DUAL END RESONANT SLOT ARRAY ANTENNA FEED

Abstract: An antenna with a dual end resonant slot array feed improves the bandwidth performance of a resonant slotted waveguide planar array antenna. The dual end resonant slot array feed includes a tee junction (28) which may be either an E-plane or H-plane, two waveguide sections (32, 34), and two E-plane waveguide bends (36, 38). The two waveguide sections (32, 34) are formed by a septum (40) mounted in a slotted waveguide (42) for separating the input tee junction (28), from the slots (44) of the slotted waveguide. The ends of the septum (40) coacting with the ends of the waveguide to form the E-plane waveguide bends (36, 38). Thus, resonant feeding of the series-slot waveguides (50) is achieved by the opposing traveling waves thereby eliminating the need to use resonant short circuits, cavities, or folded short circuits. Further direct coupling to the series slots (44) directly adjacent to the E- or H-plane feed point (50) is avoided by introducing the septum (40) between the feed point (50) and the row of slots (44).

Description: This invention relates to slotted array antennas and more paricularly to a dual end resonant slot array feed for a resonant slotted waveguide planar array antenna. In the past slotted array antennae have been fed by single end feed mechanisms. When a waveguide section is fed at one end, a waveguide short at the opposite end sets up a standing wave in the waveguide. Shunt or series slot elements are located at appropriate points on the standing wave pattern (voltage or current peaks, respectively) to cause radiation with the correct amplitude and phase. Over a band of frequencies, the standing wave pattern in the waveguide varies relative to the location of the slots, causing errors in the slot amplitudes and phases. The magnitude of these errors increases in a direct relationship to the deviation of frequency from the design center frequency. The magnitude of the errors also increases with the length of the waveguide, and hence the number of slots. For waveguides having four or more slots, the usable bandwidth of a single end feed is on the order of ± 1 percent. To improve the bandwidth relative to a single end feed, E-plane and H-plane tee feeds have been used. The E-plane tee feed is in essence, two single end feeds joined at their respective feed points by an E-plane waveguide tee; improvement is caused by reducing the length (and number of slots) associated with each of the two single end feeds. The problem with the E-plane feed is that in order to maintain equal slot spacing one slot must lie directly under the E-plane tee. Owing to mutual coupling to the E-plane tee, this slot suffers a variation in phase and amplitude over the frequency band which differs significantly from the other slots in the array. This significantly different set of phase/amplitude errors for the slot under the E-plane feed largely offsets any bandwidth advantages that otherwise would have been obtained by using the E-plane tee. By substituting an H-plane (shunt) tee for the E-plane (series) tee, the feed point for the slot waveguide can be located half way between two slots instead of directly over the slots. Nevertheless, because the H-plane feed must be about one-half wavelength wide (to avoid waveguide cutoff effects), the feed couples to the two slots, to yield essentially the same bandwidth limitations as the E-plane feed. For a large array antenna, the bandwidth typically has been limited to less than 2.5% using one of the above methods owing to the need to keep the manifold complexity within reasonable bounds. Both the amplitude and phase of the aperture illumination begin to be significantly degraded at + 1 % of the center frequency. The single end feed for a resonant waveguide array is described in a number of texts on antennas. For more detailed information pertaining to single end feeds reference may be made to Johnson and Jasik's "Antenna Engineering Handbook, Second Edition, 1984 & 1961, Chapter 9. In US-A-2 981 948, Kurtz, there are disclosed phased array slot antennas for simultaneous lobing having at least one line of slots along a waveguide driven by sum and difference signals at different places along the guide. Metallic interfaces are provided separating slots along the same waveguide driven at two places to form separate array elements. In US-A-3 293 647, Crumpen, there are disclosed slot antennas along waveguides having non-resonant feeds at both ends for producing two oppositely inclined beams of radiation. In JP-A-59 32205, Nippon Hoso