The radiation pattern of a small corrugated conical horn with a wide flare angle has been calculated by means of a Kirchhoff-Huygens integration. An assumption was made about the phase relation between the TE and the TM parts of the spherical hybrid mode in the horn. The solutions of the characteristic equation became dependent on the electrical length of the horn. Calculations and experiments showed a good agreement. © 1969, The Institution of Electrical Engineers. All rights reserved.