AbstractâA novel broadband flip-flop low profile Reflectarray. (RA) is designed for Ka-Band based on the principle of dual side printed substrate. First, the ...
Flip-Flop Low Profile Wideband Reflectarray Antenna For Ka-Band Muhammad M. Tahseen, and Ahmed A. Kishk ECE Department, Concordia University, Montreal, Canada Abstract—A novel broadband flip-flop low profile Reflectarray (RA) is designed for Ka-Band based on the principle of dual side printed substrate. First, the reflected wave phase curve is obtained by varying the patch size on top layer while energy is coupled through a bottom slot of equal size to the patch. Such a cell provides 3600 degress reflected phase with almost linear behavior. Second, the element is flipped and analysis for reflected phase when square slot is varied on top layer while the complementary patch, is varied in the bottom layer. Both methods provide full 360 degrees phase range. In both methods, a small air gap is introduced below substrate to add GND plane on bottom. The proposed methods provide broadband using the thinnest available substrate. Both designs achieve good performance in term of Half Power Beam width (HPBW), Side Love Level (SLL), cross polarization and gain bandwidth (at 30 GHz). The first 15*15 RA design provides, HPBW of 6.6 degrees, SLL -20 dB, cross polarization -25 dB down than copolar component, 1dB gain bandwidth of 14.5 % and 3-dB bandwidth of 23.2 % centered. Similarly, the second flipped 15*15 RA design provides, SLL of -17 dB, cross polarization of -25 dB down than copolar component, 1-dB gain bandwidth of 11.5 % and 3-dB bandwidth of 21 %.
I. I NTRODUCTION High gain antennas with narrow HPBW are desired for long distance communication e.g. satellite, radio astronomy, radar and etc. Normally for these requirements, traditional arrays with array theory method and parabolic antennas with geometrical optics principle, are designed. Traditional arrays require complex feeding network, which are difficult to design for large arrays operating at high frequencies. The feeding Transmission Lines (TL) network increase the antenna losses through the long path for the RF signal as well as the possible radiation from network beds and discontinuity, which in turn affect the radiation characteristics of the array and reduce the antenna gain and efficiency. Parabolic reflector meets with the requirement but have non-planar curved reflector surface and bulky size. Recently, planar surface RA has been proposed as an alternative to parabolic reflectors, which are in low cost, light weight and easier to design and provide the advantages of both reflector and arrays [1]. The basic principle of RA is to convert incident wave with spherical wave front, transmitted from feed at focal point, to planar wave front at RA surface in transmit mode while converting back the planar wave front to spherical wave front in receiving mode working on the principle of focusing energy to a focal point of reflector using geometrical optics. Several methods have been proposed to fulfill this job, such as varying patch size, circular and square rings size variation, slot in
ground plane variation, using multilayer unit cell with slot and patch, multilayer cells with air gap layer used in between to increase phase range and bandwidth, changing effective dielectric constant of the substrate by drilling holes in it and many other methods [1], [2], [3], [7], [5]. It is normally stated that the bandwidth of RA is controlled by two major factors: the element bandwidth and non-constant path delay between RA elements [1], [3], [2], [6]. The narrow band elements used in RA designing will end up in decreasing the RA bandwidth e.g. patch antenna (3-5 % bandwidth). Different methods such as stacked patches with variable patch length [3], multi-resonant dipoles [8] has been proposed to widen RA bandwidth. These methods has reached in providing 10-20 % of RA bandwidth using different geometries with some tradeoff. It is observed that most of the researchers rely on using multilayer structure for widening bandwidth but in that case antenna size, weight, cost and losses increase. It is effective that to get wide RA bandwidth, elements selected should have large bandwidth. It is important to describe that the BW will be higher if the