Kyokai, published in Patent Abstracts of Japan, Vol.8, No.119, (E-248)(1556), there is disclosed an array antenna having slots in concentric rings on a flat disc-shaped waveguide. The slots are driven from the circumference of the disc and a terminating resistor is provided at the centre of the disc. Accordingly, it is an object of this invention to provide a slotted array antenna having substantially increased frequency bandwidth. Another object of the invention is to provide a feed for improving substantially the bandwidth performance of the slot array over that obtained using a single end feed. Yet another object of the invention is to improve substantially the amplitude and phase accuracy of the aperture illumination of the slot array antenna. According to the present invention there is provided a slotted waveguide antenna for transmitting or receiving rf energy including a straight rectangular unobstructed waveguide having slots in one face thereof and connected at both ends to respective rectangular waveguide feeders disposed at right angles to the slotted waveguide, so that rf energy can be applied to or derived from both ends of the slotted waveguide, characterised in that in use, equal amounts of rf energy appear simultaneously and co-phasally at the ends of the slotted waveguide, said slots are spaced one half of the wavelength in the waveguide apart so that said slotted waveguide is resonant, a rectangular waveguide section equal in length to the slotted waveguide is disposed parallel to the slotted waveguide on a second face remote from the slotted face, the waveguide section is joined at its ends respectively to the waveguide feeders to form a loop of an integral number of wavelengths of rf energy of the operating frequency in the waveguide antenna and an input/output waveguide is coupled by a tee junction to the middle of the waveguide section. Other objects and features of the invention will become more readily apparent from the following detailed description when read in conjunction with the accompanying drawings in which: Figure 1 is an exploded view of a slot antenna array; Figures 2a and 2b are prior art realizations of slotted waveguide antennas; Figures 3a and 3b are views of a dual end series slot feed using, respectively, an E-plane tee feed and an H-Plane tee feed according to the present invention; Figures 4a and 4b are respectively, a side view of the E-plane waveguide bend and a top view of the matched H-plane tee junction; Figures 5a and 5b are charts, respectively, of the radiation current amplitude distribution for an 8 slot waveguide section using the invention, and of the radiation current phase distribution for an 8 slot waveguide section using the invention; and Figures 6a and 6b are charts, respectively, of measured slot output voltage amplitude and slot output voltage phase (degrees) compared to slot 3 of a 5 slot array. Figure 7 is a view showing the combination of two dual end series slot feeds. Referring now to Figure 1, a planar slotted array antenna 10 comprises a polarizer 12, a longitudinal shunt slotted plate 14, a rotational series slotted plate 16, and manifold 18. The series waveguide excites a row of series slots 17 which couple RF power into the shunt waveguides. (The series waveguides are not visable in this Figure, as they are located on the back side of 16.) The shunt waveguide excites the shunt slots, which are the radiating elements. All of the slots are spaced one half waveguide wavelength (λ g /2) from the adjacent slots fed by the same guide. One form of a prior-art waveguide feed system for the series slots is shwon in Figure 2a. Each of the series slot waveguides 24 is fed at one end by a feed manifold 18. A waveguide short-circuiting wall 23 at the opposite end of the waveguide sets up the standing wave needed for proper excitation of the series slots. In certain applications, variable phase shifters 22 may be added to electronically scan the antenna's radiation pattern. In another form of the prior art, the series slots are fed as shown in Figure 2b. Here an E-plane waveguide tee 100 divides RF energy between two series slot waveguides 102 and 104, through E-plane tees 114 and 116. Waveguide shorts 106 at the outer ends of waveguides 102 and 104 set up the appropriate standing waves so that the series slots 108, 110, 112 etc., couple energy to the front face of the antenna. For a proper standing wave, the waveguide short 106 must be one-half wavelength from the end slot in the waveguide, as shown. Similar λ g /2 waveguide shorts are needed at the opposite ends of