What are the disadvantages of horn ANTENNA?

Directional diagram width depends on the physical size of antenna. In order to get beam radiation antennas are made with a long horn. It's better when it is in proportion to an aperture square, i.e. of a peak cross-section LH or LE, and a directional diagram width is in inverse proportion to LH or LE in the first power.Therefore in order to make a directional diagram N times narrower the aperture width must be broadened by N times and the horn must be lengthened by N2 times. The horn must be excessively lengthened to get a higher gain which is very inconvenient. In this case mirrors and lenses are used in the horn aperture.Use of a horn as an irradiator of a parabolic antenna presents a whole new device which is distinguished for its narrow directional diagram and low level of side lobes.