Dual depth aperture chokes for dual frequency horn equalizing E and H-plane patterns

Abstract: The antenna includes a coupling portion to permit coupling to a communication device. An inner portion is coupled to the coupling portion, and includes a first choke having a depth which extends substantially parallel to a central longitudinal axis of the antenna and a width which extends in a radial direction of the antenna. The depth and the width of the first choke are set so that the first choke will operate at the first frequency. An outer portion is coupled to the inner portion, wherein the outer portion has a maximum diameter in the radial direction which is greater than the maximum diameter in the radial direction of the inner portion. The outer portion includes a second choke which also has a depth to extend substantially parallel to the central longitudinal axis of the antenna, and a width which extends in the radial direction. The depth and the width of the second choke are greater than the depth and the width of the first choke, and are set so that the second choke will operate at the second frequency.

Description: Background of the Invention The present invention relates generally to horn antennas, and, more particularly, to horn antennas capable of operating at two or more separate frequencies and capable of providing equalized E and H plane patterns at each of the frequencies. In the communication field, a number of systems exist which require antenna systems to be capable of operating at two or more separate frequencies. For example, in military and commercial satellite systems, it is common for the uplink signal from a ground station to the satellite to have a first frequency while the downlink signal from the satellite to the ground station has a second frequency. Commercial and military Ka-Band communication satellites are one example of this where the uplink frequency is 20GHz and the downlink frequency is 30GHz. In the past, communication satellite systems such as those mentioned above have handled the two frequencies by using reflector antenna systems in the satellite which are designed with an antenna feed (for example, a feed horn) and a reflector system (generally using a primary reflector and a sub-reflector). In such an arrangement, separate horn antennas are often used as the feeds, with one horn antenna provided for each frequency to be covered. On the other hand, various systems have been developed using a single horn operating at dual frequencies. USP 3,938,159, USP 4,785,306 and USP 5,003,321 are three examples of such dual frequency feed horns that can be used in a satellite communication system. However, these arrangements are somewhat complicated to construct, and are not readily adaptable to equalizing the E and the H plane patterns at the different frequencies. In their studies, the inventors considered the possibility of using a corrugated horn operating at two or more separate frequencies such as the above-noted 20GHz and 30GHz frequencies in the Ka-Band. Corrugated horns (i.e., horns where corrugated recesses are provided which each have a depth extending radially to the central axis of the horn) have an advantage in being able to readily provide antenna patterns that are equal in the E and H planes by effectively terminating substantially all of the current parallel to the inner wall of the horn (so that the horn will have the same boundary conditions that exist for the E field perpendicular to the wall). To this end, the inventors designed and studied a corrugated horn such as shown in Figure 1. In the arrangement shown in Figure 1, a corrugated horn 10 has a plurality of corrugated recesses 12 that gradually increase in depth and width from an inner portion of the horn to an outer portion. By virtue of the different depths, the center frequency of each of the recesses 12 will be slightly different than that of the adjacent recess 12. Typically, the depth is set at λ/4 to tune to the desired frequency. The width of each corrugation recess 12 determines the bandwidth of that particular recess around the center frequency. Thus, by properly designing the depth and the width of each of the recesses 12, the horn of Figure 1 can provide continuous coverage of a desired frequency band. Also, by properly setting the depth and width of the corrugation recesses, equalized E and H plane patterns can be provided within that frequency band, as noted above. In further considering this structure, the inventors studied the possibility of providing two or more groups of corrugation recesses 12 in a conventional horn such as Figure 1, to thereby construct a horn which would operate at two distinct frequency bands (e.g., centered around 20GHz and 30GHz, for example), while providing equalized E and H plane patterns at each of these separate frequency bands. However, after considering this, the inventors noted a fundamental problem which would exist with such an arrangement. Specifically, as shown in Figure 1, the electrical aperture of the corrugated horn 10 would be limited to the inner diameter of the horn. Because of the corrugation recess construction, this inner diameter will be substantially smaller than the actual maximum physical diameter of the horn. In other words, the corrugated horn 10 of Figure 1 has a significantly larger physical aperture than its electrical aperture. This can be a serious drawback, particularly in terms of size and weight considerations which are involved in construction of a satellite antenna. Also, the relatively large physical diameter of such a horn could serve as a significant constraint in reflector systems used in satellites wherein a plurality of feed horns might be located adjacent to one another to provide multiple coverage beams from a single