Compact waveguide antenna array and feed

A compact waveguide antenna array feed system provides antenna element ports spaced along an array face by less than one free-space wavelength in at least one dimension, while retaining a thickness in a direction perpendicular to the array face of less than one and one-half free-space wavelength.

FIELD OF THE INVENTION

The present invention relates generally to waveguide antenna feeds and array antennas.

BACKGROUND

Antennas provide for the radiation or reception of electromagnetic signals. An antenna may be characterized in terms of gain, beamwidth, or more specifically in terms of the antenna pattern which is a measure of the antenna gain as a function of direction. Simple antennas, such as dipoles or horns are well known, and find use in a number of applications. Simple antennas, however, are generally limited in terms of performance, providing limited gain/directivity and shaping of the radiation pattern.

Antenna arrays use a number of simple antenna elements to provide increased gain and directivity over what can be achieved using a single antenna element. In reception, signals from the individual elements are combined with appropriate phases and weighted amplitudes to provide the desired antenna pattern. Antenna arrays are also used in transmission, splitting signal power amongst the elements, again using appropriate phases and weighted amplitudes to provide the desired antenna pattern. Transmission lines or waveguides can be arranged to provide the desired phasing and combination/splitting of signals, and such an arrangement of transmission lines or waveguides is referred to as an antenna array feed.

At microwave frequencies, antenna feed design can be difficult when small element-to-element spacing relative to the wavelength is desired and potential for high losses. Microwave feed designs often use waveguide because of the lower loss provided. A waveguide feed typically includes a number of bends and twist sections to provide correct phasing to the elements of the array. These bends and twists take up large amounts of space, however, making waveguide feeds relatively bulky.

As a specific example, an antenna array can be constructed using horn antennas all facing in a common direction. The horn antennas are fed by a waveguide feed which includes a number of splitters and bends, for example in a corporate feed structure. The waveguide feed typically extends a large distance (i.e., many wavelengths) behind the antenna array. Typically waveguide feeds are designed for a specific array size (e.g., 1-by-8 array containing 8 radiating antenna elements), and as such, are not easily adapted to a larger array (e.g., 2-by-16 array containing 32 radiating elements) without significantly increasing the overall size and/or complexity of the feed network.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop a compact waveguide antenna array feed.

In one embodiment of the present invention, a compact antenna array feed includes a first signal port facing in a first direction and a plurality of antenna ports all facing in a second direction. The second direction is substantially orthogonal to the first direction. The antenna ports are arranged in an array and spaced from each other by less than one free-space wavelength in at least one direction. A waveguide corporate feed includes E-plane tees and H-plane bends to couple the signal port to the plurality of antenna ports. The waveguide corporate feed extends less than one and one-half free-space wavelength in a third direction substantially opposite to the second direction.

In another embodiment of the present invention, a compact antenna array feed includes a plurality of antenna ports arranged in an antenna array defining an array surface, wherein the antenna element ports are spaced less than one free-space wavelength in at least one dimension along the array surface. A waveguide corporate feed interfaces a signal port to the plurality of antenna element ports. The waveguide corporate feed includes a plurality of E-plane junctions and H-plane bends. The waveguide corporate feed extends less than one and one-half free-space wavelength in a direction perpendicular to the array surface.

Another embodiment of the present invention is a method of making a compact antenna array. The method can include fabricating an antenna element assembly having a plurality of antenna elements disposed along an antenna face. The antenna elements can be separated by less than one free-space wavelength in at least one direction along the antenna face. A feed network can be fabricated which provides a plurality of antenna ports corresponding to the antenna elements and having a waveguide corporate feed structure. The feed network can couple the plurality of antenna element ports to a signal port. The antenna element assembly and the feed network assembly can be joined to form the compact antenna array.

DETAILED DESCRIPTION

FIGS. 1(a) and1(b) provide a perspective illustration of an antenna array in accordance with a first embodiment of the present invention. The antenna array, shown generally at10, includes eight horn antenna elements12. The horn antenna elements are arranged in a plane with the open faces facing in the radiation direction14and defining an array surface. The horn antenna elements can include a septum polarizer40to provide circular polarization.

