- reduce cross-polarization levels produced by reflector feeds - produce nearly identical E- and H-plane patterns of feeds

Transcription

1 Corrugated Horns Motivation: Contents - reduce cross-polarization levels produced by reflector feeds - produce nearly identical E- and H-plane patterns of feeds 1. General horn antenna applications 2. Types of horns 3. Edge diffraction 4. Corrugated horns 5. Types of corrugations 6. Dual-band horns Literature [1] A.D. Olver, P.J.B. Clarricoats, A.A. Kishk, L. Shafai, Microwave Horns and Feeds, IEEE Press, 1994 [2] P.J.B. Clarricoats, A.D. Olver, Corrugated Horns for Microwave Antennas, Peter Peregrinus Ltd, 1984 [3] C.A. Balanis, Antenna Theory Analysis and Design, 2 nd ed, John Wiley and Sons, 1997 [4] C.A. Balanis, Advanced Engineering Electromagnetics, John Wiley and Sons,

2 1. General horn antenna applications 1) Stand alone applications : - LMDS (Local Multipoint Distribution Systems) - Beam forming arrays - Calibration horns 2) Reflector antenna feeds -Terrestrial point to point communications - Radio-astronomy - Sat-com such as VSAT (Very Small Satellite Antenna Terminal) and DBS (Direct Broadcast by Satellite) - Radar systems Wide-band corrugated horn Astro Research Corporation, Yokohama, Japan 2

3 2. Types of horns 1) Pure Mode Horns - TE and TM type aperture modes - Inferior performance to later developed horns - Used in arrays and as calibration horns 2) Corrugated Horns - Developed in response to the need for horns with symmetric radiation patterns and low cross polarization - Hybrid type aperture modes - Used in demanding applications due to good performance 3

4 3) Choked Waveguide Horns - Developed to approach corrugated horn performance with a geometry that is easier to manufacture - Can yield high efficiency over an acceptable bandwidth 4) Aperture-Matched Horns - Outer flange is contoured to minimize diffractions and to match waveguide modes to radiating modes - Performance can be comparable to corrugated horns if specifications are reduced Curved surface attached to horn walls 4

5 5) Dielectrically Loaded Horns - A metal horn partially filled with dielectric is used to generate hybrid modes - Useful at millimeter wave frequencies - Phase center sensitivity of short wide angle horns is improved with dielectric lenses - Dielectric rod radiators operate with hybrid modes 6) Multimode Horns - Not just the dominant mode present in the aperture fields - Higher order modes used to compliment the dominant mode and improve performance - Typically narrow band in operation due to frequency sensitive structures in horn that generate higher order modes 7) Dual-band Horns - Support two separate frequency bands of operation - Increased level of complexity in horn design - Frequency band separation may imply overmoding of the structure for the higher band - Necessitates propagation separation of the two bands 5

6 3. Edge diffraction Regular horn antennas excite a significant amount of cross polarization due to diffraction from the edges of the horn. In the aperture, the electric field is truly vertical only in the center planes. At off-center locations, the field is slightly curved, thus leading to diffracted fields in many directions. E- and H-plane diffraction at edges of aperture, horn and reflector antennas [4] 6

7 4. Corrugated horns Corrugated horns create a hybrid-mode pattern in the aperture which straightens out the E-field and reduces diffractions from the edges. Ideally, the wave radiating from the aperture is no longer bound by the aperture edges and, therefore, does not cause edge diffraction. In order to achieve a symmetrical radiation pattern with low sidelobes and low cross polarization, a nearly linear horn aperture field distribution is required. In addition to low cross polarization, horn antenna requirements are: Symmetrical radiation pattern Coinciding E- and H-plane phase centre Optimized radiation pattern taper for illumination and spillover efficiency Feed horn with hybrid mode aperture fields preferable, to match to reflector focal fields 7

