We consider resonant wavelike dark matter conversion into low-frequency radio waves in the Earth's ionosphere. Resonant conversion occurs when the dark matter mass and the plasma frequency coincide, defining a range m_{DM}∼10^{-9}-10^{-8}  eV where this approach is best suited. Owing to the nonrelativistic nature of dark matter and the typical variational scale of the Earth's ionosphere, the standard linearized approach to computing dark matter conversion is not suitable. We therefore solve a second-order boundary-value problem, effectively framing the ionosphere as a driven cavity filled with a positionally varying plasma. An electrically small dipole antenna targeting the generated radio waves can be orders of magnitude more sensitive to dark photon and axionlike particle dark matter in the relevant mass range. This Letter opens up a promising way of testing hitherto unexplored parameter space that could be further improved with a dedicated instrument.