Quality Factor and Absorption Bandwidth of Electrically Small Lossy Structures

A new quality factor (Q abs) characterising power absorption by electrically small structures ranging from antennas to lossy objects is defined. In analogy to the radiation quality factor (Q rad), which is evaluated using the antenna radiated fields in transmitting mode, Q abs is evaluated using the fields scattered by the absorbing structure. Similar to the known relationship between the antenna matching bandwidth (B match) and Q rad, it is rigorously shown that for an electrically small lossless receiving antenna Q abs is inversely proportional to the absorption bandwidth (B abs) of the antenna. Based on a circuit model, it is then conjectured that the same B abs–Q abs relation is also valid in the cases of electrically small lossy antennas and objects that do not have terminals. Numerical examples are shown to demonstrate the validity of the presented B abs–Q abs relation.