reflected phase curve in unitcell environment varies linearly, which reduces the abrupt errors and provide fabrication tolerance. Here, a broadband Ka-Band linearly polarized low profile dual side substrate printed flip-flop RA, is designed. The single element is analyzed separately with both sides patch and slot variation, in CST unit cell periodic environment. Two different phase curves are obtained in unitcell analysis. One by varying patch size on top and have back side equal size slot variation, while for second phase curve, the substrate is flipped and square slot on top varies and equal size patch varies on the back side. The unitcell has size of 0.6λ ∗ 0.6λ. The proposed unitcell is analyzed for oblique incidence and found that the accurate results can be obtained when the effect of angle of incidence is taken into account. The proposed method bypass other bandwidth improving methods, and it provides over 20 % antenna bandwidth in both flipped cases. Both RA antenna cases provide, wide band, low reflection at port, low SLL and low cross polarization. II. U N -F LIPPED (PATCH TOP AND S LOT B OTTOM ) R EFLECTARRAY D ESIGN A. Reflected Phase Curve with Patch Size Variation Single element with electrical size of 0.6λ∗0.6λ is designed in CST using Rogers RT 5880 material with dielectric constant 2.2 and thickness of 0.127 mm. The patch antenna on the top surface is varied in size to obtain reflected phase curve
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Fig. 3. 15 * 15 simulated RA model (un-flipped) Fig. 1. Un-Flipped and flipped unitcells geometry for RA design 0
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Fig. 4. Normalized feed pattern in E and H plane
Fig. 2. Reflected phase vs. patch size variation
while the complementary slot varies in bottom as shown in Figure 1(a). The double square rings at the top and bottom corners are used to create coupled resonance as the patch size increase that contributes in increasing the overall reflected phase range. The single element is simulated in periodic environment along x and y direction and y-polarized wave incidence is used. The reflected phase and amplitude curves versus patch size are shown in Figure 2. The reflected phase curve slope variation is smooth similar to linearity approaching maximum of 4000 degrees reflected phase. B. Reflectarray Model and Simulation Results A 15*15 RA with the electrical size of 9λ ∗ 9λ is designed in CST for full wave analysis as shown in Figure 3. To achieve the required specifications in RA, the feed is considered as the most important part of the structure so it is designed in a way to satisfy required outcome. A conical horn with symmetric radiation pattern in E and H plane shown in Figure 4, and with FOD = 0.337, is used to excite elements of the RA. The maximum gain of 26.5 dB is realized at center frequency 30 GHz with 1-dB gain bandwidth of 14.5 % and 3-dB gain bandwidth of 23.2 % is obtained. There are certain factors that hinder on high aperture efficiency of reflectarray e.g. feed blockage, feed loss, element loss, dielectric loss, polarization mismatch loss, and aperture loss. The gain variation versus frequency is shown in Figure 8. The normalized radiation pattern of un-flipped (patch top and slot bottom) RA for Eplane and H-plane is shown in Figure 5. Equations 1 and 2 are used to get required phase distribution φ(xi , yi ) on lens located in X-Y plane, to direct the beam towards (θ0 , φ0 ). The propagation constant in free space is k 0 and di is the distance
from the feed to the individual element location (xi , yi ) [1]. The total phase delay from feed to a fixed plane in front of the aperture, must be constant. φ(xi , yi ) − k0 (di − sinθ0 (xi cosφ0 + yi sinφ0 )) = 2πN (1) di =
(xi − xf )2 + (yi − yf )2 + (zf )2
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III. F LIPPED (S LOT TOP AND PATCH B OTTOM ) R EFLECTARRAY D ESIGN A. Reflected Phase Curve with Slot Size Variation Unitcell is flipped to analyze the reflected phase variation when slot on top surface varies in size as shown in Figure 1(b). The reflection phase and amplitude curves are shown in Figure 6. Flipped unitcell also provides complete 3600 degree reflected phase deviates from linearity but still promising to design RA with good fabrication tolerance.
