  Natural modes of helical structures are treated by using the periodic dyadic
Green's functions in cylindrical coordinates. The formulation leads to an
infinite system of one-dimensional integral equations in reciprocal (Fourier)
space. Due to the twisted structure of the waveguide together with a
quasi-static assumption the set of non-zero coefficients in reciprocal space is
sparse and the formulation can therefore be used in a numerical method based on
a truncation of the set of coupled integral equations. The periodic dyadic
Green's functions are furthermore useful in a simple direct calculation of the
quasi-static fields generated by thin helical wires.
