Super-Directive, Efficient, Electrically Small, LowProfile Antenna based on Compact Metamaterial Nebil KRISTOU, Jean-François PINTOS Kouroch MAHDJOUBI CEA-LETI, MINATEC Campus, 38054 Grenoble, France Nebil.KRISTOU@cea.fr IETR, Université de Rennes 1, 35042 Rennes, France kouroch.mahdjoubi@univ-rennes1.fr Abstract— Artificial Magnetic Conductors (AMC) present an attractive substrate for low profile antennas which can take advantage of intrinsic zero reflection phase response to boost antenna performance without the need for thick quarter wave backplane. In this paper, a modified version of the Egyptian Axe Near-Field-Resonant-Parasitic (NFRP) dipole antenna is placed over compact AMC based on square spiral metallic patches and a capacitor network beneath the ground plane. Simulation results demonstrate that the NFRP antenna is electrically small (ka1=0.498) with a low-profile (total height = 0.07Ȝ0) when placed over compact AMC (ka2=1.29). It achieves super-directivity (6.52 dBi), high realized gain (5.7 dBi) and high radiation efficiency (83.7%). II. A. Antenna and AMC design The selected antenna is a modified version of the Egyptian Axe Dipole (EAD) antenna as proposed in [2], it consists on a driven element printed on the bottom-side of the substrate and a near field resonant parasitic (NFRP) element printed on the topside. The resonant behavior is dependent on the capacitive load; the dipole’s self-inductance and capacitance; and the resonant lengths of the dipoles arms [2]. The NFRP element takes the same shape as the driven element for impedance matching purposes and their dimensions are co-designed to achieve the desired reactive and resistance matching. Finally, two lumped capacitors of 0.1 pF are introduced on both antenna sides. These capacitors are used to adjust the impedance value once the antenna is placed over the metasurface. The proposed modified NFRP antenna shown in Fig. 1 is based on the Rogers Duroid 3003 (İr = 3, tan δ=0.0013) substrate with a thickness of 0.75 mm. The dimensions of the NFRP element are given in Fig. 1 and summarized in Table I. Keywords—Artificial Magnetic Conductor (AMC); metasurface; Electrically Small Antenna (ESA); Super-directive antenna; high gain antenna; low-profile antenna. I. INTRODUCTION Electrically small antennas (ESA), because of their compact sizes, continue to be of great research interest for potential wireless applications. There have been many efforts to find the best tradeoff between size and performances for ESAs, Successful miniaturized antenna designs, with high directivities, have been reported recently based on some type of metastructure [1]. Artificial Magnetic Conductors (AMC) present an attractive substrate for low profile antennas which can take advantage of intrinsic zero reflection phase response to boost antenna performance without the need for thick quarter wave backplane. However, for an effective AMC structure, several unit cells are required. As a result, the whole antenna system usually has a relatively large electrical size. Instead of the typical square or rectangular shape, a circular AMC structure is designed to minimize the volume (radius) of the smallest sphere enclosing the antenna structure. The selected unit cell, as detailed in [3], is based on square spiral metallic patch and a capacitor network beneath the ground plane. In this paper, a modified version of the Near Field Resonant Parasitic (NFRP) antenna [2] is chosen to be positioned over a compact AMC based on high miniaturized unit cell proposed in another work [3]. The NFRP antenna is selected because of its smal electrical size (ka < 0.5), high efficiency (ηrad > 90%), operational bandwidth (1.8%), and dipole radiation characteristics. The aim of this paper is to propose a superdirective, efficient, electrically small, low-profile NFRP antenna based on compact AMC metasurface. We note that, the term “electrically small” will mean ka < 0.5, where a is the radius of the smallest sphere enclosing the entire antenna system and k=2ʌ/Ȝ, Ȝ being the free-space wavelength. Note that all of the numerical simulations reported in this paper were performed using ANSYS Electronic Desktop (HFSS). ,((( NFRP ANTENNA OVER COMPACT AMC (a) (b) Fig. 1. Top view of the proposed system: (a) Egyptian axe NFRP dipole antenna with key parameters. (b) NFRP antenna over compact AMC TABLE I. DIMENSIONS (MILLIMETERS) OF THE NFRP ANTENNA DESIGN NFRP outer radius: a 24.8 Driven outer radius: e 21.8 NFRP inner radius: b 21.8 Driven inner radius: f 20.8 NFRP gap length: c 10 Driven gap length: g 37 NFRP strip width: d 1.5 Driven strip width: h 2 $36 Unit cell dimensions are 11.8×11.8 mm2. The same substrate (RO 3003) is used for top and bottom layers of the cell. The air gap layer is used to ensure the minimum thickness needed with a lightweight structure. The overall thickness is about 7.75 mm (Ȝ0/40). All capacitances at the bottom layer are fixed to C=0.2 pF with serial ohmic resistance Rres=1 Ohm. The AMC structure is optimized to be an in-phase reflector, i.e., to act as an artificial magnetic conductor, near 960 MHz, the frequency of interest. antenna over the compact AMC and confirm the superdirectivity behavior. 