Horn antenna arrangement

A horn antenna arrangement comprising an H-plane sectoral horn wherein, with reference to a cylindrical co-ordinate system having a rectilinear z-axis which is normal to a reference plane parallel to the H-plane, the sectoral horn has a wide angle of flare about the z-axis in the reference plane, said angle of flare being not greater than 360 degrees, the sectoral horn being bounded over the whole of said angle of flare by conductive surfaces spaced apart in the z-axis direction and conductively connected to conductive planar side surfaces arranged radially to the z-axis at each end of the angle of flare, and wherein the aperture of the horn substantially conforms to an imaginary reference surface which is cylindrical about the z-axis, in combination with a feeder waveguide formed between substantially orthogonally disposed first and second pairs of parallel spaced conductive surfaces, said feeder waveguide extending from the throat of the sectional horn and being provided with launching means for launching radio-frequency energy along said feeder waveguide towards said horn substantially only in a fundamental mode over an operating frequency range, characterized in that electromagnetic energy is launched by said launching means so as to propagate along said feeder waveguide substantially only in the fundamental TE(1,0) waveguide mode characterised by a planar wavefront, and in that said feeder waveguide includes a mode-converting section having an input at which the waveguide has an input of planar elongate cross-section transverse to the direction of flow of said radio frequency energy along the waveguide, which is comprised of said orthogonally disposed pairs of parallel spaced conductive surfaces, and the longer width dimension, taken along a longitudinal median axis of the input cross-section, is at least four times the height in a direction orthogonal to said median axis, the H-plane of said fundamental TE(1,0) waveguide mode in said cross-section being parallel to said longitudinal median axis, said mode-converting section having an output cross-section of circumferential form transverse to the direction of radio frequency energy flow which conforms substantially to an imaginary cylindrical reference surface whose axis is the z-axis, and said output cross-section having longer and shorter boundaries substantially parallel to the H-plane of the sectoral horn and to the z-axis, respectively, said output cross-section corresponding to the throat of the sectoral horn, and the waveguide forming said mode-converting section is so shaped that the path length for the flow of said radio frequency energy therethrough is substantially the same for all respective propagation paths parallel to the local energy propagation direction in the mode converting section and each connecting a respective pair of corresponding points in said cross-sections at the respective ends of the mode-converting section, the arrangement being such that substantially only the lowest order horn mode TM(0,1) is excited in the sectoral horn by said radio frequency energy.