Fig. 5. Normalized E and H-plane pattern of simulated (a) flipped (b) unflipped RA
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Fig. 6. Reflected phase vs slot size variation in flipped unitcell
Fig. 7. Reflected phase for different incidence angles
B. Normal and Oblique Incidence The oblique incidence on unitcell is investigated shown in Figure 7, depicts the effect of the angle of incidence on the reflected phase. The Analysis indicates that reflected phases by the RA elements are affected by the oblique incidence excitation of elements from focal point especially. The elements with larger dimensions are highly effected by the angle of incidence when it increases from 200 degrees to onward. The accurate results can be accomplished when effect of the angle of incidence is taken into account during RA designing. C. Simulation Results The antenna is re-simulated for full wave analysis when it is flipped. Now in this situation, slot is varying on top while the patch is on the back side. The maximum gain of 26 dB is realized at center frequency 30 GHz with 1-dB gain bandwidth of 11.5% while 3-dB gain variation is found 21%. The gain variation versus frequency is shown in Figure 8. The normalized radiation pattern for E-plane and H-plane is shown in Figure 5, where SLL in E-plane becomes higher than Hplane but still below -16 dB. IV. C ONCLUSION This paper has presented design and analysis of novel low profile flip-flop linearly polarized Ka-Band RA based on dual side printed substrate. In the first case, single element with square patch on front side is varied to get reflection wave phase curve, while complementary slot varies at the same time. This configuration provides total phase range of 4000 degrees with almost linear slope variation. The linearity in phase curve shows tendency to provide wide band in RA antenna
Fig. 8. Gain variation with frequency of simulated un-flipped and flipped RA
performance. In the second case, substrate is flipped and new reflected wave phase curve is obtained for the required phase compensation at RA surface. Flipped geometry also provides full cycle of 3600 degrees phase curve, which confirms that the RA will perform well in this environment too. These phase information has been used in designing reflectarray in such a way that when the required phase is compensated at each location then it contributes in reflecting the wave towards a particular direction. A 15*15 reflectarray antenna with electrical size of 9λ ∗ 9λ has been designed in CST for un-flipped and flipped structures. A potter conical horn with symmetrical radiation pattern in both planes is used to excite array elements, placed at focal point for broadside radiation. The antenna performance has been assessed at 30 GHz. The un-flipped designed RA antenna provides, 1-dB gain bandwidth of 14.5% and 3-dB gain bandwidth of 23.2%, HPBW of 6.60 degrees, gain of 26.5 dB, SLL is -20 dB down, cross polarization is -25 dB below than co-polarization and the highest aperture efficiency of 45% has been attained. On the other hand, the flipped RA antenna provides, 1-dB gain bandwidth of 11.5% and 3-dB gain bandwidth of 21%, HPBW of 70 degrees, maximum gain of 26 dB, SLL of -17 dB down, and the maximum aperture efficiency of 41%. R EFERENCES [1] J. Huang and J. A. Encircle, “Reflectarray Antennas” John Wiley & Sons Inc., Hobo ken, NJ, 2007 [2] David M. Polar, Stephen D. Tarkington, and H. D. Rigors, “Design of Millimeter Wave Micro strip Reflectarrays”, EERIE Transactions on Antennas and Propagation, Vol. 45, No. 2, 1997. [3] Encircle, J. A, “Design of two-layer printed reflectarray using patches of variable size”, IEEE Transaction on Antennas and Propagation, Vol, 49, pp. 1403-1410, 2001. [4] J. Haung, “ Analysis of Microstrip Reflectarray antenna for micro spacecraft application” The telecommunications and Data Acquisition Report, P153-P173, 1995. [5] Mohammad Reza Chaharmir, Jafar Shaker, Nicolas Gagnon, and David Lee, “Design of Broadband, Single Layer Dual-Band Large Reflectarray Using Multi Open Loop Elements”, IEEE Transactions on Antennas and Propagation, Vol. 58, No. 9, Sep, 2010. [6] D. M. Pozar, “Bandwidth of Reflectarrays”, IEEE Electronics Letters, Vol. 39, No. 21, October 2003. [7] Jose A. Encinar and J. Agustin Zornoza, “Broadband Design of ThreeLayer Printed Reflectarrays”, IEEE Transaction on Antennas and Propagation, Vol, 51, No.7, July 2003. [8] H. Deguchi, K. Mayumi, M. Tsuji, and T. Nishimura, “Broadband singlelayer triple-resonance microstrip reflectarray antennas,” in Proc. EuMA, pp 29-32, Italy, 2009.