0 -5 |S11| [dB] -10 -15 -20 -25 NFRP antenna over AMC B. Results and discussion The simulated return loss of the NFRP antenna alone is shown in Fig. 2(a) (black curve). It resonates at the frequency fr= 990 MHz, where the minimum |S11| = -19 dB and the -10 dB bandwidth is 18 MHz (1.8%). Radiation efficiency is 96%, peak directivity and peak realized gain are respectively 2.16 dBi and 1.91 dBi. At f0=960 MHz, the NFRP antenna alone is not matched to 50 with a minimum |S11| = -2 dB. Radiation efficiency is 90% while peak directivity and peak realized gain are respectively 2.17 dBi and -2.62 dBi. -30 0.95 1 Frequency [GHz] 1.05 (a) 0 30 330 10 60 5 [dBi] 0 300 10 5 -5 0 90 -5 -10 270 -10 Directivity Realized Gain -15 The NFRP antenna, with a ka1 = 0.498, is placed 15 mm (Ȝ0/20) above the AMC structure as shown is Fig. 1(b). The AMC structure has a finite diameter equal to 128 mm (Ȝ0/1.6). This gives ka2 = 1.29 at the resonance frequency f0 = 960 MHz. Simulated return loss of the NFRP antenna over the AMC surface is shown in Fig. 2(a) (red curve). The resonance frequency is shifted from 990 MHz to 960 MHz because of the presence of the AMC surface in the extreme near field of the NFRP antenna. The return loss of the entire system reach -32 dB with 9 MHz (1%) of -10 dB bandwidth at the operating frequency f0. Radiation results are shown in Fig. 2(b) and Fig. 2(c). At frequency of interest f0, high peak directivity (6.52 dBi) and peak realized gain (5.7 dBi) more than double the peak realized gain of the NFRP alone (1.91 dBi without AMC) are achieved, while the radiation efficiency and total efficiency are 83.7% and 83.5%, respectively. Fig. 2(c) shows the simulated radiation pattern of the entire system compared to that of the NFRP antenna alone. -20 0.9 0.95 1 Frequency [GHz] 120 240 1.05 NFRP antenna over AMC 150 210 NFRP antenna 180 (b) (c) Fig. 2. Simulation results: (a) Comparison of S|11| of the NFRP antenna aver AMC and NFRP antenna alone (b) Peak directivity and realized gain of the proposed system. (c) Radiation patterns at 960 MHz for phi=0°. Directivity [dBi] 15 10 5 0 Harrington Limit NFRP antenna over AMC -5 -10 In Fig 3. the peak directivity of the designed electrically small NFRP antenna over the proposed compact AMC surface at f0 = 960 MHz is compared to Harrington limit of directivity [4]. Considering Fig 3. It is observable that the proposed system achieves a very high directivity (6.52 dBi) which exceeds the Harrington limit. Then, the proposed system can be considered as a super-directive antenna. III. NFRP antenna -35 0.9 0 0.5 1 1.5 ka 2 2.5 3 Fig. 3. Peak directivity of the proposed NFRP antenna over the AMC compared to the normal directivity Harrington limit [4]. REFERENCES [1] CONCLUSION AND FUTURE WORK [2] In this work, a super-directive (6.52 dBi), efficient (ηrad= 83.7%), electrically small (ka1=0.498), low-profile (total height = 0.07 Ȝ0) NFRP antenna based on Compact AMC (ka2=1.29) is designed and simulated. Nevertheless, a deterioration in the frequency bandwidth of the NFRP antenna is noted which drops from 1.8 % without AMC to 1 % with the AMC. The next step will be the prototyping and characterization of the NRFP [3] [4] P. Jin and R. W. Ziolkowski, "High-Directivity, Electrically Small, LowProfile Near-Field Resonant Parasitic Antennas," in IEEE Antennas and Wireless Propagation Letters, vol. 11, no. , pp. 305-309, 2012. P. Jin and R. W. Ziolkowski, "Linearly and circularly polarized, planar, electrically small, metamaterial-engineered dipole antennas," 2010 IEEE Antennas and Propagation Society International Symposium, Toronto, ON, 2010, pp. 1-4. N. Kristou, J-F. Pintos, K.Mahdjoubi , "Miniaturized Tunable Artificial Magnetic Conductor for Low LTE Band,” 2017 IEEE International Symposium on Antennas and Propagation (APSURSI), San Diego, California, USA, 2017. R. F. Harrington, "Effect of antenna size on gain bandwidth and efficiency " J. Res. Nat .Bur., vol 64-D, pp 1-12, Jan/Feb. 1960.
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