  A popular absorbing structure, often referred to as Perfect Metamaterial
Absorber, comprising metallic periodic pattern over a thin low-loss grounded
substrate is studied by resorting to an efficient transmission line model. This
approach allows the derivation of simple and reliable closed formulas
describing the absorption mechanism of the subwavelength structure. The
analytic form of the real part of the input impedance is explicitly derived in
order to explain why moderate losses of the substrate is sufficient to achieve
matching with free space, that is, perfect absorption. The effect of the
constituent parameters for tuning the working frequency and tailoring the
absorption bandwidth is addressed. It is also shown that the choice of highly
capacitive coupled elements allows obtaining the largest possible bandwidth
whereas a highly frequency selective design is achieved with low capacitive
elements like a cross array. Finally, the angular stability of the absorbing
structure is investigated.
