IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
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Design and Implementation of Dual-Frequency Dual-Polarization Slotted Waveguide Antenna Array for Ka-Band Application Teng Li, Hongfu Meng, Member, IEEE, and Wenbin Dou, Senior Member, IEEE
Abstract—In this letter, a dual-frequency dual-polarization slotted waveguide antenna array at Ka-band is proposed. A inclined slot array working at 35 GHz, cut in the narrow wall of rectangular waveguides, is employed for horizontal polarlongitudinal slot array working at 30 GHz, ization (HP). An etched on the broadside of ridged waveguides, is employed for vertical polarization (VP). The two arrays are interlaced with each other and fed from the opposite sides. Two novel bend interconnected structures are invented to connect the radiating waveguides and feeding networks, which simplify and compact the antenna structure and improve the aperture efficiency. The measured 10-dB reflection coefficient bandwidth of HP array is 688 MHz, and 658 MHz for VP array. The maximum gain is 25.4 and 24.8 dB for HP array and VP array, which correspond to the aperture efficiency of 36.3% and 43.05%, respectively. The sidelobe levels of both arrays are below 17.6 dB. Index Terms—Antenna arrays, dual frequency, dual polarization, feeding network, slotted waveguide.
I. INTRODUCTION
M
ODERN radar and communication systems demand to design an antenna with polarization-agile ability and even frequency-agile function since the polarization or frequency diversity can significantly improve the system performance. Therefore, the synthetic aperture radar (SAR) systems, multiple-input–multiple-output (MIMO) system, and scatterometer (SCA) have a requirement on dual polarization. In order to realize these characteristics, microstrip antennas have been widely studied [1]–[4]. As frequency increases to millimeter-wave band, the slotted waveguide antenna is a very promising candidate due to low loss, high power capacity, thermal stability, and high mechanical strength. Different types of dual-polarization slotted waveguide antennas have been researched [5]–[10]. The feeding networks of these antennas are either unsuitable for Ka-band applications or occupy the antenna aperture, which decreases the aperture efficiency, especially for a subarray feeding system in a large array. In this letter, a dual-frequency dual-polarization slotted waveguide antenna array is presented. The design procedure of radi-
Manuscript received May 21, 2014; revised June 14, 2014; accepted July 04, 2014. Date of publication July 09, 2014; date of current version July 21, 2014. The authors are with the State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2337355
Fig. 1. Configuration of the proposed antenna array. (a) Top view. (b) Front view and details of the bend structure.
ating elements is also introduced. The complexity of the feeding networks is decreased by using two invented bend interconnected structures. This compact multilayer structure is especially suitable for the feeding system of a subarray since no aperture is occupied. The simulations were obtained by the commercial software HFSS. The proposed antenna array was fabricated with aluminum and was tested to confirm its design validity. II. ANTENNA CONFIGURATION AND DESIGN The proposed antenna has an aperture size of mm and a thickness of 18.5 mm. It consists of a horizontal polarization (HP) array working at 35 GHz and an vertical polarization (VP) working at 30 GHz, as depicted in Fig. 1. The HP in the -direction is realized by alternant inclined slots cut in the narrow wall of rectangular waveguides, and the radiation polarization in the -direction is canceled by the adjacent slots, assuring a low cross polarization. The VP in the -direction is
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Fig. 2. Equivalent circuit model of slots in waveguide.