8 Analysis E x Let be the co-polarized (desired) polarization and the cross-polar E y (unwanted) component. Assume that for small flare angles (<20 deg), the corrugated horn can be viewed as a periodically corrugated waveguide. Then the electric field of the dominant mode in a hybrid-mode waveguide can be written as: X -Y Ex = AJ 1 0( Kr) - AJ 2 2( Kr) cos 2f kr X -Y Ey = AJ 2 2( Kr) sin 2f kr Ef X =-j Y H z 1 z Hf Y = j Z E F F 1 K = transverse wavenumber A, A Z 1 2 F = amplitude coefficients 1 = = free-space impedance Y F 8

9 A linear field distribution (zero cross polarization) is achieved if X-Y=0: - X and Y are finite and equal - X=0 and Y=0 Balanced hybrid-mode condition The balanced hybrid-mode condition requires: Ef X =- j YF = 0 Ef = 0 H z Hf Y = j ZF = 0 Hf = 0 E z Can be achieved if a sufficient number of corrugations per wavelength is chosen Can be achieved by corrugations which are a quarter wavelength deep Also note: The cross-polar component decays with 1/kr 1. Therefore, larger apertures exhibit better cross-polar performance. They are also less frequency sensitive. 9

10 Modal characteristics of corrugated waveguide Slow-wave region Fast-wave region b b b = < 1 k b = > 1 k fast wave (waveguide) slow wave (periodic structure) S=0: Balanced hybrid-mode condition. Magnitude of ratio E z / H z equals free-space wave impedance; high-frequency cutoff for EH 11 S= : Waveguide acts as if a continuous metal wall is present at corrugation boundary. Corresponds to a slot depth of /2 and, therefore, is used at throat transition. The HE 11 mode is the dominant mode. At the balanced hybrid-mode condition, lower modes (EH 11 ) go to high-frequency cut-off. HE 11 propagates as a fast wave through the point S= until it reaches /k=1. Thus the short-circuit condition can be used at the throat of the horn in order to match the circular waveguide s TE 11 mode to the HE 11 mode. 10

11 Analysis (optimization) of corrugated horns The Mode-Matching Technique (MMT) is most often used. It provides good results on mode conversion along the length of the horn. The Method of Moments (MoM) is used to compute radiation characteristics. If the aperture is smaller than 1.5 wavelengths, then the currents on the flange of the horn become significant and need to be accounted for. Example: Mongiardo, Ravanelli, IEEE Trans MTT, May

12 Transition Circular Waveguide to Corrugated Conical Horn Whenever the physical geometry of a guide changes (here from smooth-wall circular waveguide to corrugated waveguide), there is a potential for higher-order mode excitation. Since the axial symmetry of the horn is preserved, only modes with similar dependence on can be excited: HE 11 can only lead to EH 1m and HE 1m. Generally, for semi-flare angles of less than 70 o, the higher-order modes are in cut-off. Transition at the throat is done with half-wavelength-deep first corrugations. A smooth conical section before the corrugation improves wide angle horn return loss. 12

13 Transitioning corrugations of different depths This occurs in the main section of the horn where slot depths are gradually changed into a quarter wavelength. Therefore, higher order modes are generally not in cutoff. EH 1m modes are exited with greater intensity than HE 1m modes. 13

14 Corrugation depth limitation When the corrugation depth is less than a quarter wavelength, the EH 11 mode can propagate. This mode, if present in the aperture fields, will radiate a high level of cross-polarized field components. Excitation of this mode can be reduced by using a gradual change from the half wavelength slot depth to the quarter wavelength depth, over not less than a wavelength. Change in flare angle A change in flare angle will excite mainly HE 1m modes. The amount of excitation is proportional to the difference in flare angles of adjoining sections. It is almost independent of the actual nominal flare angle values. Abrupt change in flare angles Continuous change in flare angles Profiled corrugated horn 14