reflector system. The article "A dual frequency, dual polarized feed for radioastronomical applications", by M. E. J. Jeuken et al., Nachrichtentechnische Zeitschrift, August 1972, West Germany, Vol. 25, No. 8, pages 374-376, discloses a dual frequency antenna comprising an outer part and a central part being coaxially arranged with respect to each other. The central part is designed to operate at a specific higher frequency, namely 4995 MHz, while the outer part is designed to operate at a specific lower frequency, namely 610 MHz. The outer and central parts comprise chokes, which are arranged at the respective outer ends. The outer and central parts are connected with an intermediate tube-like part. EP 0 421 757 A2 discloses a microwave antenna consisting of a radiating cylinder, the front end of which providing a radiating aperture. The radiating aperture is surrounded by a ring of chokes consisting of a plurality of concentring channels, the depths of which being shaped such that each choke is set to one quarter wavelength of a frequency the respective choke is tuned for. GB 1 219 872 discloses a horn radiator comprising a waveguide of circular cross section, wherein an end of the waveguide defines a radiating aperture. Connected to the waveguide externally of the radiating aperture is a device including a member having a face lying in the plane and an aperture in the face. A plurality of ring-like radiation limiters is mounted on the member and coaxially arranged with the member's aperture. The limiters and the member are extending from the plane in a direction opposite the radiation aperture. The limiters are in the form of co-axial annular recesses. Object of the Invention It is an object of the present invention to provide a horn capable of operating at two or more separate frequencies. It is a further object of the present invention to provide a horn antenna capable of operating at two or more separate frequencies while providing substantially equalized E and H plane patterns at each of the different frequencies. It is a further object of the present invention to provide an antenna horn capable of operating at two or more separate frequencies and providing substantially equalized E and H plane patterns, wherein the electrical aperture of the horn is close in size to the physical aperture. It is a further object of the present invention to provide a horn antenna capable of operating at two or more frequencies which is easy to construct, compact in size and capable of providing equal E and H plane patterns at multiple separate frequencies. Summary of the Invention To achieve this and other objects, a horn antenna according to claim 1 is provided. Brief Description of the Drawings Figure 1 shows a conventional corrugated horn studied by the inventors in developing the present invention. Figure 2 shows a perspective view of a preferred embodiment of the horn constructed in accordance with the present invention. Figure 3 is a simplified cross-section of a horn constructed in accordance with the present invention to operate at two separate frequencies. Figure 4 is a sectional view taken from the line 4-4 of Figure 2 showing details of a preferred embodiment of the present invention. Figure 5 is an illustration of a horn constructed in accordance with the present invention used in a satellite reflect antenna system. Detailed Description of the Invention Figure 2 provides an overall perspective view of a horn 20 constructed in accordance with a preferred embodiment of the present invention. As will be described in detail below, the horn 20 of this embodiment is constructed as a conical horn having a plurality of chokes 22 arranged concentrically within the horn to have depths which extend substantially parallel to the longitudinal central axis 24 of the horn. The widths of these chokes 22 extend substantially radially, noting that the horn is preferably rotationally symmetrical about the longitudinal axis 24. The diameter of the horn gradually increases from a connecting portion 26 which permits connection to an input or an output element (for example, a circular waveguide) of a communication device (for example, a receiver and/or transmitter). As will be discussed below in more detail, the chokes 22 are arranged to operate in separate frequency bands, wherein the higher frequency operation takes place in the chokes closest to the connecting portion 26, while the lowest frequency operation takes place in the chokes closest to the maximum aperture of the horn. Thus, for example, the horns can operate at two or more separate frequency bands centered around 20GHz and 30GHz if the system is used in a Ka-Band communication satellite system as discussed above. With regard to the terminology used in the present description, it is noted that the term "separate frequencies" is intended to refer to two discrete frequencies which are separated from one another by a range of frequencies. In other words, this would include situations such as discussed above wherein the "separate frequencies" are 20GHz and 30GHz. Of course, some degree of bandwidth would be associated with each of the separate frequencies. As such, the term "separate frequencies" is intended to refer to situations where the bandwidths of the separate frequencies are not sufficiently large that the frequencies effectively blend into one another to form a continuous range of frequencies. Similarly, the term "frequency band" is intended to refer to a discrete frequency, such as 20GHz, and a predetermined bandwidth around this discrete frequency. For example, in the case of 20GHz, the term "frequency band" could include 19.99GHz to 20.01GHz. In other words, with this definition, the frequencies 20GHz and 30GHz, with their respective bandwidths, are