Spacecraft direction-finding measurements on terrestrial non-thermal radio continuum or myriametric radiation have shown that there are two source regions - the dayside plasmapause and the morning magnetosheath. One theory proposed for the production of myriametric radiation is mode-coupling of upper hybrid waves to left hand polarised ordinary mode waves at the level where the wave frequency equals the plasma frequency. Intense upper hybrid waves are observed by Geos-1 to be closely confined to the magnetic equator at the plamapause and beyond. It can be shown that inherent in the proposed coupling process is a beaming effect such that the resultant L-O waves are excluded from a zone centred on the magnetic equator. Results from the Geos-1 spacecraft confirm the presence of a shadow zone whose extent is frequency dependent as predicted by theory.