Planar Sleeve Antenna with Left-handed Choke Structure Takatsugu Fukushima1, Naobumi Michishita1, Hisashi Morishita1, and Naoya Fujimoto2 Graduate School of Science and Engineering, National Defense Academy, Kanagawa, Japan 2 Defense Electrics Division, Hitachi Kokusai Electric Inc., Tokyo, Japan [email protected] Abstract—A planar composite right/left-handed (CRLH) transmission line (TL) is applied to a choke structure of a sleeve antenna. The dispersion relation of the CRLH TL is designed as the left-handed (LH) branch at around 600 MHz. |S11| characteristic and radiation patterns are calculated and compared with those of the conventional sleeve antennas. The length of the LH choke structure is 0.06 wavelength at 636 MHz. 800 lu z Keywords—Sleeve antennas, planar structure, choke, composite right/left-handed transmission line, leakage current. wi y wg3 li wc 600 I. INTRODUCTION (a) Left-handed mode 500 400 300 200 A sleeve antenna has one of the most simple structure so that it has been employed in experiments. The most simple structure of the sleeve antenna is composed of an inner conductor that extends a quarter wavelength from the center conductor of a coaxial cable. In this structure, leakage current flows through the outer conductor of the coaxial cable radiates undesired waves which deform the radiation pattern [1]. To suppress the leakage current, a choke structure a quarter wavelength in length is attached to the outer conductor of the coaxial cable. The radiation pattern of the sleeve antenna with the choke structure is similar to that of a dipole antenna. Air line 0 30 60 90 120 150 180 p [degrees] (b) Fig. 1 (a) unit cell of the proposed composite right/left-handed transmission line and (b) dispersion relation with li = 18.0 mm, wi = 0.5 mm, wc = 0.8 mm, wg3 = 14.0 mm, lu = 5.0 mm, Ca = 5.6 pF. II. PROPOSED PLANAR CRLH TL Fig. 1 (a) shows a unit cell of the proposed planar CRLH TL. The unit cell composed of chip capacitor and PCB with metal patterns. Thickness and permittivity of the PCB are 3.0 mm and 3.8, respectively. The thin metal strips of which length is li work as shunt inductance. The chip capacitors work as series capacitance. Wave propagates z direction. The center metal strip of which width is wg3 is replaced with a coplanar waveguide (CPW) when the CRLH TL is applied to a choke structure of a planar sleeve antenna. Fig. 1 (b) shows the simulated dispersion relation of the proposed planar CRLH TL. Since the phase velocity has the different sign from the group velocity, the LH branch is confirmed at around 600 MHz. A choke structure becomes large when the operation frequency is low because the resonant length corresponds to the physical length of the antenna configuration. Therefore, miniaturized sleeve antennas have been proposed [2]. To produce easily, this antenna was designed to be a planar structure which is able to be made by printed circuit board (PCB). In the field of metamaterials, the composite right/lefthanded (CRLH) transmission line (TL) has been proposed [3]. The CRLH TL was applied to a choke structure of a sleeve antenna [4]. The length of the choke structure was 0.05 wavelength. However, the antenna could not produce easily because of its volumetric and complex structure. In this paper, planar CRLH TL was proposed for a choke structure of a planar sleeve antenna. The CRLH TL was designed with the left-handed (LH) branch at around 600 MHz. The radiation patterns and current distributions on the outer conductor of the coaxial cable were calculated to verify the operation of the choke structure. These characteristics were compared with those of conventional planar sleeve antennas. 978-1-5386-3284-0/17/$31.00 ©2017 IEEE 700 Chip capacitor (Ca pF, 16 mm 8 mm) Frequency [MHz] 1 III. PROPOSED PLANAR SLEEVE ANTENNA Fig. 2 (a) shows the proposed antenna. This antenna is composed of a PCB with metal patterns and a coaxial cable for feeding. The coaxial cable was designed 50 Ohms. The metal patterns are composed of a grounded CPW, an extension of a center strip conductor and an LH choke structure. Fig. 2 (b) shows the LH choke structure, which is composed of five unit cells of the proposed CRLH TL. The LH choke structure is attached on the ground pattern of the CPW. The upper and the lower end of the choke are short-and open-circuited, respectively. 