  This paper presents a boundary-integral equation (BIE) method for the
calculation of poloidal axisymmetric magnetic fields applicable in a wide range
of ac frequencies. The method is based on the vector potential formulation and
it uses the Green's functions of Laplace and Helmholtz equations for the
exterior and interior of conductors, respectively. The work is particularly
focused on a calculation of axisymmetric Green's function for the Helmholtz
equation which is both simpler and more accurate compared to previous
approaches. Three different approaches are used for calculation of the Green's
function depending on the parameter range. For low and high dimensionless ac
frequencies we use a power series expansion in terms of elliptical integrals
and an asymptotic series in terms of modified Bessel functions of second kind,
respectively. For the intermediate frequency range, Gauss-Chebyshev-Lobatto
quadratures are used. The method is verified by comparing with the analytical
solution for a sphere in a uniform external ac field. The application of the
method is demonstrated for a composite model inductor containing an external
secondary circuit.
