IEICE TRANS. COMMUN., VOL.E95–B, NO.9 SEPTEMBER 2012

2991

LETTER

Numerical Analysis of Monopole Multi-Sector Antenna with
Dielectric Cylinder
Yuto SUZUKI†a) , Student Member and Naoki HONMA† , Member

SUMMARY
This letter proposes a monopole multi-sector antenna with
dielectric cylinder, and shows some results of simulations that examined theantenna characteristics. The dependency of radiation characteristics on relative permittivity εr shows the lens eﬀect with increase of εr . Furthermore,
the characteristics of the proposed antenna are improved by optimizing thetermination conditions at the quiescent antennas. The backlobe level islower than −10 dB. Also, the vertical HPBW and the conical HPBW arearound 70.5˚ and 63.4˚, respectively. The optimization improved the actual
gain by 2 dB. It is found that the diameter of the proposed antenna is 1/3rdthat of the conventional one.
key words: millimeter-wave band, lens antenna, multi-sector antenna, quiescent elements

1.

Introduction

Recently, the data transmission rate is increasing in communication systems. The conventional communication system has limited access to the frequency bands. Millimeter wave (mm-wave) bands are many potentially available
bands, such as 57 ∼ 66 GHz for transmission system [1].
Therefore, it is expected that mm-wave systems can realize very high transmission rates. Also, RF-systems can beeasily miniaturized because the wavelength of mm-wave isshort. Hence, System-On-Package (SOP) which is the integration of antenna and RF-system is proposed [2]. For suchRF-systems, constructing the antenna within the dielectric
substrate is useful because it can be easily configured with ahigh manufacturing accuracy.
However, high path loss at mm-wave is a serious problem since it degraedes the communication quality. Therefore, a high-gain antenna is required to overcome this problem. A beam switchable antenna can be one of the solutions
for achieving high gain to any direction and has been studiedin many works [3]–[7]. But these antennas cannot be integrated with RF-system because their diameter is too large.
In this letter, a monopole multi-sector antenna with dielectric cylinder is proposed. The proposed antenna is constructed by several monopole elements, ground plane, anddielectric cylinder. The dependency of radiation characteristics on relative permittivity εr are analyzed to show the lens
eﬀect with increase of εr . Also, the eﬀect of wavelength
shortening in dielectric cylinder on the size of antenna isstudied. Furthermore, the characteristics of the proposed anManuscript received January 27, 2012.
Manuscript revised May 8, 2012.
†
The authors are with the Graduate School of Engineering,
Iwate University, Morioka-shi, 020-8551 Japan.
a) E-mail: t2311036@iwate-u.ac.jp
DOI: 10.1587/transcom.E95.B.2991

tenna are improved by optimizing the termination conditions
at the quiescent antennas. It is clarified that the proposed antenna is eﬀective in achieving switched-beam antenna thathave compact configuration. In this study, Finite-Diﬀerence
Time-Domain (FDTD) analysis is used to take account of
the three dimensional dielectric components.
2.

Proposed Antenna

Figure 1 shows a monopole multi-sector antenna with dielectric cylinder. The dielectric cylinder is arranged on
the ground plane. Six monopole elements are uniformly
arranged along the cylinder sidewall. Six-sector is assumed considering the simplification of the RF-circuit. The
monopoles can be configured by using through holes in thesubstrate. Dielectric has the two eﬀects. One is able to focus radio wave such as lens, and the other is that shortening
wavelength. These eﬀects achieve high-gain and miniaturized antenna. The directivity can be controlled by switching
the feed port.
The antenna elements except the feed antenna are unconnected to the feed line. Therefore, these elements canbe regarded as the quiescent antennas. In this study, thequiescent antennas are used to improve the radiation characteristics. The reactive loads are connected to the quiescent antenna ports. The current on the quiescent antennas ischanged by the reactive loads. Hence, the directivity of the
proposed antenna is changed. Also, the reactive loads at thequiescent antenna ports are switched when the feed port is
switched. Therefore, the similar directivity can be obtained
when each antenna element is fed.
ESPAR antenna, whose feed element is at the center

Fig. 1 Structure of monopole multi-sector antenna with dielectric
cylinder.

c 2012 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. COMMUN., VOL.E95–B, NO.9 SEPTEMBER 20122992

of the circular parasitic antennas, has similar configuration,
but its maximum actual gain is lowered to 4.26 dBi when it
is configured in the dielectric material [8]. The feed elementin the proposed antenna is arranged along the cylinder sidewall. The dielectric cylinder and other quiescent elements
work together like the director in Yagi-Uda antenna. By thiseﬀect, the high actual gain can be achieved even when theantenna is configured within the substrate.
3.

