Résumé Observations of static noise on 17.6 Mc/s extending over a long period reveal that cosmic signals registered one hour before sunset and one hour and three hours after sunset present on this frequency a difference 1 in intensity of the order of 0.6 dB, the after sunset signal being the weaker. For working frequencies closer to the critical frequency the effect is reversed. This effect may be explained by the closing up of the reception cone for a given critical frequency as a result of the diminishing of the altitude of the F region with the coming of the night. This effect is not appreciable when the presence of the centre of the Galaxy in the cone counterbalances the loss, i.e. during the summer months.