Multiband Antenna Employing Multiple Metamaterial Reflectors

1. Introduction

Mobile wireless communication systems must now offer higher bit rates because of the increasingly widespread use of smartphones and tablet terminals. Mobile wireless communication systems such as cellular networks commonly utilize multiple frequency bands to achieve high system capacity. In addition, cellular systems use a sector configuration to improve frequency efficiency [1].

A conventional sector antenna is shown in Fig. 1. It has a radiator and metal reflector for each frequency band used in mobile wireless communication systems. As these systems begin handling more frequency bands, it is expected that the increased number of antennas and the larger space needed for these systems will become a problem.

Fig. 1. Conventional sector antenna.

2. Multiband sector antenna employing multiple metamaterial reflectors

NTT Access Network Service Systems Laboratories is developing a small-volume multiband sector antenna employing multiple metamaterial reflectors. Metamaterial reflectors have a periodic structure and consist of dielectric rods. They also have electromagnetic band gap (EBG) characteristics, meaning that they can reflect/transmit electromagnetic waves according to the frequency band [2]. The EBG characteristics enable the reflectors to act as a band-stop filter or a band-pass filter.

The concept of the proposed N-band sector antenna employing N metamaterial reflectors and a multiband radiator is shown in Fig. 2(a). The center frequency of the desired frequency band is defined as f_n (f₁ > f₂ > … > f_N). Metamaterial reflector #n reflects the electromagnetic waves of f_n similarly to the way a metal reflector does. On the other hand, metamaterial reflector #n transmits the electromagnetic waves of specific frequency bands (f_n+1, f_n+2, …, f_N, N > n) as if the reflector was transparent. Each metamaterial reflector reflects/transmits different frequency bands, so the proposed antenna can radiate multiple frequency bands in an area with a small footprint.

The reflectors are square, and their side lengths are indicated by L_n. The distance between the multiband radiator and the front surface of each metamaterial reflector is s_n, as shown in Fig. 2(b). Metamaterial reflector #N, which is furthest from the multiband radiator, can be implemented as a metal reflector in the proposed antenna.

Fig. 2. Concept of multiband sector antenna employing multiple metamaterial reflectors.

The proposed concept yields better design prospects than those of other design concepts because when the reflected frequency bands change or the number of frequency bands increases, we only need to change the EBG bands or increase the number of metamaterial reflectors in the proposed antenna to adapt to the situation.

3. Woodpile metamaterial

We applied woodpile metamaterial [3] to the reflectors of the proposed multiband sector antenna. The woodpile metamaterial structure consists of layers of dielectric rods and is illustrated in Fig. 3. The rods have a base of w_n1× w_n2, where w_n1 is the rod depth along the X-axis, and w_n2 is the rod width along the Y- and Z-axes. Layer B (D) is at right angles to layer A (C) in space, and the woodpile metamaterial consists of four layers of rods. The spacing between rods in each layer is indicated by a_n. The rods in layer C (D) are set to the spacing of half of a_n from the rods of layer A (B).

Fig. 3. Woodpile metamaterial structure.

4. Triple-frequency-band antenna

We fabricated a triple-frequency-band sector antenna with the same 90^° beamwidth that radiates at 800 MHz, 2 GHz, and 4 GHz in order to verify the concept of the proposed antenna [4]. The prototype is shown in Fig. 4. The triple-frequency-band sector antenna comprises a multiband radiator, metamaterial reflector #1 (reflects electromagnetic waves at 4 GHz), metamaterial reflector #2 (reflects electromagnetic waves at 2 GHz), and a metal reflector (reflects electromagnetic waves at 800 MHz). The multiband radiator is separated from the reflectors by distance s_n, which is a quarter of the wavelength of each frequency band. That is, s₁ = 18.8 mm, s₂ = 37.5 mm, and s₃ = 93.8 mm. The multiband radiator consists of a biconical antenna and a dual-band sleeve dipole antenna [5].

Fig. 4. Prototype antenna.

The metamaterial reflectors consist of ceramic rods with a relative permittivity of 9.6 and a loss tangent of 3.5 × 10⁵. The parameters of both metamaterial reflectors are w₁₁ = 4.7 mm, w₁₂ = 8.6 mm, a₁ = 37.5 mm, w₂₁ = 14.1 mm, w₂₂ = 14.6 mm, and a₂ = 75.0 mm. The side lengths of the reflectors are L₁ = 83.6 mm, L₂ = 164.6 mm, and L₃ = 375.0 mm.

The EBG characteristics of the metamaterial reflectors are plotted in Fig. 5. It should be noted that each EBG characteristic corresponds only to the respective metamaterial reflectors. The center frequencies of the EBG band in the measurements and the electromagnetic field simulations are in good agreement. A finite array was used in the measurements, and consequently, the transmission characteristics degraded in comparison to those in the electromagnetic field simulations in which an infinite array was considered.

Fig. 5. EBG characteristics of woodpile reflectors.

The radiation patterns on the horizontal plane for each frequency band are shown in Fig. 6. The proposed antenna has excellent directivity in each frequency band, and the measurement results match the electromagnetic field simulation results well. The beamwidth for each frequency band is indicated in Table 1. The proposed antenna achieves a beamwidth of approximately 90^° at 800 MHz, 2 GHz, and 4 GHz, as designed.

Fig. 6. Radiation patterns of proposed antenna on horizontal plane.

Table 1. Beamwidth for each frequency band in horizontal plane.

5. Summary

We developed a novel multiband sector antenna employing multiple metamaterial reflectors and a multiband radiator for a sector antenna in mobile wireless communication systems. A triple-frequency-band sector antenna that radiates at 800-MHz, 2-GHz, and 4-GHz bands was designed, and a prototype was fabricated. We clarified the feasibility of the proposed small-footprint antenna in measurements and simulations.

References

	Hideya So Engineer, NTT Access Network Service Systems Laboratories. He received the B.E. from Tokyo University of Science in 2009 and the M.E. from Tokyo Institute of Technology in 2011. He joined NTT Access Network Service Systems Laboratories in 2011 and has been engaged in research and development (R&D) of antennas for future wireless access systems. He is a member of the Institute of Electronics, Information, and Communication Engineers (IEICE) and the Institute of Electrical and Electronics Engineers (IEEE).
	Atsuya Ando Research Engineer, NTT Access Network Service Systems Laboratories. He received the B.S. in electronic engineering from Kitami Institute of Technology, Hokkaido, in 1988, the M.S. in information engineering from Hokkaido University in 1990, and the Dr. Eng. in system information engineering from Tsukuba University, Ibaraki, in 2013. Since joining NTT Wireless Systems Laboratories in 1990, he has been researching and developing personal and base station antennas for wireless mobile communication systems. From 2000 to 2003, he was with the ATR Adaptive Communications Research Laboratories in Kyoto, where he was involved in analyzing antennas for wireless ad-hoc network systems. He is currently engaged in R&D of base station antennas using metamaterials. He is a member of IEICE and IEEE.
	Takatoshi Sugiyama Senior Research Engineer, Supervisor, Group Leader, NTT Access Network Service Systems Laboratories. He joined NTT in 1989 and has conducted R&D on forward error correction, interference compensation, CDMA, and MIMO-OFDM technologies for wireless communication systems such as satellite, wireless LAN, and cellular systems. He is currently responsible for the R&D of intelligent interference compensation technologies, radio propagation modeling, and multiband antenna design for future wireless communication systems. He currently serves as the Vice Chair of the Technical Committee on Satellite Communications in IEICE. He is a senior member of IEICE and a member of IEEE.