The effectiveness of a dipole source within the ionosphere is determined relative to a ground based source of the same dipole moment. The earth-to-ionospheric cavity is excited best if dip angles of the geomagnetic field are in excess of 15° to 20°. In the optimum orientation the axes of electric and magnetic dipoles are transverse to the geomagnetic field; axial electric dipoles are particularly ineffective. For frequencies in the VLF range the relative effectiveness of an ionospheric source is about 10% at nighttime and 1% at daytime. In the ELF range the relative effectiveness is from 20 to 100%.