The horn antenna elements12are fed by a compact waveguide corporate feed16coupled to the horn elements (via twist sections38, explained further below) at antenna element ports18of the feed. The waveguide corporate feed has a thickness dimension20, which extends approximately one free-space wavelength in a direction opposite the radiation direction14. As will be explained further below, using the presently disclosed techniques, compact waveguide corporate feeds can be constructed with a thickness dimension that remains constant for virtually any number of radiating elements. The free-space wavelength is referenced to an operational frequency for which the antenna array is intended to operate. It will be appreciated that antennas may, however, be actually operated at frequencies that differ from the operational frequency, including frequencies which are significantly different than the operational frequency.

To provide this compact size, the waveguide corporate feed uses E-plane tees with integrated H-plane bends. The waveguide corporate feed includes a signal port24, which is coupled to a primary E-plane tee26, which in turn is coupled to two secondary E-plane tees28which are coupled to a set of four tertiary E-plane tees30. Integrated into the tertiary E-plane tees are H-plane bends32to feed the antenna element ports18.

By constructing the waveguide corporate feed16entirely from waveguide sections, losses are reduced as compared to a design incorporating two-conductor transmission lines (e.g., microstrip or coaxial cable). Furthermore, because the waveguide corporate feed uses only E-plane tees (rather than a combination of E-plane and H-plane tees), overall loss is reduced even further.

Because of the compact nature of the waveguide corporate feed16, the antenna element ports18, and hence the antenna elements12, can be spaced apart by less than a free space wavelength. This provides a cleaner antenna pattern, as grating lobes are avoided. For example, the spacing in the azimuth direction34can be less than a free space wavelength, the spacing in the elevation direction36can be less than a free space wavelength, or the spacing in both directions can be less than a free space wavelength.

Disposed between the waveguide corporate feed16and the horn antenna elements12are twist sections38. For example, twist sections of length of about one-half wavelength are included, increasing the overall thickness of the entire feed system to about one and one-half wavelengths when the twist sections are included. The twist sections help to provide proper phase alignment at the antenna element ports18. For example,FIG. 2illustrates an E-field vector diagram of the feed showing the E-field directions through the tees26,28,30and at the inputs to the H-plane bends32. It can be seen that a 180 degree phase shift exists between each of the arms of the tertiary E-plane tees30. Accordingly, phase alignment at the horn antennas can be provided by including 90 degree twist sections38at the output of the H-plane bends, with the pair of twist sections on each tertiary tee30twisting in opposite directions.

As an alternative to twist sections38, combinations of straight and ridged waveguide can be used to provide zero and 180 degree phase shifts. In such a case, since no rotation is provided between the waveguide corporate feed16and the antenna elements12, the waveguide corporate feed may be rotated 90 degrees relative to the array elements.

The compact size of the antenna feed is enabled in part by the use of highly integrated E-plane tees and H-plane bends. The spacing between conventional E-plane tees is several wavelengths in length, in part to help avoid coupling between higher order waveguide modes, which can adversely impact frequency bandwidth and return loss. In contrast, a one-to-four junction that combines the functions of seven components (three tees and four bends) into a single integrated component has been designed and is shown inFIG. 3. The one-to-four junction50provides four antenna ports51separated by less than one free-space wavelength in all directions, yet extends less than one free-space wavelength behind53the four ports. This highly compact design not only provides significant space savings in the antenna feed, but also orients the main port at a right angle to the plane of the four branch ports, making it possible to connect multiple one-to-four junctions together using coplanar tees to provide a scaleable feed network that maintains the same relative thickness (depth) regardless of the size of the antenna array.

The one-to-four junction50includes a first E-plane tee52having a main port54and two arm ports56. A second E-plane tee58has a main port60integrated into a first one of the two arm ports of the first E-plane tee. A third E-plane tee62has a main port64integrated into a second one of the two arm ports of the first E-plane tee. Four H-plane bends66are integrated into the arm ports68of the first E-plane tee and second E-plane tee. Integrating the tees and bends together helps to provide the compact size. The one-to-four junction may include steps69within the waveguide to help improve the VSWR/return loss, increase the frequency bandwidth, or both.

