Multiport multimode slot antenna
&#39; United States Patent [191 Walker et al.  
 [451 Apr. 8, 1975 MULTIPORT MULTlMODE SLOT ANTENNA [75] Inventors: Niles A. Walker, Castro Valley,  
 Calif.; Carlton H, Walter, Columbus, Ohio [73] Assignee: The United States of America as represented by the Secretary of the United States Air Force, Washington. DC.  
 [22] Filed: Aug. 1, 1973 [2]] Appl. No; 384,531  
 [58] Field of Search 343/767, 783, 786, 854, 343/768, 789  
 [56] References Cited UNITED STATES PATENTS 3,518.683 6/1970 Jones 343/783 Primary E.\&#39;aminerEli Lieberman Attorney, Agent, or Firm-Harry A. Herbert, Jr.; William Stepanishen 57 ABSTRACT A slot antenna apparatus for exciting higher order modes to obtain beam scanning and pattern control with a relatively small aperture.  
 10 Claims, 23 Drawing Figures SiiiU 2 [1F 4 r oEm XJMOd ill-1073M v BEN MULTIPORT MULTIMODE SLOT ANTENNA BACKGROUND OF THE INVENTION The present invention relates broadly to a slot antenna apparatus, and in particular, to a multiport multimode slot antenna having broad band capabilities for exciting higher order modes.  
  The slot antenna basically is a complete metallic enclosure which contains a radio freqency source and a break in the wall in the form of a slot which permits radiation. The length of the slot is typically a half wavelength and the width is much smaller. As the slot approaches this length, the radiation tends to augment just as it does with the linear conductor when it approaches this same resonant length. A slot antenna may be fed by a transmission line directly connected across its narrow dimensions, by a resonant cavity behind it or in other ways, This type of antenna is particularly useful in airborne applications where flush mounting provides a unique advantage.  
 SUMMARY OF THE INVENTION The present invention utilizes a multiport multimode slot antenna apparatus to provide beam scanning and pattern control in higher-order mode excitation. The multimode slot antenna is a continuous aperture array which simultaneously excites higher modes in a relatively small aperture that is formed by an open waveguide. A trifurcated waveguide multimode antenna is also provided comprising three dielectrically loaded wave guides which are placed side by side and fed into one guide. In this manner, a high degree of pattern control can be accomplished by properly weighting the linear combination of individual modes.  
 It is one object of the invention, therefore. to provide a multiport multimode slot antenna apparatus for exciting higher order modes in an antenna having a relatively small aperture.  
  It is another object of the invention to provide a multiport multimode slot antenna apparatus having a high degree of pattern control by properly weighting the linear combinations of individual modes.  
  It is yet another object of the invention to provide a multiport multimode slot antenna apparatus having a continuous aperture array to provide beam scanning and pattern control.  
  These and other advantages, features and objects of the invention will become more apparent from the fol lowing description taken in connection with the illustrative embodiment in the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a is a top view. partly schematic, of the slot antenna apparatus and feed system.  
  FIG. lb is a side sectional view, partly in schematic, of the slot antenna apparatus showing a dielectric,  
  FIG. It is a schematic diagram of the slot antenna apparatus and feed system exciting H H: and Hm; waveguide modes,  
  FIG. Za-c is a&#39; graphical representation of the patterns of the individual modes both measured and calculated,  
  FIG. 3a is a top view, partly schematic, of the trifurcated waveguide multimode antenna apparatus.  