realized by longitudinal slots etched on the broadside of ridged waveguides. The radiating waveguides of the two arrays are interlaced with each other. The VP array is fed by feeding networks on the left, and the HP array is on the right. The spacing along the waveguide direction between the radiating elements is or , where and are the guide wavelength in the radiating waveguide of HP array and VP array, respectively. In order to suppress the grating lobe, the spacing between the radiating waveguides should be less than , which means , where is the height of radiating waveguide of HP array, is the width of radiating waveguide of VP array, and is the thickness of waveguide wall. In view of various factors, the parameters are chosen as follows: mm, mm, mm, and mm. The width of all slots is 0.6 mm limited by processing technology. Furthermore, the 20-dB sidelobe level (SLL) of Taylor continuous distribution with is employed for the two arrays in both - and -directions. A. Parameter Extraction for the Radiating Element The two types of radiating element can both be modeled and characterized by equivalent shunt admittance on transmission line [11], as shown in Fig. 2. As an initial design consideration, it is essential for the admittance of a single radiating slot module to satisfy the requirement of impedance matching and array synthesis. The normalized admittance of the radiating slot can be obtained from [12] (1) where the -parameters are calculated from a two-port single-slot module. Assuming all radiating elements to be ideal point sources, the radiating structure consisting of radiating slots and radiating waveguides is longitudinal symmetrical structure. Therefore, only the up or down half-structure is taken into account. The up half-waveguides of the HP array are named from 1 to 5, and those of the VP array are named from 1 to 4, as depicted in Fig. 1(a). For this two-dimensional array, the effect from antenna structure and mutual coupling should be considered. Consequently, we attempt to directly use the entire array model for parameter extraction. Equation (1) is based on a two-port single-element model, and it also can be used in an array model, which means 10 and 8 wave ports are required in the HP and VP array, respectively. However, the more wave ports are introduced, the more
Fig. 3. Parameters extraction curves. (a) Resonant slots cutting depth and normalized admittance versus rotation angle for the HP array. (b) Resonant slots length and normalized admittance versus slot offset for the VP array.
computer resources and time are required. Therefore, we use the end-fed array model to solve this problem, but we cannot directly use (1) to obtain the normalized admittance in this situation. To simplify matters, all radiating slots of the parameter extraction array model are in the same dimensions. Therefore, the equivalent admittance of each slot in the same waveguide can be approximately considered the same. According to the transmission line theory and the equivalent circuit model of slots in waveguide, the slots are shunt-connected, and the total normalized shunt admittance in waveguide is given by [13] (2) is the where is the amount of slots in waveguide and reflection coefficient of port . When the imaginary part of equals zero, it means the slots excited by port are almost resonant. It is obvious to see that the obtained is an average value of slots in waveguide , and it can be calculated by fixing the reference planes of waveguide ports to the center of the radiating slots using the full-wave simulator HFSS. For the inclined slots in HP array, the extracted parameter curves about (represents the resonant cutting depth of slots in waveguide ) and versus (represents the rotation angle) are summarized in Fig. 3(a). It is found that the normalized admittance curves become diverged with the increase of rotation angle. The mutual coupling between the waveguides becomes
LI et al.: DUAL-FREQUENCY DUAL-POLARIZATION SLOTTED WAVEGUIDE ANTENNA ARRAY FOR Ka-BAND APPLICATION
more significant when radiating slots rotated at a wide angle. In addition, the curves , , and have the similar tendency, which is different from the curves and owing to the different inclined directions of slots. For the longitudinal slots in VP array, the curves about (represents the resonant length of slots in waveguide ) and versus (represents slot offset) are summarized in Fig. 3(b). It can be observed that the differences between curves are indistinct, which are quite different from the HP array, because the mutual coupling effect between the adjacent waveguides is reduced by the radiating waveguide structures of HP array. The initial dimensions of radiating slots can be obtained from the curves of Fig. 3 according to the Taylor distribution [12]. It is realized by different inclined angle for HP and different shift for VP.
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Fig. 4. Photograph of the proposed antenna array.