Because the tees and bends are very close together, there is significant coupling between the components, making optimization of the overall dimensions difficult. Optimization of the design for a specific frequency can be performed using computer aided design techniques. For example, the present design was optimized for operation at 14.875 GHz using both MiG WASP-Net and CST Microwave Studio to perform three-dimensional electromagnetic (EM) simulations. To obtain the final design, over 1000 iterations to optimize dimensions were automatically performed by the design tools. A model was parameterized so that physical dimensions were realizable (i.e., no negative dimensions). The independent parameters were used as optimization variables. Based on past experience, intuition and best design practices, these parameters were given initial conditions. Cost functions were written in order to maximize efficiency. This information was then fed into an optimizer to find a minimum of the cost function. As there is no way to guarantee that the solution found is a global minimum, the optimizer was restarted with new initial conditions in an effort to find a better solution, until acceptable results were obtained.

FIGS. 4(a) and4(b) show the one-to-four junction50used as a feed for a small horn antenna array70. The horn antennas72are coupled to the one-to-four junction through waveguide twist sections74. The waveguide twist sections and one-to-four junction can include steps76to provide for increased frequency bandwidth. Steps78can also be included within the aperture80of the horn to provide increased bandwidth and a better impedance match to free space. Optimization of the locations of the steps within the one-to-four junction, waveguide twist sections, and horn apertures can be performed similarly as described above. If desired, double steps can be used to provide even greater frequency bandwidth.

Using the one-to-four junction as a building block, a compact feed can be expanded or scaled for larger size arrays. For example, the basic one-to-four junction can feed a set of four antenna elements. Arrays having 4, 8, 16, 32, etc antenna elements can easily be formed by combining multiple one-to-four junctions using additional E-plane tees. For example,FIG. 5illustrates a 32-element array90having a compact feed91built using eight of the one-to-four junctions50. The feed includes a tree-like arrangement of coplanar E-plane tees, including a primary tee92, two secondary tees94, and four tertiary tees96to provide one-to-eight splitting/combining between the main feed port97and the one-to-four junctions. The feed also contains multiple E-plane bends, including two primary bends93and four secondary bends95. The one-to-four junctions are oriented to allow connection to the arms of the tertiary tees, and provide an additional four-way split/combine, such that all 32 antenna elements are fed with equal amplitude and phase (assuming the inclusion of the twist sections).

Also shown inFIG. 5is a “scorpion tail” that includes an H-plane bend99and twist section98to bring the waveguide towards the front of the feed and feed a coaxial transition providing the main feed port97of the feed91. This provides a more convenient positioning of the main feed port in some applications. For example, in one embodiment, this places the main feed port close to the center of gravity of the antenna90, which, in turn, ensures proximity to an RF rotary joint in an antenna pedestal, thus reducing feed losses through the antenna pedestal.

Larger antenna arrays can, of course, be constructed using additional E-plane tees. For example,FIG. 6illustrates a rear view block diagram of a rectangular antenna array feed100having sixty-four ports102using similar principles. The rectangular array uses a plurality of the one-to-four junctions50as described above, intercoupled by a plurality of E-plane tees104to the array feed106. It can be seen that using the compact feed structure, additional increases in array size can be easily accommodated by adding additional E-plane tees, without increasing the thickness of the compact feed network, since all of the E-plane tees are coplanar. Inter-element spacing can be maintained at less than one free-space wavelength (e.g., about 0.9 wavelengths), while the thickness of the feed (not including the twist sections) is roughly the H-plane width of the waveguide (e.g., less than one free-space wavelength).

More generally, arrays having sizes of 4*N, wherein N is a positive integer, can be formed by using a combination of 2-way and/or 3-way E-plane junctions and other higher-order E-plane junctions (e.g., 5-way) as well. For example, a 12 element array can be formed using three of the one-to-four junctions and a one-to-three E-plane splitter/combiner. Arrays of size 4, 8, 16, 32, or in general 4*2N, wherein N is a positive integer, can be formed using two-way E-plane tees. In all of these cases, the thickness of the compact feed network may remain the same, regardless of the size of the array. This is of great advantage over conventional feed networks which typically become increasingly large in multiple dimensions as array size is scaled.

