waves reflected from surrounding objects.
Interference
Constructive interference, in which radio waves are "lined up" or in phase, leads to extensions or protrusions on the antenna footprint. Destructive interference, in which the waves are out of phase and cancel each other out, leads to pits or holes in the footprint.
Measurement
Because of the irregular shape of the antenna footprint, an RFID tag may be readable or not readable based on tiny changes in location or orientation. A device called a signal analyzer can be used to measure the precise outline of the antenna footprint.
Tags: antenna footprint, reader antenna, interference which, radio waves, RFID reader, RFID reader antenna