  FIG. 3b is a side sectional view, partly schematic, of the antenna apparatus in FIG. 3a showing the air cavity and the dielectric,  
  FIG. 4 is a graphical representation of typical three mode scan patterns, I  
  FIG. 5a is a front view of .a three feed broad band multimode antenna apparatus,  
  FIG. 5b is a side view of the broad band antenna FIG. 5a showing the dielectric,  
  FIG. 6a-g is a graphical representation of the far field patterns and the broad band multimode antenna of FIG. 5a and 5b,  
  FIG. 7 is a graphical representation of scan patterns for the broad band antenna of FIGS. 5a and 5b at a frequency of 1230 MHz,  
  FIG. 8 is a graphical representation of the optimum scan pattern for the broad band antenna of&#39;FlGS. 5a and 51) at a frequency of 1230 MHz,  
  FIG. 9 is a schematic diagram of the basic adaptive feedback system,  
  FIG. 10a is a graphical representation of three mode patterns employing pattern constraint for broad side radiation, and  
  FIG. 10b is a graphical representation of three mode patterns also employing pattern constraints but at an angle 6 equal to 45.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT The multimode slot antenna is, in essence, a continuous aperture array as compared to the conventional array of discrete apertures. The basic concept of the multimode antenna is that of simultaneously exciting higher-order modes in an aperture such as formed by an open ended waveguide. Each mode, in a broad sense, can be considered analogous to the output of each element in an array. Thus, for example, a three I The far field pattern F,,(k cos9) is found by taking the fourier transform of A,,(z) which results in A (z) sin 2 (2) F (k cose) cos 6 The linear combination of N modes may be written as for the source distribution and F &#39;(k c056) for the far field pattern. The W,,s are the complex weighting coefficients. The conditions under which the directivity function associated with the patterns can be optimized was done using three probes to simultaneously excite the H H and H waveguide modes. The feed system for accomplishing mode excitation and the antenna are shown in FIGS. 1a, b, c. In FIGS. 1a, b, waveguide 10 is a cavity forming means and conductive plate 11 has an opening 12 therein. Probe feeds 13, 14, 15, FIG. 1a and probes l4 and 15 FIG. 11), extend into cavity 10. An excitation source 16 which is schematically shown in FIG. 10 feeding waveguide 10 is conventional and well known in the art. The input signal to probes 13-15 may be varied in power and phase to provide beam control and scanning. The far field patterns for this feed arrangement are shown in FIGS. 2a-c.  
  The next feed system that was investigated for the three mode antenna was a trifurcated waveguide. This was merely three dielectrically loaded waveguides 24-26 placed side by side and feeding into one guide 27. Each dielectrically loaded guide is capable of supporting its dominant mode. The configuration of this antenna is shown in FIGS. 3a, b. The conductive plate 20 has an opening 21 therein. The far field patterns for this antenna are comparable with those shown in FIGS. 2a-c. These two antennas have comparable scan characteristics. Typical scan patterns for the two configurations are shown in FIG. 4.  
  The two antennas described above use probe feeds to excite modes in a dielectrically filled waveguide. This feed system is inherently narrow band. For example.  
 for n ode for n even the trifurcated system has only a 20 MHz impedance bandwidth (VSWR 2:1) centered at 1038 MHz. Greater dielectric loading to increase the number of modes, say to five, would be expected to give even less bandwidth.  
  Turning now to FIGS. 5a,b there is shown a multimode antenna with greater bandwidth. For a single feed at the center, this configuration reduces to a conventional T-Bar slot. The multimode antenna shown is a three mode configuration but is not limited to three modes. Due to the bar 30 in the cavity, a liquid rubber potting compound capable of being poured. was used for dielectric loading in an actual model that was built and tested for operation at about 1,000 MHz. The relative dielectric constant of the compound is 7.7. The ground plane 32 has an opening 33 therein.  
  Far field patterns with the three outputs combined in-phase and no adjustment made in amplitudes are shown in FIGS. 6a-g. The frequency sweep covers a 3:1 bandwidth with minimum VSWR occuring at 1230 MHz. The scan capability of the antenna is shown by the patterns in FIG. 7 which were taken at a frequency of 1,230 MHz. These scan characteristics were accomplished by varying only the phasing between the multiport terminals. When the weighting coefficients at the multiports are adjusted properly, the beam can be both scanned and shaped. A typical pattern is shown in FIG. 8 where the beam has been scanned 32 off broadside and has a half power bandwidth of 38. This is the type of performance that one could expect from a conventional array of three half wave slots spaced about a half wave length apart with a total aperture of about two wave lengths. Here it has been obtained with a single aperture less than one wavelength long with three modes. Thus, the multimode operation of this antenna has been realized.  
  Measurements have shown that mode purity is not essential to realize effective multimode operation. In principal. if one had N outputs to a multimode slot antenna in which N modes could propagate. the outputs could be properly combined with suitable weighting coefficients to give results identical to the optimum combination of pure modes. provided the outputs were linearly independent. The far field pattern functions for the N outputs F,,, n=l ...N are related to the H,,, model pattern functions F &#34;=1. ...N by the non-singular linear transformation &#34;ill The physical significance of Eq. is that the output voltage at each port is some linear combination of the far-field pattern function of the N modes. The far field pattern of the multimode antenna becomes N. F(k 0056 2 F5 (k case) Comparing Eq. (6) with Eq. (4) it may be seen that by the equation Hence Eq. (8) is of the same form as Eq. (6) for N=3. Therefore, a particular optimization of S(t) will yield the same type of result that would be obtained had it been applied to the far field pattern function F(k cos 0).  