both waveguides 102 and 104, but only one-quarter wavelength on space is available for each of these shorts (since a constant series slot spacing of λ g /2 is imposed by the array grid). λ g is the wavelength in the waveguide at the operating frequency. Therefore, prior art antennas have employed a folded waveguide short 118 in which a 180 degrees E-plane bend is used to gain the needed spacing λ g /2 between the shorting wall and the last slot. Such folded shorts are only an approximation to a true waveguide short circuit; folded short circuits limit the array frequency bandwidth, and introduce numerous fabrication and assembly problems for the antenna. Slots 110 and 112, being located directly under the E-plane tees 114 and 116, respectively, exhibit direct coupling effects to the tee, which results in phase and amplitude errors for these slots. These slots thus become another bandwidth limiting element in the antenna. Referring now to Figures 3a and 3b, the dual end series slot feed 26 includes a tee junction which may be either an E-plane tee junction 28 (Fig. 3a) or an H-plane tee junction 30 (Fig. 3b), two waveguide sections 32 and 34, and two E-plane waveguide bends 36 and 38. The two waveguide section 32 and 34 and the E-plane bends are formed by a septum 40. The septum 40 is placed across waveguide 42 to separate all (n) slots 44 from the tee junction. The two E-plane waveguide bends 36 and 38 are formed by the space between ends 46 and 48 of the septum 40 and the ends of the waveguide 42 which space interconnects the two waveguide sections 32 and 34. The thickness of the septum 40 is much less than the wavelength in order to minimize the antenna thickness. The total length of the waveguide loop is approximately equal to nλ g , where n is equal to the number of slots. The series resistances of the slots 44 are selected to present an impedance that is matched to the imput waveguide 50. It will be appreciated from the foregoing description that a typical design of the dual end slot array feed is based on the following rules: 1. The H-plane or E-plane tee is separated from the slots by a septum. The E-plane tee (Fig. 3a) is located on the top of a series slot white the H-plane tee is located at the middle of two series slots (Fig. 3b). 2. The sum of the normalized resonant slot resistances of all n series slots in one unit is equal to 2. 3. The waveguide loop length is approximately equal to nλ g . 4. Between two arrays of n₁ and n₂ series slots where n₁ > n₂ a waveguide length equal to (n₁-n2) λ g /2 is required to be connected to the tee junction input of the array with n₂ slots. 5. H-plane or E-plane tee junctions shall not be offset by more than ± 0.01%λ g . The improved performance of the dual end feed is demonstrated by theoretical analysis of a waveguide with 8 series slots using ideal H-plane tee junction and E-plane waveguide bends. The slots are identical and their normalized resistances are equal to 0.25. The radiation current distribution compared to the ideal current is shown in Figures 5a and 5b, and are computed for ± 1.8% off the center frequency. The set of symmetrical curves are computed for the tee junction at the center while the unsymmetrical results are computed for the tee junction at a half guide wavelength off from the center. It is to be noted that the radiation current amplitude and phase variations are only 0.16 dB and 9.5 degrees, respectively, for the symmetrical feed over a 3.6% bandwidth. These variations in radiation current distribution increase to 0.44 dB and 13 degrees when the tee junction is offset by λ g /2. A comparison of the single end and dual end feed theoretical performances for the 8 slot array is shown in Table 1. These results are computed for 3.6% bandwidth. Obviously, the dual end feed provides an improvement in bandwidth performance as compared to the single end feed. EXAMPLE A dual end series slot feed was fabricated using the E-plane waveguide bend of Figure 4a and the H-plane tee junction of Figure 4b. A 16.5 GHz center frequency waveguide section with 5 unequal slots was employed. The dimensions of the waveguide 42 (Fig. 4a) were 0.496" by 0.155". (1 inch = 2.54 cm). For the E-plane waveguide bend, the thickness (t) of the septum 40 was 0.032", and the space "W" was 0.177". For the H-plane tee junction (Fig. 4b) the input 50 was 0.496" wide, with a tuning stub 52 which is 0.025" high and having a 0.138" diameter positioned 0.637" from the end of waveguide section 32. Waveguide section 32 has a width of 0.496" and a T shaped matching vane 54 centered with