considered as two separate frequency bands, notwithstanding the fact that they are both within the overall Ka-Band. To put this another way, what is intended is to define two frequency ranges which are separate from one another by another range of frequencies (even though they might exist within an overall frequency band such as the Ka-Band), as opposed to covering a large range such as all of the frequencies between 20GHz and 30GHz. Figure 3 is a simplified illustration of the present invention which is provided to facilitate understanding of the principles involved in the present invention. In this figure, the depth and width dimensions are exaggerated for purposes of illustration. In Figure 3, the connection portion 26 is constructed as a tapered transition coupled to a circular waveguide 28 which can operate as an exciting port. In the embodiment using 20GHz and 30GHz as the center frequencies, the circular waveguide 28 can be used both to receive the 20GHz signal from the horn to provide these signals to a satellite receiver and to transmit the 30GHz signal from the satellite transmitter to the horn to be transmitted as a downlink signal. On the other hand, if higher frequencies are involved, a coaxial feed, or some other feed mechanism, could be provided in conjunction with a waveguide. It is also noted that any type of connection would be used, and the invention is not limited to the illustrated tapered connection. An inner portion 30 is coupled to the connection portion 26 to provide the high frequency component of the horn 20. An outer portion 32 is coupled to the inner portion 30 to provide the low frequency component of the horn 20. Between these two portions 30 and 32, the chokes 22 (see Figure 1) are constructed to be broken down into a group of first chokes 34 and a group of second chokes 36. As can be seen in Figure 3, the depth and width of the first chokes 34 are significantly smaller than the depths and widths of the second chokes 36 so that the inner portion 30 will operate at a higher frequency. More specifically, the depths and widths of the first chokes gradually increase from the smallest one, immediately adjacent to the connection portion 26, to the largest one, immediately adjacent to the outer portion 32. In this way, a frequency band of operation is provided. For example, if the high frequency of 30GHz is intended, a central one of the first chokes 34 can be constructed with a depth tuned to resonate at 30GHz. Those first chokes 34 which are closer to the connection portion 26 can be tuned to have progressively higher center frequencies (by having smaller depths), while those first chokes 34 closer to the outer portion 32 can be tuned to have progressively lower center frequencies (by increasing the depth). The width of the first chokes 34 control the bandwidth of operation of each of the first chokes 34 around its particular center frequency. Thus, by adjusting the depths and widths suitably, a continuous frequency range of, say, 29.99GHz to 30.01GHz can be provided to ensure satisfactory operation at the 30GHz frequency by allowing a slight bandwidth to account for minor variations in the downlink signal. By way of example, this can be accomplished by using five of the first chokes 34 and setting the widths of the respective chokes to provide sufficient bandwidth around each of the center frequencies so that, as a whole, the five chokes will completely cover the frequencies between 29.99GHz and 30.01GHz. It should be noted that the depths of the chokes should be significantly greater than the widths in order to provide proper choke operation. Typically, the widths of the chokes can be set between λ/10 and λ/20, although the invention is not limited to this. Of course, the greater the width of the choke, the broader the bandwidth of the particular choke. With regard to spacing, the chokes should, in general, be spaced to avoid electrical interference between them. This will depend on the frequency and bandwidth of operation of each choke. Finally, the number of chokes used in either the inner or outer portions (or any internal portions, for that matter) determine the overall total bandwidth of that portion (with each choke covering a small band within the larger overall band). In this same way, the depth and width of the second chokes 36 of the outer portion 32 can be varied to provide coverage of a frequency range of, say, 19.99GHz to 20.01GHz to ensure adequate reception of the 20GHz uplink signal. With regard to this, it is noted that the present invention is intended to operate at two separate frequencies (or frequency bands), such as 20GHz and 30GHz which are substantially different from one another. It is noted, of course, that these frequencies are provided herein only for purposes of example, and that the present invention can operate at various frequencies as desired. For example, the present invention is also very well suited for operation at frequencies within the X-Ku-Band. It is further noted that the horn has been described as a dual frequency horn solely for purposes of convenience, and it could readily be constructed to operate at three or more separate frequencies by adding a middle section between the inner portion 30 and the outer portion 32, with chokes of the one or more middle sections being tuned to intermediate frequencies. Also, although the above description sets forth an arrangement for receiving one frequency and transmitting another frequency, the present invention can be used for receive-only systems or transmit-only systems using two or more frequencies as well. Generally, the chokes will be substantially designed to have a depth equal to λ/4 for the center frequency that they are particularly tuned to. One advantage of using chokes, similar