B. Feeding Network Design The conventional feeding network is using a waveguide with center coupling slots without bends. Due to the width limitation of radiating waveguide and the complexity of slotting on the ridges, it is difficult to directly use the conventional structure. The side-fed network is feasible, but occupies the aperture. Therefore, we bend and broaden the radiating waveguide to the next layer without occupying aperture, as depicted in Fig. 1(b), and the guide wavelength is unchanged. The alternating inclined coupling slots are still used for power splitting. For the HP array, the radiating waveguides are fed from the right end, and the feeding waveguide is end-fed. However, it is difficult to feed radiating waveguides through coupling slots from feeding waveguide below since their broadsides are orthogonal to each other and only the narrow wall of the radiating waveguides can be used. Thus, a novel bend interconnected structure placed between the coupling slot and the radiating waveguide is invented. The left end of the bend structure is short-circuit with spacing to the coupling slot. The energy is coupled from the bend structure to the radiating waveguide through the connecting window, and the inductive block is used for wide impedance matching. Due to the adjacent radiating slots inclined in the opposite direction, the radiating waveguides should be excited out of phase. Therefore, the bend structures are spaced in the -direction alternately to create a 180 phase difference. On the contrary, the radiating waveguides of the VP array are fed from the left end. A longitudinal feeding slot, the same mechanism as the VP array radiating slot with to the short end, on the broadside of feeding waveguide is used to feed coupling waveguide in the center, where is the guide wavelength of feeding waveguide. The coupling power is determined by the length and offset of the feeding slot. Because of the width limitation of radiating waveguide, we use single-ridged waveguide for compressing dimensions. The bend structure consisting of two stepped ridges is introduced to transform the ridged waveguide from narrow to broad. The size of steps can be adjusted to obtain a wide impedance bandwidth. The stepped ridges are cascaded back to back; therefore, the radiating slots and coupling slots can be located at the opposite side of the ridge. This structure simplifies the design and manufacture by avoiding cutting ridge around slots. The right end of
Fig. 5. Return loss and isolation of the proposed antenna array.
the bend structure is short-circuit with spacing coupling slot.
to the
III. RESULTS AND DISCUSSION According to the initial design dimensions obtained from the parameter extraction curves, the antenna array is optimized slightly to achieve the required performance. The photograph of the fabricated antenna array is shown in Fig. 4. The results in Fig. 5 show the simulated and measured return loss and isolation of the proposed antenna array. The HP array exhibits a bandwidth of 688 MHz with a return loss below 10 dB, which agrees well with the simulation. However, there is about a 200-MHz frequency shift in the VP array owing to the additional feeding slot located at the edge of feeding waveguide, which is more sensitive to the fabrication tolerance (mainly the welding tolerances). In spite of this, the measured 10-dB return-loss bandwidth is about 658 MHz, which is the same as the simulation result. The isolation between the two arrays is better than 40 dB over the whole band since their orthogonal polarizations and the filter affects each other. Comparing the measured results of two arrays, the fabrication tolerance within mm is acceptable. The E-plane and H-plane normalized radiation patterns of the proposed antenna array are depicted in Figs. 6 and 7. According to the figures, the measured radiation patterns have good agreements with the simulation results. The radiation patterns in -plane are unsymmetrical because the radiating waveguides are end-fed and the antenna is not symmetrical in -direction. The measured SLLs are better than 20 dB, except the SLL in
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
E-plane of HP array. It deteriorates to approximately 17.6 dB due to the effect of bend structure and the accumulative errors caused by the radiating slots. Nevertheless, the achievement of the SLLs demonstrates the desired Taylor distribution and the presented parameter extraction method. The measured gains of HP array and VP array are 25.4 and 24.8 dB, which correspond to the aperture efficiency of 36.3% and 43.05%, respectively. In addition, the antenna array exhibits a less than 25-dB crosspolarization level at Ka-band. IV. CONCLUSION The dual-frequency dual-polarization slotted waveguide antenna array for Ka-band has been designed, constructed, and tested. The results indicate the effectiveness of the proposed design procedure and feeding structures. The array parameter extraction method can be used for the other types of slotted waveguide arrays. Two invented bend structures simplify the design of feeding networks and improve the aperture efficiency. Moreover, these structures are very suitable for the subarray feeding system of a large antenna array. In summary, this proposed antenna is an attractive candidate for large dual-polarization antenna arrays. REFERENCES Fig. 6. Normalized radiation patterns of HP array. (a) E-plane. (b) H-plane.
Fig. 7. Normalized radiation patterns of VP array. (a) E-plane. (b) H-plane.
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