 The adaptive processor which is shown in FIG. 9 provides the feedback rule which controls the weighting coefficients W W W to achieve a certain output optimization. A typical application of the multimode adaptive system would be for communications in a signal environment containing interference. The operation of this system may be understood as follows. The output, S(r). is assumed to consist of a desired signal plus interference and noise. From this output we subtract a reference signal. R(r). to produce the error signal, (l). If the reference signal were equal to the desired portion ofthe output. the error signal would then just be equal to the interference and noise. The adaptive processor is designed to change the weighting coefficients in such away that the error signal is minimized. This is equivalent to adjusting the weights until there is a pattern null in the direction of any incoming interference while pointing the beam maximum in the general direction of the desired signal. The adaptive processor 40 may be any general purpose computer which is programmed to compare the error signal AU) against the three input signals S (l). S (1) and 5 (1) to adjust W W and W,, to obtain a null.  
 DET a 0.  
  An indication of the type of performance that may be expected from a three-mode adaptive antenna system is shown in FIGS. 10 a, b. These measured patterns resulted when constraints as to beam maximum and nulls were imposed. FIG. 10a is the pattern for a desired signal arriving at broadside with an interfereing signal at 48 or 48 or both. FIG. 10b is the pattern for a desired signal at 45 and interference at 38.  
  The use of a multimode antenna as described above provides a very flexible antenna system that has the performance of a larger antenna in the form of a conventional array. The three-feed multimode slot has very good characteristics including 3:1 impedance bandwidth. pattern control properties, and substantial size reduction. The concept should be useful for five or more modes. Mode purity. ie. each part corresponding to a particular mode is not necessary in general. An application for which the multimode antenna is particularly well suited is an adaptive antenna system for communication in an interference environment. This type of system could also be used where it is desired to mount the antenna on an irregular structure. The antenna system would automatically adjust its pattern to correct for structural effects. Still another possible application is an array of multimode antennas mounted on a conformal surface. Computer control would permit antenna selection to enhance coverage and adjust the pattern to correct for surface variation.  
  Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.  
 We claim:  
  1. A slot antenna apparatus for exciting higher order modes comprising in combination:  
 a conductive plate having an opening therein,  
 cavity formimg means secured to said conductive plate in alignment with said opening, said cavity forming means defining a single microwave cavity, plurality of probe feeds inserted into said caviity forming means to excite higher order modes, said plurality of probe feeds being arranged in said single microwave cavity in a predetermined configuration with respect to each other, said plurality of probe feeds comprise a first, second and third probe feed, said first and second probe feeds being in a plane parallel to said conductive plate. said cavity forming means having an open and a closed end. said open end being in contact with said eonductive plate. said first and second probes positioned at a first predetermined distance from said closed end. said third probe feed being positioned at a second predetermined distance from said closed end, and  
 a dielectric material disposed within said cavity forming means, said dielectric material surrounding said plurality of probe feeds, said dielectric material having a dielectric constant greater than one.  
  2. A slot antenna apparatus as described in claim 1 wherein said cavity forming means comprises a first, second and a third dielectrically loaded waveguide juxtapositioned and feeding into a fourth waveguide. said fourth waveguide being air filled, said plurality of probefeeds comprising a first probe feed inserted into said first dielectrically loaded waveguide, a second probe feed inserted into said second dielectrically loaded waveguide and a third probe feed inserted into said third dielectricallyloaded waveguide.  
  3. A slot antenna apparatus as described in claim 1 wherein said dielectric constant equals 14.  
  4. A slot antenna apparatus as described in claim 1 wherein said first predetermined distance equals A03/2 and said second predetermined distance equals AOl/Z.  
  5. A slot antenna apparatus as described in claim 2 wherein said first, second, and third probe feeds are respectively centered in said first, second, and third dielectrically loaded waveguides.  
  6. A slot antenna apparatus as described in claim 2 wherein said first, second, and third dielectrically loaded waveguides are equal in length to said fourth waveguide.  
  7. A slot antenna apparatus as described in claim 2 wherein the dielectric constant of said first, second, and third dielectrically loaded waveguide is 14.  
  8. A slot antenna apparatus as described in claim 7 wherein said dielectric constant equals 7.7.  
  9. A slot antenna apparatus as described in claim 8 wherein said plurality of probe feeds comprise three probe feeds spaced equidistant within said cavity forming means.  
 10. A slot antenna apparatus as described in claim 9 further including a T- bar connected between said three probe feeds.