to the case of using corrugations such as described for Figure 1, is that they serve to permit equalization of the E and H field plane patterns at each of the frequencies. On the other hand, if the horn is to be used as a feed for a reflector system, the actual beam widths for the patterns of the horn for each of the two frequencies should generally be different since the reflection system itself will reflect the patterns differently depending on the difference in frequencies. In other words, if the beam width from the horn is to be identical for both frequencies, it will be reflected such that the beam width for the higher frequency will be greater than the beam width for the lower frequency (assuming that the diameter of the reflecting surface will be the same for both frequencies) . Therefore, in a reflector system, the beam width for the different frequency patterns from the horn should be set so that the ultimate patterns reflected from a primary reflector of the antenna system will have equal beam widths. Unlike the corrugated horn arrangement shown in Figure 1, the present invention has the significant advantage of providing an electrical aperture which is close in size to the physical aperture. As shown in Figure 3, this can be the case because the axial direction of the depth of the chokes permits the electrical aperture to extend almost to the extreme physical edge of the horn. Essentially, the electrical aperture is defined by the inner diameter of the largest choke while the physical diameter can be defined by the outer diameter of the largest choke. Thus, only the wall thickness between the inner and outer diameters of the largest choke will define the difference between the electrical aperture and the physical aperture. Since the electrical aperture determines the antenna gain, this permits a significant increase in the antenna gain within the size constraints for which the antenna system is designed. As an example of actual size, the embodiment shown in Figure 3 can be constructed to have a maximum horn outer diameter (i.e., the physical aperture) of 3.6 inches while the electrical aperture of the outermost choke will be 3.4 inches. Therefore, the electrical aperture differs from the physical aperture only by 0.2 inches. Incidentally, with regard to the physical and electrical aperture size in terms of wavelength of the operating waves, the physical and electrical apertures in this particular case are about 6λ (based on λ = 0.6 inches for the 20GHz frequency). Generally, the apertures can be set between λ and 10λ, although this is not intended to be limiting. Figure 4 is a cross-section of the horn shown in Figure 1, illustrating a preferred embodiment of the present invention. In this embodiment, a total of 29 chokes 22 are provided for dual frequency operation at frequency bands 20GHz and 30GHz. In this arrangement, circular beams are created since the particular horn is designed for generation of circular beams between a satellite and a ground station. On the other hand, the present invention is not limited to conical, or circular beams, and could be used with other arrangements, for example, rectangular, or pyramidal, horns. Also, solely for purposes of example, it is noted that the horn shown in Figure 1 can be extremely compact, having another diameter of 1.125 inches at the input of the coupling portion, a maximum outer diameter of 3.6 inches at the horn opening, and a total length of about 11.5 inches. Preferably, the horns constructed in accordance with the present invention will be made with extremely light but strong material. For example, very thin nickel (for example, as thin as 0.005 inches) could be used in constructing the preferred embodiment shown in Figures 1 and 4. However, other materials could also be used, such as aluminum, if desired. Figure 5 shows a satellite Cassegrain reflector system for a satellite antenna in which the present invention can be used. More specifically, a plurality of horns 20 of the present invention can be used with the sub-reflector 38 and the primary reflector 40 to generate a plurality of circular beams from the primary reflector 40 to separately cover different portions of the earth's surface. Generally, in the preferred Ka-Band system using 20GHz for the uplink signal and 30GHz for the downlink signal, this system will be designed to generate circularly symmetrical beams having a half power beam width of 9°. Of course, these dimensions are solely for purposes of example. Also, if rectangular, or pyramidal, horns were used, it is possible to generate non-circular beams to cover different shaped areas on the earth's surface. Although the present invention is very useful as a feed horn for an antenna system in a satellite, it can be readily be used in other antenna systems as well, including, for example, ground stations or TVRO systems (i.e., television receive only systems). In addition, it is noted that the present invention can be used with a variety of reflector systems, including, but not limited to, offset, Cassegrain, front-fed, side-fed and Gregorian reflectors. The above description sets forth a horn antenna that is capable of providing an electrical aperture which is nearly as large as the physical aperture, while, at the same time, providing operation at two or more frequencies with equalized E and H plane patterns for each of the frequencies. Another advantage of the present invention is that it is relatively easy to construct, in comparison with the relatively complicated structures previously used for obtaining dual frequency operation, and, due to the minimum number of parts required, is relatively maintenance free. This, of course, is particularly important in satellite antenna design where maintenance is quite difficult.