FIG. 7illustrates a dual polarization compact feed200, capable of both left-hand circular polarization and right-hand circular polarization operation. A first compact feed network202is coupled to first sides204of septum polarizer horns (not shown) via straight waveguide208and straight ridged waveguide210sections to provide 180 degree phasing correction. The first compact feed network includes integrated E-plane tee and H-plane bends as described above and a first signal feed port212. A second compact feed network203(shown with shading) is offset from the first compact feed network and is coupled to second sides214of the septum polarizers via straight waveguide216and ridged waveguide218sections. The second compact feed network provides a second signal feed port220. The second compact feed network is similar to the first compact feed network, except that the phasing correction sections are slightly longer to place the second compact feed network in position behind the first compact feed network. Depending on which signal feed port212,220is used, left-hand or right-hand circular polarization can be generated.

While various feed designs have been illustrated above using rectangular waveguide, the same principles outlined above for the design of the compact waveguide antenna feed can be applied to a design using circular or elliptical waveguides.

Finally, a method for fabricating a compact antenna array is described. The method may include fabricating an antenna element assembly having a plurality of antenna elements disposed along an antenna face. The antenna elements are separated by less than one free-space wavelength in at least one direction along the antenna face. For example, the antenna elements may be fabricated by casting, stamping, electroforming, stereo lithography (SLA), or otherwise forming a conductive material into the shape of the antenna elements. If desired, the antenna elements may include twist or straight/ridged waveguide sections to provide phase rotation. For example,FIG. 8illustrates an antenna element assembly300corresponding to the antenna element portion ofFIG. 5. The antenna element assembly includes a septum-polarizer horn section302and a waveguide twist section304. A flange306disposed at the feed end of the waveguide twist section allows assembly with a corresponding compact waveguide feed network.

The method can also include fabricating a feed network assembly having a plurality of antenna element ports corresponding to the antenna elements. The feed network assembly includes a waveguide corporate feed structure to couple the plurality of antenna element ports to a signal port. The overall thickness of the feed network assembly can be less than one free-space wavelength in a direction perpendicular to the antenna face (or less than 1.5 free-space wavelengths if integrated twist sections are included). For example, the feed network assembly can be fabricated by casting, stamping, electroforming, stereo lithography (SLA), or otherwise forming a conductive material into the shape of the feed network assembly.

As a specific example, a solid is formed in the shape of an interior portion of the corporate waveguide feed structure, and is then plated with a conductive material to form an outer surface of the waveguide corporate feed structure. If desired, the solid may be dissolved to expose the interior portion of the waveguide corporate feed structure, or the solid may be left in place as a dielectric material.

Another example involves utilizing the stereo lithography apparatus/assembly (SLA) process to fabricate the antenna array structure (both the antenna element assembly and the feed network assembly). The SLA parts are constructed layer by layer using a ceramic-filled epoxy material, which hardens when exposed to UV light. The final SLA parts are then plated with noble metals (e.g., silver) to metalize the parts for good RF performance.

For example,FIG. 9illustrates a feed network assembly350corresponding to the feed network portion ofFIG. 5. The feed network assembly includes a mating surface352that mirrors the flange306of the antenna element assembly300. Waveguide portions354corresponding to the H-plane bends and E-plane junctions of the feed are included in the mating surface.

The compact antenna array is completed by joining the antenna element assembly and the feed network assembly together. For example, the flange306of the antenna element assembly300may be attached to the mating surface352of the feed network assembly350. The surfaces may be bonded together with a conductive glue or epoxy. As another example, brazing or welding may be performed.

The conductive material used to form the compact antenna array may be, for example copper or silver. Alternately, low-loss conductive materials (e.g., copper or silver) may be coated onto a structure formed of other materials (e.g., aluminum, SLA material). If desired, passivation (e.g., anodizing or gold plating) of the conductive material may be performed.

Summarizing and reiterating to some extent, the disclosed antenna array feed techniques provide a very compact, yet scalable antenna feed architecture. The one-to-four junction building block tightly integrates junctions and bends to provide a convenient arrangement of ports. Multiple one-to-four junctions can be ganged together using conventional E-plane tees to provide a wide range of array sizes. While illustrated primarily using horn antennas and rectangular waveguide, the same basic approach can be applied to other waveguide types and antenna configurations. The foregoing examples are necessarily limited in complexity in the interest of brevity. Alternate arrangements of a compact antenna array feed similar to the above examples will occur to one of skill in the art.

It is to be understood that the above-referenced arrangements are illustrative of the application for the principles of the present invention. It will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims.