Electromagnetic field from simulated double-headed mid-gap streamers is computed. The effect of the applied electric field is studied by considering electric fields higher than the conventional breakdown threshold (‘overvoltage’). The obtained results show a 20 dB decay at 3.8 GHz for conventional breakdown electric field, 35 GHz for ambient electric fields five times the conventional breakdown and 170 GHz for fields ten times the conventional breakdown electric field. The limitation in nature of the intense electric fields suggest further experiments using a radio receiver at frequencies of more than 10 GHz in order to investigate the origin of the detected x-rays in laboratory sparks.