Longitudinally slotted conical horn antenna with small flare angle

The radiation and crosspolarisation characteristics of a longitudinally slotted conical horn antenna with small flare angle, excited in the modified TE11 mode, are investigated. To this end the electromagnetic wave propagation in a longitudinally slotted circular waveguide is studied first, Using a wall reactance model. Because of the need to introduce mode-dependent wall reactances in the present structure, conditions for orthogonality are given. Propagation characteristics and field line patterns are presented. It turns out that the modified TE11 mode in this guide, used as an aperture antenna, produces a main radiation lobe which can be made nearly symmetrical around the axis over a wide frequency range, namely by a proper choice of the slot depth. As a typical example the maximum crosspolarisation level in the main lobe is lower than-30 up to-45 dB, dependent on slot depth and frequency range. Experiments carried out with a slotted conical horn antenna with small flare angle confirm the theoretical results.