739 AP-S 2017 3.5 Chip capacitor (Cb pF, 16 mm 8 mm) wg2 Unit cell 5.0 lcc z y x Coaxial cable z wg1 li lcw wc z y x y z y z y x (a) (a) (b) 330 z y Unit: mm (b) Fig. 4 Conventional planar sleeve antenna (a) without the choke and (b) with the quarter-wavelength choke structure. Fig. 2 (a) Proposed planar sleeve antenna with (b) the lefthanded choke structure. 0 85.0 1.0 200.0 wcw 22.3 1.0 wi gg1 200.0 Printed circuit board 1.5 3.5 115.0 lu 10.8 lew 103.0 Chip capacitor (Ca pF, 16 mm 8 mm) 0z Without choke 10 30 0 300 -10 Quarterwavelength choke |S11| [dB] -20 270 -20 -30 -30 90 x -20 636 MHz -30 550 60 -10 -10 240 120 0 600 650 Frequency [MHz] (a) 700 10 210 180 150 Left-handed choke Current [A/m] (b) Fig. 3 (a) |S11| characteristic of the proposed antenna and (b)radiation patterns of planar sleeve antennas in zx plane. 10.0 6.3 4.0 2.5 1.6 1.0 z y (a) Fig. 3 (a) shows |S11| characteristic of the proposed antenna. The parameters are li = 18.0 mm, wi = 0.5 mm, wc = 0.8 mm, wg3 = 14.0 mm, lu = 5.0 mm, wcw = 4.0 mm, gg1 = 2.0 mm, wg1 = 3.5 mm, wg2 = 1.0 mm, and lcw = 10.8 mm, Ca = 5.6 pF, Cb = 3.9 pF and the metal strips are copper. The proposed antenna resonated at 608 MHz. (b) (c) Fig. 5 Current distributions of the planar sleeve antenna (a) without the choke, (b) with the quarter-wavelength choke structure, and (c) with the LH choke structure. IV. CONCLUSION A planar CRLH TL was proposed as a choke structure. The dispersion relation was designed with an LH branch at around 600 MHz. The CRLH TL was applied to the choke of a planar sleeve antenna. The proposed antenna was made to resonate at 636 MHz. The radiation pattern of this proposed antenna is similar to that of a conventional planar sleeve antenna with a quarter-wavelength choke structure. The length of the choke structure was 0.06 wavelength, at 636 MHz. To verify the effect of the LH choke structure, radiation patterns were calculated. Conventional planar sleeve antenna without the choke and with a quarter-wavelength choke structure in Fig. 4 were also calculated for comparison. Fig. 3 (b) shows the radiation patterns in zx plane. In the case without the choke, the radiation towards the upper part of the antenna was suppressed due to the undesired leakage radiation. The radiation patterns of the planar sleeve antenna with the quarterwavelength and with the LH chokes were shaped as a figure eight. REFERENCES [1] Fig. 5 shows the current distributions of each antenna at 636 MHz. As shown in Fig. 5 (a), a current of 2.5 to 4.0 A/m flowed through the lower part of the outer conductor of the planar sleeve antenna without the choke. As shown in Fig. 5 (b), the quarter-wavelength choke structure suppressed the leakage current on the outer conductor. The current on lower part of the outer conductor become lower than 1.0 A/m. As shown in Fig. 5 (c), the LH choke structure also suppressed the leakage current. The current on lower part of the outer conductor was smaller than 1.0 A/m. [2] [3] [4] 740 M. Taguchi, S. Egashira, and K. Tanaka, “Sleeve Antenna with Ground Wires,” IEEE Trans. Antennas & Propag., vol. 39, no. 1, pp. 1322-1330, Jan. 1991. Y. Nishioka, T. Fukasawa, S. Makino, and Y. Sunahara, “Small Planar Sleeve Antenna for Portable Wireless Terminal,” Proc. A・ P Conf. IEICE., Japan, Rep. B-1-43, 2001. C. Caloz and T. Itoh, “Application of the transmission line theory of lefthanded (LH) materials to the realization of a microstrip LH transmission line,” IEEE-APS Int. Symp. Dig., Vol. 2, June 2002, pp. 412-415. T. Fukushima, N. Michishita, H. Morishita, N. Fujimoto, “Sleeve Antenna with Left-handed Choke Structure,” Int. Symp. on Antennas and Propg., Okinawa, Japan, pp. 484-485, 2016.