Claims: A horn antenna (20) for operating at a plurality of separate frequency bands comprising: a coupling portion (26) to permit coupling of the horn antenna to a communication device (28); an inner portion (30) coupled to the coupling portion (26) and including a plurality of first chokes (34), wherein depths of the first chokes (34) extend substantially parallel to a central longitudinal axis (24) of the horn antenna (20) and widths of the first chokes (34) extend in a radial direction of the horn antenna (20); and an outer portion (32) having a maximum diameter in the radial direction which is greater than a maximum diameter in the radial direction of the inner portion (30), the outer portion (32) comprising a plurality of second chokes (36) having depths which extend substantially parallel to the central longitudinal axis (24) of the horn antenna (20) and widths which extend in the radial direction, wherein the depths and the widths of the second chokes (36) are greater than the depths and the widths of the first chokes (34); characterized by the outer portion (32) being coupled to the inner portion (30) in line in relation to the central longitudinal axis (24), the plurality of first chokes (34) operating in a first frequency band; and the plurality of second chokes (36) operating in a second frequency band different than the first frequency band. A horn antenna (20) according to claim 1, wherein the second frequency band is lower than the first frequency band. A horn antenna (20) according to claim 1 or 2, wherein the second frequency band includes 20 GH 2 and the first frequency band includes 30 GHz. A horn antenna (20) according to one of the claims 1 to 3 comprising: at least one middle portion coupled between the outer portion (32) and the inner portion (30), the middle portion including a plurality of third chokes operating in a third frequency band, which is separate from the first and second frequency bands, and having depths, which extend substantially parallel to the central longitudinal axis (24) of the horn antenna (20), and widths, which extend in the radial direction, wherein the depths and the widths of the third chokes are greater than the depths and the widths of the first chokes but less than the depths and the widths of the second chokes. A horn antenna (20) according to claim 4, wherein the third frequency band is lower in frequency than the first frequency band but higher in frequency than the second frequency band. A horn antenna according to one of the claims 1 to 5, wherein the plurality of the first chokes (34) are provided to extend between the coupling portion (26) and the outer portion (32), the first choke closest to the coupling portion (26) has a first depth, the first choke furthest from the coupling portion (26) has a second depth, greater than the first depth, and the depths of the other first chokes located between the first choke closest to the coupling portion (26) and the first choke farthest from the coupling portion (26) gradually increase in depth from the first depth to the second depth to provide gradually decreasing center frequencies within the first frequency band. A horn antenna (20) according to one of the claims 1 to 6, wherein the plurality of the second chokes (36) are provided in the outer portion (32) to extend away from the inner portion (30), the second choke closest to the inner portion (30) has a first depth, the second choke farthest from the inner portion (30) has a second depth, greater than the first depth, and the depths of the other second chokes located between the second choke closest to the inner portion (30) and the second choke farthest from the inner portion (30) gradually increase in depth from the first depth to the second depth to provide gradually decreasing center frequencies within the second frequency band. A horn antenna (20) according to claim 1, wherein each first choke (34) has a depth substantially equal λ/4 for a center frequency the first choke (34) is tuned to, and each second choke (36) has a depth substantially equal λ/4 for a center frequency the second choke (36) is tuned to. A horn antenna (20) according to one of the claims 1 to 8, wherein the depths and widths of the first chokes (34) are set to substantially equalize the E and H plane patterns of the horn antenna (20) for the first frequency band, and the depths and widths of the second chokes (36) are set to substantially equalize the E and H plane patterns of the horn antenna (20) for the second frequency band. A horn antenna (20) according to one of the claims 1 to 9, wherein the maximum electrical aperture of the horn antenna (20) is substantially equal to the maximum inner diameter of the outer portion (32), and the maximum physical aperture of the horn antenna (20) is substantially equal to the maximum outer diameter of the outer portion (32). A horn antenna (22) according to one of the claims 1 to 10, comprising means for providing electromagnetic waves in the first and second frequency bands to the horn antenna (20). A horn antenna (20) according to one of the claims 1 to 11, wherein the inner portion (30) is coupled to the coupling portion (26) in line in relation to the central longitudinal axis (24). A horn antenna (20) according to one of the claims 1 to 12, wherein the diameter of the horn antenna (20) gradually increases from the coupling portion (26) to the outer portion (32).

Inventors: CHANDLER CHARLES W, EM MAKKALON, CHANDLER, CHARLES W., EM, MAKKALON
Applicants: NORTHROP GRUMMAN CORP, NORTHROP GRUMMAN CORPORATION
Application year: 2000
Main classification: H01Q13
IPC codes: H01Q13, H01Q5