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We derive a steady-state, electromagnetic solution for collisional plasma and apply it to computing the total conductance for a vertically stratified ionosphere interrogated by a 3D signal with finite transverse wavelength, which we compare to the field-line-integrated conductivity, finding significant differences on all scales investigated. The approximate solution is derived from an exact linearization of the electromagnetic 5-moment fluid equations, and writing down the integral form of the driven steady-state solution, which is expressed as a sum over modal contributions associated with the eigenvectors/eigenvalues of the set of equations. We find the eigenvectors/eigenvalues numerically and use them to argue that only two of the modes are capable of transmitting energy through the ionosphere. Approximating their integrands with analytic functions yields a solution valid for homogeneous plasma, expressed in terms of the eigenvectors/eigenvalues for the two modes. The solution may be understood using wavepacket concepts. Transmission line theory is then used to extend the homogeneous-plasma solution to a solution for the vertically-stratified ionosphere. We review how transmission line theory contains electrostatic theory as a special case, and show that our solution reproduces electrostatic theory exactly when the eigenvectors/eigenvalues have the right properties, which requires that they be artificially modified. Otherwise we find short-wavelength, mode-mixing, and wave-admittance effects with broad relevance to ionospheric science, and to global modeling of the Sun-Earth system. Since resonant systems can be very sensitive, quantitative accuracy may require tuning, but the physical conclusions seem unavoidable and to affect all transverse scales.