  In this letter, the bandwidth of high-impedance surfaces (HISs) is discussed
by an equivalent circuit approach. Even if these surfaces have been employed
for almost 10 years, it is sometimes unclear how to choose the shape of the
frequency selective surface (FSS) on the top of the grounded slab in order to
achieve the largest possible bandwidth. Here, we will show that the
conventional approach describing the HIS as a parallel connection between the
inductance given by the grounded dielectric substrate and the capacitance of
the FSS may induce inaccurate results in the determination of the operating
bandwidth of the structure. Indeed, in order to derive a more complete model
and to provide a more accurate estimate of the operating bandwidth, it is also
necessary to introduce the series inductance of the FSS.We will present the
explicit expression for defining the bandwidth of a HIS, and we will show that
the reduction of the FSS inductance results in the best choice for achieving
wide operating bandwidth in correspondence with a given frequency.