respect to the input 50. The T has a length of 0.222" and a thickness of 0.030". Tests showed that the VSWR of the E-plane waveguide bends is less that 1.10 over a 6% bandwidth, and the input VSWR of the H-plane tee junction is less than 1.18 over the same bandwidth. The measured output voltage amplitude and phase from the slots are shown in Figures 6a and 6b. The slot output voltages are measured from a set of identical waveguides in which the RF power is coupled through the series slots. It will be noted from Figure 6a that the measured voltage amplitudes are consistently evenly distributed over a wide bandwidth. The length of slot 2 is slightly too short (owing to fabrication errors) such that the amplitude falls off at the low frequency. The phase plot (Fig. 6b) was obtained by normalizing to the phase of slot 3, i.e., the phase of slot 3 = 0. All the phases track very well except the first slot. However, the largest discrepancy (at 16.0 GHz) over a 6 % bandwidth is only 17 degrees. Two dual end slot array feeds 42 (Fig. 7) having different number of slots 44 in their arrays of slots n1 and n2 (where n1 > n2) can have their tee junctions 50 connected to waveguide sections 56 and 58. Waveguide sections 56 and 58 are connected to a power divider 60 of manifold 18. Between the two arrays of n1 and n2 series slots where n1>n2, a waveguide length equal to (n1-n2) λ g /2 is required to be connected to the tee junction input of the array with n2 slots. Although only a single embodiment of the invention has been described, it will be apparent to a person skilled in the art that various modifications to the details of construction shown and described may be made without departing from the scope of this invention. For example, while most of the descriptions have addressed the feeding of series slot elements in the broad wall of a rectangular waveguide, the method is equally applicable to both shunt and series slots in waveguides of arbitrary cross-section. Also, it will be understood by those skilled in the art that this antenna will operate reciprocally, having the same characteristics whether transmitting or receiving, despite the fact that the antenna has been described above primarily as a transmitting antenna.

Claims: A slotted waveguide antenna for transmitting or receiving rf energy including a straight rectangular unobstructed waveguide (34) having slots (44) in one face thereof and connected at both ends to respective rectangular waveguide feeders (36,38) disposed at right angles to the slotted waveguide, so that rf energy can be applied to or derived from both ends of the slotted waveguide, characterised in that in use, equal amounts of rf energy appear simultaneously and co-phasally at the ends of the slotted waveguide (34), said slots are spaced one half of the wavelength in the waveguide apart so that said slotted waveguide is resonant, a rectangular waveguide section (32) equal in length to the slotted waveguide (34) is disposed parallel to the slotted waveguide on a second face remote from the slotted face, the waveguide section (32) is joined at its ends respectively to the waveguide feeders (36,38) to form a loop of an integral number of wavelengths of rf energy of the operating frequency in the waveguide antenna and an input/output waveguide (50) is coupled by a tee junction (28,30) to the middle of the waveguide section (32). The antenna according to claim 1 wherein said tee junction is an E-plane tee junction (28) coupled to the middle of a first side of said waveguide section (32). The antenna according to claim 1 wherein said tee junction is an H-plane tee junction (30) coupled to the middle of a sidewall of said waveguide section (32). The antenna to claim 2 or 3 wherein the waveguide feeders (36,39) form E-Plane end bends joining the ends of the waveguide section (32) to the slotted waveguide (34). The antenna according to claim 2 wherein said E-plane tee junction 28 has an offset of not more than about ±0.01 λg from a slot of said slotted waveguide (34). The antenna according to claim 3 wherein said H-plane tee junction (30) has an offset of not more than ±0.01 λg from a vertical plane between two slots of said slotted waveguide (34). The antenna according to anyone of the preceding claims wherein the slots (44) in the slotted waveguide (34) are series slots and the sum of the normalized resonant slot resistances of said slots is about 2.

Inventors: RICHARDSON, PHILLIP N., YEE, HUNG Y.
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1986
Main classification: H01Q21
IPC codes: H01Q21, H01Q13, H01P5