Optimization Method

In the following optimization method, the iterative verification of the directivity is required, but it will cost significant
CPU time for the total process. In this study, the directivity
is calculated by the directivity synthesis method [9]. Thus,
the directivities with certain loads are estimated by post processing with very small computation time.
The feed antenna is connected to a 50 Ω feed line.
The quiescent antennas are connected to the reactive loads,
X = (X2 , X3 , · · · , X6 ). The quiescent antennas at the symmetrical positions are connected to same reactive loads, i.e.,
X2 = X6 and X3 = X5 . The reason includes the realizationof symmetrical beam on the azimuth plane, and easy fabrication of the RF-circuit by reducing the number of the reactive loads. The reactive loads are optimized evaluating theradiation characteristics of the proposed antenna. The radiation characteristics are evaluated using the steepest gradientmethod [10]. Now define F1k as the evaluation function of
the 1st antenna port at k step. The gradient can be expressedas⎛ k⎞∂F k ⎟⎟⎜⎜ ∂F ∂F k
gk1 = ⎜⎜⎝ 1 , i , · · · , i ⎟⎟⎠ ,(1)∂X2 ∂X3∂X6
where ∂Xi is the minor change of the reactance. Next, define
X k and X k+1 as the reactances at k and k + 1 steps, respectively. The reactances for step k + 1 can be expressed as
X k+1 = X k + αgk1 , where α is the constant to climb the gradient. Thus, the gradient is considered to next step reactances.
Therefore, reactances will approach to an optimal value stepby step.
By using the steepest gradient method, it tends to be
optimized to the specific parameter by the setting of the evaluation function. For example, the antenna can be mainly optimized for a certain property, such as gain, sidelobes, and soon, and it ends up with the antenna having unwanted characteristics. One of the solutions for this is setting the physically feasible goal for the optimization process. To design
the sector antenna, the beam shape of the mainlobe is important. In this study, the reference HPBWs (Half-PowerBeam Widths) are defined in advance. The reference gain is
calculated by
G0 =

K
,
θ0 φ0

φ0 is the reference HPBW on the conical plane [11]. The
conical plane is defined in the plane with the mainlobe andthat θ is the constant [7]. In this simulation, the parameters
are set to K = 3.4 × 104 , θ0 = 70˚, and φ0 = 60˚ considering
circular aperture and six-sector.
The evaluation function is calculated by the gain, theconical HPBW, and the F/B (front-to-back ratio). The gainof the antenna is the most important factor in the evaluation
function since the path loss in the mm-wave bands is veryhigh. The term for gain is expressed as
fg = −|Gt − G0 |,(3)

where Gt is the gain when X is given to the quiescent antenna ports. Next, in order to consider the sector beamshape, the conical HPBW is the necessary factor in the evaluation function. The term for HPBW is expressed as
fφ = −|φt − φ0 |,(4)

where φt is the HPBW on the conical plane when changed
the reactive loads. Thus, fg and fφ become maximum if Gt
and φt approach to G0 and φ0 , respectively. Here, φ0 is 60˚
since this is the six sector antenna. The F/B is also taken into
account in the evaluation function because the suppressionof the undesired radiation can yields high-gain. The term for
F/B is expressed as fFB , which is set to − F/B. The evaluation
function is expressed as
F = wg fg + wφ fφ + wFB fFB ,where wg , wφ , and wFB are the weights to converge to thedesired characteristics, and are set to be independent of thesteps and the initial reactances. The absolute values of theterms for gain and F/B are multiplied by wg and wFB , respectively. wφ is needed to be larger value than other values because the conical HPBW has small change against thechange of reactance.
4.

