Broadband Corrugated Horn Antenna

This invention relates to antennas and more particularly to a broadband horn antenna useful for direction finding or for feeding a reflector.

The corrugated horn antenna, known also as the scalar horn antenna, is a conical or square horn antenna with coaxial corrugations or slots formed in the horn wall along axially spaced planes that are transverse to the axis of the horn. This antenna has many advantages including a circularly symmetrical radiation pattern essentially free of side lobes and a substantially constant beamwidth. The useful bandwidth of this corrugated horn, however, is approximately 1.7:1 which limits its applications. For example, there are microwave receivers currently available which may be tuned over frequency ranges of 8-12 GHz and 12-18 GHz, respectively, so that two receivers are employed to cover the 8 to 18 GHz band. It is advantageous for many reasons to have two such receivers share a single antenna but to accomplish this, the antenna must have an operating bandwidth of at least 2.25:1, i.e., it must have acceptable performance characteristics over this band.

Efforts to extend the bandwidth of the corrugated horn antenna have included forming the horn with broadband slots such as partially dielectrically loaded slots, tapered slots, or ridge loaded slots, the latter being described in a paper entitled "The Ring Loaded Corrugated Waveguide" by Y. Takeichi et al published in IEE Transactions on Microwave Theory and Techniques, December 1971, pages 947-950. While such horn constructions have resulted in some bandwidth improvement, the radiation pattern nevertheless still deteriorates at the upper end of 8 to 18 GHz band so that the antenna is unacceptable for use in high performance receiving systems operating over this band.

A general object of this invention is the provision of a horn antenna having a useful operating bandwidth of at least 2.25:1.

A further object is the provision of a corrugated horn antenna capable of operating over a band of 8-18 GHz with minimum variation in the E and H-plane beamwidth, minimum beamwidth variations as a function of frequency and low voltage standing wave ratio (VSWR).

These and other objects of the invention are achieved with a broadband corrugated horn antenna that is fed by a waveguide in which a TM.sub.11 mode suppressor means is disposed. This is based on the discovery that in addition to the effect of capacitive bandwidth of the horn corrugations on antenna operating frequency range, the generation in the horn of modes of higher order than the fundamental hybrid mode is also a bandwidth limiting factor because excitation of such higher order modes in the horn produces significant radiation pattern deterioration. The presence of conventional higher order modes in the input feed waveguide tends to couple to these higher order hybrid modes in the horn and so additional bandwidth extension sufficient to permit operation over the critical 8-18 GHz band is attained by suppression of such higher order modes in the input feed waveguide. The suppressor means comprises straight wire conductors or cylindrical resistance card for a conical horn and resistance cards for a square horn.

FIG. 1 is an elevation partly in section of a conical horn antenna system embodying the invention;

FIG. 2 is a transverse section taken on line 2--2 of FIG. 1;

FIG. 3 is a plot of actual performance of a conical horn antenna embodying the invention;

FIG. 4 is a section similar to FIG. 2 of a square horn antenna system showing resistance card mode suppressors in the square waveguide feed section; and

FIG. 5 is an enlarged section similar to FIG. 2 showing another shape of resistance card useful in the practice of the invention.

Referring now to the drawings, an antenna system 10 embodying the invention is shown in FIG. 1 and comprises a conical horn 11 with a circular aperture 12 and a circular feed port 13, a cylindrical waveguide 14 connected to port 13, a broadband tapered ridged circular-to-rectangular transition waveguide 15, a ridged rectangular waveguide 16 connected to transition waveguide 15 and a coaxial cable 17 connecting waveguide 16 to utilization apparatus 18. By way of example, apparatus 18 may comprise two receivers tunable over frequency ranges of 8-12 GHz and 12-18 GHz, respectively.

Horn 11 has an outwardly flared wall 20 with a plurality of coaxial axially spaced annular slots or corrugations 21 formed on its inner surface, the cross-sectional profile of each slot resembling a ring loaded or ridged configuration. The effect of such ring loading or ridging is to extend the capacitive bandwidth of the corrugations so that the depth of each remains between one-quarter and one-half wavelength over the operating frequency range. Other techniques for similarly extending the bandwidth of the horn are the use of partial dielectrically loaded slots or tapered slots.

Extension of the horn bandwidth by shaping the corrugations therein, however, is insufficient to permit operation of the system over the 8 to 18 GHz because excitation of hybrid modes in the horn at the upper end of that band cause an unacceptable deterioration in the radiation pattern. In order to prevent such pattern deterioration, mode suppressor means comprising straight conductors 23 and 24 are supported in parallel axially extending relation in cylindrical waveguide 14 throughout its length. Conductors 23 and 24, preferably made of brass, are disposed in the central plane of the waveguide coincident with the central plane of transition waveguide 15 containing the electric field vector. The space between conductors is approximately one-half the inside diameter of waveguide 15 and each is spaced the same distance from the waveguide wall. In order to support conductors 23 and 24 within waveguide 15, a cylinder 26 of suitable wave-transparent dielectric such as polystyrene foam bored to receive the conductors is snugly inserted in the waveguide.

An antenna system embodying the invention illustrated in FIGS. 1 and 2 having the following characteristics was built and successfully operated:

FIG. 3 is an actual plot of the radiation pattern and VSWR measurements of the above antenna over the frequency band of interest. It will be noted that both the 3 db and 10 db beamwidth plots (E and H planes) are fairly consistent as a function of frequency.

The invention may also be practiced with similar advantage with a square horn 30, shown partially in FIG. 4, connected by a square waveguide 31 to a ridged rectangular to square transition waveguide, not shown, otherwise similar to transition waveguide 15. The field distribution in square waveguide 31 is such that the magnetic field is zero along spaced transverse planes 32 and 33 so that the higher order mode suppressors may take the form of flat resistance cards 34 and 35 supported by dielectric 36 and extending in planes 32 and 33, respectively, for the length of waveguide 31. The electric field vector in waveguide 31 is normal to planes 32 and 33 so that insertion loss due to the resistance cards is negligible.

The invention may also be practiced with a conical horn 11' by a cylindrically shaped resistance card 38, see FIG. 5, supported coaxially in cylindrical feed waveguide 14' by a wave transparent dielectric 39 in radially spaced relation to the waveguide wall. The diameter of card 38 is approximately 60% of the inside diameter of the waveguide and typically is made of metallized Mylar about 0.005 cm. thick. Card 38 introduces a small insertion loss of about 1.5 db in the system which otherwise has substantially the same operating characteristics as the system of FIGS. 1 and 2.