European Patent Office © Publication number: 0 136 818 Office europeen des brevets A1 © EUROPEAN PATENT APPLICATION © Application number: 84305895.9 © Int. CI/: H 01 Q 25/04 H 01 Q 13/02 © Date of filing: 29.08.84 © Priority: 06.09.83 US 529375 © Applicant: ANDREW CORPORATION 10500 West 153rd Street Orland Park Illinois 60462(US) © Date of publication of application: 10.04.85 Bulletin 85/15 © Inventor: Knop, Charles M. Route 6, Box 279 © Designated Contracting States: Lockport lllinois(US) DE FR GB IT IML © Inventor: Ostertag, Edward L. 1918-38 Heatherway Lane New Lenox lllinois(US) © Representative: MacDougall, Donald Carmichael et al, Messrs. Cruikshank & Fairweather 19 Royal Exchange Square Glasgow G1 3AE, Scotland(GB) © Dual mode feed horn or horn antenna for two or more frequency bands. © A A microwave feed horn or horn antenna (11,12) for at least two frequency bands comprises a conical waveguide section (42) whose aperture at the large end has an inside diameter (D1) approximately equal to one wavelength in the J lower frequency band so as to produce substantially equal vvvvvv power patterns in the E and H planes in the lower frequency {. band. The slope (0)(ß) of the inside wall of said conical section ) (42) is selected to cancel the electric field at the aperture of { the horn in the higher frequency band, thereby producing X. > . y substantially equal power patterns in the E and H planes in KSvvS the higher frequency band. A pair of straight waveguide sections (40,41(40,41) ) connects the opposite ends of the conical waveguide section (42). 7- & Croydon Printing Company Ltd. The present invention relates generally to microwave antennas and, more particularly, to feed horns or horn antennas that are capable of handling two or more frequency bands. It is a primary object of the present invention to provide an improved feed horn or horn antenna that produces substantially equal E-plane and H-plane patterns in at least two different frequency bands, and yet is extremely simple and economical to manufacture. In this connection it is also an object of this invention to provide a feed horn or horn antenna suitable for simultaneous operation across two different frequency bands in both vertical and horizontal polarizations. It is another important object of this invention to provide such an improved feed horn or horn antenna which is extremely small and, therefore, minimises the horn blockage of reflector-type antennas. It is yet another object of this invention to provide an improved feed horn or horn antenna which achieves the foregoing objectives while maintaining a good VSWR (i.e. less than 1.1) and a low level of back radiation. Other objects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings. In accordance with the present invention, the fore- going objectives are realised by a microwave feed horn or horn antenna for at least two frequency bands, the horn comprising a conical waveguide section whose aperture at the large end has an inside diameter approximately equal to one wavelength in the lower frequency band so as to produce substantially equal main beam patterns (from O to about 20 dB down) in the E and H planes in said lower frequency band, the slope of the inside wall of said conical section being selected to cancel the electric field at the inside wall of the horn at its large end in the higher frequency band, thereby producing substantially equal main beams in the E and H planes in said higher frequency band, and a pair of straight waveguide sections connected to opposite ends of said conical section. In the drawings: Fig. lA is a cross-sectional view of a dual-reflector antenna utilising the feed horn according to the invention; Fig. 1B is a cross-sectional view of a parabolic antenna with a prime feed utilising the feed horn according to the invention; Fig. 2 is an enlarged longitudinal section of the feed horn of the antenna of Fig. 1: Fig. 3 is a plot of the radiation amplitude patterns, in both the E-plane and H-plane for the feed horn of Fig. 2, measured at a radius of 11" from the centre of the feed horn aperture and at a frequency of 3.95 GHz; Fig. 4 is a plot of the radiation phase patterns in both the E-plane and the H-plane, for the feed horn of Fig. 2, measured at a radius of 11" from the centre of the feed horn aperture and at a frequency of 3.95 GHz; Fig. 5 is a plot of the radiation amplitude patterns, in both the E-plane and the H-plane, for the feed horn of Fig. 2, measured at a radius of 11" from the centre of the feed horn aperture and at a frequency of 6.175 GHz; and Fig. 6 is a plot of the radiation phase patterns, in both the E-plane and the H-plane, for the feed horn of Fig. 2, measured at a radius of 11" from the centre of the feed horn aperture and at a frequency of 6.175 GHz. While the invention will be described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to those particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. Turning now to the drawings and referring first to Fig. lA, there is illustrated a dual-reflector Gregorian antenna comprising a paraboloidal main reflector dish 10, a primary feed horn 11 connected to and supported by a circular waveguide 12 extending along the axis of the dish 10, and a subreflector 13. The axis of the main dish 10 as shown in Fig. 1A is coincident with the longitudinal axis of the waveguide 12 and feed horn 11. (The term "feed" as used herein, although having an apparent implica- tion of use in a transmitting mode, will be understood to encompass use in a receiving mode as well, as is conventional in the art). In the transmitting mode, the feed horn 11 receives microwave signals via the circular waveguide 12 and launches those signals onto the subreflector 13; the sub- reflector 13 reflects the signals onto the main reflector dish 10, which in turn reflects the signals in a generally planar wave across the face of the paraboloid. In the receiving mode, the paraboloidal main reflector 10 is illuminated by an incoming planar wave and reflects this energy into a spherical wave to illuminate the subreflector 13; the subreflector 13 reflects this incoming energy into the feed horn 11 for transmission to the receiving equipment via the circular waveguide 12. As is required in Gregorian dual-reflector antennas, the focal point F of the paraboloidal surface of the main reflector is located between the main reflector dish 10 and the subreflector 13. To achieve this configuration, the subreflector 13 presents a concave reflective surface to the face of the main reflector 10. To support the sub- reflector 13 in this desired position, the subreflector is mounted on the large end of a tripod 14 fastened to brackets 15 on the main reflector dish 10. The tripod is composed of three support legs which are relatively thin and introduce only a negligible amount of VSWR and pattern degradation into the antenna system. Normally the legs of the tripod are arranged to lie outside the horizontal plane. Alternatively, the subreflector can be supported by a dielectric cone with the small end of the cone mounted on the main reflector 10, or on the waveguide 12, and the subreflector mounted on the large end of the cone. The subreflector 13 is positioned and dimensioned to intercept a large portion of the radiation launched from the feed horn 11 in the transmitting mode, and an equally large portion of the incoming radiation reflected by the main reflector 10 in the receiving mode, while at the same time minimizing blockage of the aperture of the main reflector 10. The subreflector preferably has a maximum diameter of about six wavelengths at the lowband frequency and nine wavelengths at the highband and is positioned sufficiently close to the feed horn to accomplish the desired interception of radiation from the horn. In the illustrative embodiment, the subreflector 13 is fitted with an absorber-lined shield 30 which intercepts a substantial portion of the spillover from the feed horn 11 and also reduces diffraction of microwave radiation at the periphery of the subreflector 13. For the purpose of dissipating the spillover energy intercepted by the shield 30, the inner surface of this shield is lined with an absorber material 31. The shield 30 projects from the periphery of the subreflector 13 toward the main reflector and parallel to the axis of the feed horn. Since the Gregorian configuration of the antenna utilises a concave reflective surface on the subreflector (as contrasted with, for example, the convex reflective surface utilised in a Cassegrain configuration), the shield 30 can be added to the periphery of the subreflector 13 without interfering with the signal path between the subreflector 13 and the main reflector 10. The axial length Ll of the shield 30 is limited by the surface of an imaginary cone whose apex is the common focal point F of the dual reflectors and whose base is the periphery of the main reflector (the cone surface is illustrated by the dotted line A-B, in Fig.