Abstract: The conformal mapping and the singular-integral-equation technique are used to derive the expression for the characteristic impedance of a GTEM cell.Simple and very accurate formula for the characteristic impedance based on only spherical TEM mode propagating is found in terms of complete elliptic integrals.The method enables the characteristic impedance of a GTEM cell to be evaluated accurately and easily.In addition,a simple method,i.e., a combination of conformal mapping and the method of finite difference is suggested to be used in the study of the electromagnetic field distribution of spherical TEM mode and the characteristics of the higher modes in the GTEM cell.The analysis is useful in the study of GTEM cells and broadband transmission lines.