Editing Feed horn (section)

==Principle of operation==
The feed horn minimizes the [[mismatch loss]] between the antenna and the waveguide. If a simple open-ended waveguide would be used, without the horn, the sudden end of the conductive walls causes an abrupt impedance change at the aperture, between the [[wave impedance]] in the waveguide and the [[impedance of free space]] (see [[horn antenna]] for more details).

When used with a [[offset dish antenna|offset]], [[parabolic reflector|parabolic]] or [[lens antenna]], the [[phase center]] of the horn is placed at the [[Focus (optics)|focal point]] of the reflector. The characteristic of the feed horn is usually selected with the {{no wrap|[[3 dB point]]s}} of the horn's [[radiation pattern]] falling on the edge of the reflector (the [[beamwidth]] of the horn matching the F/D ratio of the dish).{{cn|date=January 2021}} When the shape of the antenna deviates from a circular dish, the feedhorn needs to be shaped accordingly to illuminate the antenna properly.