Numerical Analysis

Figure 2 shows the analysis model. Six monopole elements
are uniformly arranged along the cylinder sidewall. #1 isthe feed antenna, and the others are the quiescent antennas.
It is assumed that all elements are connected to a 50 Ω feed(2)

where K is the constant which determine by the apertureshape and the distribution of electromagnetic on the apertureplane, θ0 is the reference HPBW on the vertical plane, and(5)Fig. 2Analysis model.

LETTER

2993
Table 1Analysis conditions.

Center frequency
Analysis frequency
Number fo time step
Feed model
Exciting pulse
Absorbing B.C.

Fig. 360 GHz57 ∼ 63 GHz
32768
Delta-gap feed
Gaussian pulse
PML 8-layers

Antenna length versus εr .

line before the optimization, i.e., Zi = 50 Ω. hd and rd are
the height and the radius of the dielectric cylinder, respectively. la and ra are the length and the radius of the antenna
element, respectively. d is the distance between the edge ofthe ground plane and the dielectric cylinder. rg is the radiusof the ground plane. For simplification, we assumed hd = laand neglected the thickness of the ground plane. The dimensions are set to rd = 0.25λ0 , ra = 0.02λ0 , d = 0.25λ0 ,and rg = rd + d, where λ0 is the wavelength in vacuum. Details of the FDTD analysis configurations are summarized inTable 1. The shape of the cell is the cubic.
Figure 3 shows the antenna lengths for the relative permittivity εr of the dielectric cylinder. εr is set to 3.0 ≤εr ≤ 7.0. λg is the eﬀective wavelength, calculated as√
λg = λ0 / εr . Dot-line is the antenna length obtainedby FDTD analysis considering the matching. The antenna
length obtained by FDTD analysis is longer than 0.25λg .
It means that the finite dielectric, the finite ground plane,
and the quiescent antennas have an impact on the eﬀective
wavelength. In the following numerical analysis, the antenna length obtained by FDTD analysis is used.
Figure 4 show the radiation patterns at 60 GHz for various relative permittivities. Where, the electric field, |Eθ |, isthe normalized by the maximum value. From the result in
Fig. 4(a), it can be confirmed that the mainlobe is tilted in
the horizontal direction. Also, from the result in Fig. 4(b),the narrow beam can be formed. Furthermore, the F/B increases when εr becomes large, but doesn’t become higher
than 10 dB.
Figure 5 shows the actual gain versus the frequency forvarious relative permittivities. It can be confirmed that theactual gain is increased by increase of εr . However, the actual gain is not changed by increasing more than εr = 5.0.
The result in Fig. 3 means that the lens eﬀect of the dielectric
cannot be eﬀective in the gain because the antenna structure
becomes smaller.
Based on the results shown above, the dielectrics with
εr = 5.0 is used in the following discussion. The termi-

Fig. 4

Radiation patterns at 60 GHz for various εr .

Fig. 5

Actual gain versus frequency for various εr .

nations of the quiescent antenna ports must be optimized
to obtain more desirable characteristics. The reactive loads
are connected to the quiescent antenna ports, i.e., Zi = jXi .
1000 trials with diﬀerent initial reactance sets are examined
to avoid the local maximum. These sets are generated randomly with the range, −50 ≤ Xi ≤ 50 Ω. For example,
the reactive loads can be constructed by the stubs on the dielectric substrate. The actual reactance contains the error
because of the manufacturing error. The reactance error becomes higher when the absolute value of reactance becomeslager. The reactances between −50 and +50 Ω have a relatively small eﬀect on the reactance error. Therefore, theranges of the reactive loads are selected from −50 to +50 Ω.
α is set to 100. Number of iterative computation is 100. The
weights are set to wg = 100, wφ = 1000, and wFB = 10.
Figure 6 show the radiation patterns at 60 GHz. It canbe seen that the backlobe level is lower than −10 dB by optimizing the reactive loads at the quiescent antenna ports.
Also, the vertical and conical HPBWs are much wider to
70.5˚ and 63.4˚, respectively. Furthermore, The F/B becomes higher than 11 dB.

