The invention concerns the field of protection of geographical zones from undesired, even hostile, intrusions by threatening moving objects. The invention concerns a bistatic radioelectric detection barrier comprising means for transmitting one or more waves through a directional antenna and means for receiving signals through a directional antenna pointed in the transmitting direction. Said antenna is mainly characterized in that it comprises auxiliary means for emitting a wave through a non-directional antenna and means for comparing the relative levels of echoes received from the wave emitted by the main directional transmission channel, and of echoes from the wave emitted by the auxiliary transmission channel, the result of the comparison enabling the echoes derived from a wave emitted in the direction of the secondary lobes of the directional transmission antenna to be identified, end thus to reduce the number of false intrusion alarms. More particularly, the invention concerns the protection of zones using or experimenting vulnerable equipment the destruction of which may prove dangerous to nearby populations.