Geometry of a model coronal loop. Within the loop the plasma density is approximately n_e, with superimposed weak random density fluctuations. A fast electron population is produced in the source region, shaded dark grey, which relaxes collisionally, producing Langmuir waves and thus radio emission. Radio emission arises from the source region of linear size d ≃ 10⁹ cm measured perpendicular to the loop axis, of the order of the typical cross-sectional size of a coronal loop. An observer at 1 AU sees a source with angular radius θ. The surrounding plasma has an exponential density profile of scale height H.