15 Higher-order mode radiation characteristics The EH 12 mode is the main contributor to cross polarization. The HE 12 mode is low in cross polarization but contributes to a high sidelobe level in the main polarization 15

16 Beamwidth and bandwidth of corrugated horns The pattern beamwidth stays relatively constant over frequency. The rate of change of the surface impedance at the corrugation boundary is relatively slow over frequency. Therefore, the bandwidth increases proportionally with aperture diameter. Input impedance matching is usually the limiting factor. A compromise needs to be reached between input match and generation of higher order modes. 16

17 Beamwidth and bandwidth of corrugated horns 17

18 Example: Ku-band feed horn for offset dual-reflector system Hombach and Kühn, IEEE Trans AP, Sep Comparison theory and measurements in 45-degree plane Cross polarization and return loss in operational frequency bands 18

19 Ku-band offset dual-reflector system (Hombach, Kühn, IEEE Trans AP, 1989) GTD Geometrical Theory of Diffraction PO/PTD Physical Optics / Physical Theory of Diffraction Far Field (FF) or Near Field (NF) of subreflector illuminating main reflector 19

20 Example: Ku-band profiled corrugated horn Clarricoats, Dubrovka and Olver, IEE Proc. MAP, Dec Cross-polar patterns are for 45-degree plane 20

21 5. Types of corrugations The type of corrugations used in a corrugated horn depends on a number of factors and usually leads to a compromise involving - Design specifications - Space requirements - Fabrication complexity Slots in -direction or perpendicular to conical geometry used for wide-flareangle horns Slots in radial direction used for medium to lowflare-angle horns or profiled horns Slots in axial direction used for ease of fabrication and reduced performance wide-flareangle horns Ring-loaded slots used to reduce slot depth, e.g., in throat section. 21

22 Examples: Corrugated horn with all ring-loaded slots Horn with ring-loaded slots in throat section only. Note that without ring-loading, slots 1 and 2 would be of depth 2d. Corrugated horn model in CST s Microwave Studio 22

23 Example: X-band corrugated horn with coaxial rings Deguchi, Watanabe, Tsuji, Proc. EuMC, Sep Cross-pol measurements at 45 degrees 23

24 6. Dual-band horns Dual-band horns are required when two separate frequency bands of operation are required within the same feed horn structure. Dual-depth corrugated horns can be used when the frequency band separation is small enough to avoid overmoding in the higher frequency band. Lower band between EH 11 and EH 12 high-frequency cutoff; upper band beyond EH 12 highfrequency cutoff Dual-depth corrugated horn (Kühn and Philippou) 24

25 Example: 11.9/17.5 GHz dual-band corrugated horn Kühn and Philippou, Proc. 14th EuMC,1984 Amplitude and phase patterns in the 45-degree plane 11.9 GHz 17.5 GHz Due to the negligible phase variation over the main beam, corrugated horns are also referred to as scalar horns. 25

26 Dual-band horns with large frequency band separation Since a corrugated horn cannot simultaneously support two bands with large frequency separation, the upper frequency band is usually radiated by a dielectric rod antenna. However, such an arrangement requires a feeding circuitry which diplexes the two frequency bands (OMJ ortho-mode junction). James, Clark, Greene, IEEE Trans. AP, May

27 Example: Ku/Ka-band Feed Thiart, Rambabu, Bornemann, Proc. European Microwave Association, Dec Ku-band: 10.7 GHz to GHz Ka-band: 29.5 GHz to 30.0 GHz Polarization: Vertical and horizontal for both Ku- and Ka-band 27

28 Example: Corrugated Horn Array Detectors of Planck Space Telescope in the focal region of the reflector system ( ESA). The largest horn (lower right) is for 30 GHz, the smallest (close to center) for 857 GHz. Jensen, Nielsen,Tauber, Martin-Polegre, AP Mag,Oct

29 Example: Micro-Machined Corrugated Horn Array GHz polarimeter application (William Duncan, NIST, Boulder, CO) 29