IEICE TRANS. COMMUN., VOL.E95–B, NO.9 SEPTEMBER 2012

2994

tric cylinder has been proposed. The radiation characteristics for relative permittivity εr of the dielectric cylinder
were analyzed. It showed the lens eﬀect with increase ofεr . Furthermore, by optimizing the termination conditions
at the quiescent antennas, the backlobe level is lower than
−10 dB at 60 GHz. Also, the vertical and conical HPBWs
are around 70.5˚ and 63.4˚, respectively. The optimization
improved the actual gain by 2 dB. It is found that the diameter of the proposed antenna is 1/3rd that of the conventional
one. These results proved that the proposed antenna is eﬀective in achieving switched-beam antenna that have compact
configuration.
References

Fig. 6

Fig. 7

Radiation patterns at 60 GHz.

Actual gain and |S 11 | versus frequency.

Figure 7 shows the actual gain and |S 11 | versus frequency. It can be seen that the feed port is matched enoughto 50 Ω feed line even when the quiescent antenna ports are
terminated by the reactive loads. It is found that the actual
gain of the optimized antenna is 6.54 dBi at 60 GHz that is
2 dB higher than the gain without the optimization, and it
outperforms the gain of the work [8].
The proposed antenna size is compared to the conventional one [7]. It is found that the diameter of the proposed
antenna is 1/3rd that of the conventional one. Therefore, theproposed antenna is eﬀective in downsizing the switchedbeam antenna.
5.

Conclusion

In this letter, a monopole multi-sector antenna with dielec-[1] S.K. Yong and C.-C. Chong, “An overview of multigigabit wirelessthrough millimeter wave technology: Potentials and technical challenges,” EURASIP J. Wireless Communications and Networking,vol.2007.
[2] K. Lim, S. Pinel, M. Davis, A. Sutono, C.-H. Lee, D. Heo, A.
Obatoynbo, J. Laskar, E.M. Tantzeris, and R. Tummala, “RFsystem-on-package (SOP) for wireless communications,” IEEE Microw. Mag., vol.3, no.1, pp.88–99, March 2002.
[3] T. Maruyama, K. Uehara, T. Hori, and K. Kagoshima, “Rigorous analysis of transient radiation mechanism of small multi-sector
monopole Yagi-Uda array antenna using FDTD method,” Int. J. Numerical Modeling, vol.12, no.4, pp.341–351, 1999.
[4] T. Seki and T. Hori, “Cylindrical multi-sector antenna with selfselecting switching circuit,” IEICE Trans. Commun., vol.E84-B,
no.9, pp.2407–2412, Sept. 2001.
[5] M. Yamamoto, K. Ishizaki, M. Muramoto, K. Sasaki, and K. Itoh, “Aplanar-type five-sector antenna with printed slot array for millimeterwave and microwave wireless LANs,” IEICE 2000 Int. Symp. Antenna and Propagation, vol.1, pp.206–208, Aug. 2000.
[6] N. Tenno, A. Miura, T. Itoh, M. Taromaru, and T. Ohira, “A fundamental study on a switched-beam sector slot-array antenna in
60 GHz band,” Int. Symp. on Antennas and Propagat., Singapore,Nov. 2006.
[7] N. Honma, T. Seki, K. Nishikawa, K. Tsunekawa, and K. Sawaya,“Compact six-sector antenna employing three intersecting dualbeam microstrip Yagi-Uda arrays with common director,” IEEE
Trans. Antennas Propag., vol.54, no.11, pp.3055–3062, Nov. 2006.
[8] Q. Han, B. Hanna, and T. Ohira, “A compact espar antenna with planar parasitic elements on a dielectric cylinder,” IEICE Trans. Commun., vol.E88-B, no.6, pp.2284–2290, June 2005.
[9] Y. Suzuki, M. Obara, and N. Honma, “Radiation pattern optimization using quiescent elements in circular array antenna with cylindrical dielectric load,” Int. Symp. on Antennas and Propagat., Jeju,
Oct. 2011.
[10] J.W. Bandler, “Optimization methods for computer-aided design,”
IEEE Trans. Microw. Theory Tech., vol.MTT-17, no.8, pp.533–552,
Aug. 1969.
[11] J.D. Kraus, Antennas, 2nd ed. McGraw-Hill Electrical Engineering
Series, New York, 1988.