Large conducting space structures in low Earth orbit will have a nonnegligible motionally induced potential across their structures. The induced current flow through the body and the ionosphere causes the radiation of Alfv¿n and lower hybrid waves. This current flow is taken to be ac, and the radiated power is studied as a function of the ac frequency. The current may be ac because of either inductive coupling from the power system on the structure or active modulation. A space station--like structure and tether are studied. For the space station structure the radiation impedance is partially high for frequencies in the tens of kilohertz range, which suggests that the space station may be an efficient source of lower hybrid waves. The tether is also shown to be a generator of VLF waves up to source ac frequencies in the megahertz range. The implications for these two structures are discussed. ¿ American Geophysical Union 1988