Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Xplore Compliant ©2010 IEEE

Simulation on the 4 x 5 Antenna Arrays at 450 MHz

Azhari Asrokin, Anas Abas, Rizal Helmy Basri Access Network Technology

Telekom Research & Development Sdn Bhd Cyberjaya, Selangor, Malaysia

azhari.asrokinanashelmy@tmrnd.com.my

Norman Jamlus Product Design & Development

Telekom Research & Development Sdn Bhd Cyberjaya, Selangor, Malaysia

norman@tmrnd.com.my

Abstract—This paper discussed the simulation and analysis for developing array antenna operating from 450 MHz to 470 MHz using bended dipole antenna developed in Telekom Research & Development, Cyberjaya. The simulation design includes power divider design determination of cable’s length and the effect of tapering amplitude of the array antenna to the overall radiation pattern. At the end, results will be analyzed and discussed for further future implementation of realizing the antenna.

Keywords-dipole; CDMA; array; repeater; PCB antenna

I. INTRODUCTION Coded Division Multiple Access (CDMA) system is quite

an old system coming from USA in late 1980s [1]. CDMA have gone through improvements throughout the years and currently in Malaysia, Telekom Malaysia (TM) is using the CDMA2000 1x system along with CDMA2000 EV-DO [1]. The CDMA network in Malaysia which is provided by Telekom Malaysia is targeted to serve customers in rural areas [2]. To reduce the cost of building up new base station and the base station operation while increasing the profit by providing better coverage service to the users, the use of CDMA repeaters is needed [3].

To support TM business operation, Access Network Technology Program in TM R&D had designed antennas for CDMA system starting with Yagi-Uda antennas and currently with antenna for repeaters application. For a repeater system, 2 types of antennas needed which is the donor antenna and the subscriber or serving antenna [4]. The donor antenna should have a high gain so that radio frequency (RF) signal from the closest base station can be received by the repeater. The subscriber antenna should have foot print pattern depending on the needs of the area either it should be small or big (in terms of beamwidth). The use of dipole PCB antenna array design is investigated to achieve a high gain antenna for CDMA 450 MHz.

The material for the antenna is printed circuit board (PCB) using Flame Retardant-4 (FR-4) as the substrate with thickness of 1.6 mm, FR-4 permittivity (εr) 4.7, tangent loss (tan δ) 0.019 and copper thickness 0.035 mm. The metal plate use as the base plate or grounding is made of aluminium with thickness of 2 mm.

II. METHODOLOGY

A. Software The simulation was done in CST 2010 Studio Suite [5]

which comes with Microwave Studio to simulate 3 dimensional or planar designs and Design Studio to integrate all of the antenna parts for the complete result.

B. Single Element Design The antenna element design is based on half wavelength

dipole antenna [6]. The antenna is designed on FR-4 PCB board which the width, W of the antenna changes the input impedance of the antenna. The length, L is around 0.36λ. The L is much shorter than suggested in [6] is because the width of the dipole was increased to get the input port almost to 50Ω which sequentially reduce the dipole’s length to achieve resonance [6].

Figure 1. Dimension for bended dipole antenna

The motivation for designing a bended dipole is due to the physical restriction when installing the antenna to the base station tower which needs the antenna to be as small as possible to reduce wind loading, reduce scenery pollution and the most important of all to make it easy for installation. Details design of the bended dipole is discussed in [7].

C. Antenna Array In CST Microwave Studio, from a radiation pattern of a

single antenna, we can predict the radiation pattern of the array (currently limited to rectangular array) without simulating the whole array which will save a lot of simulation time. As shown in figure 2, to form a 4 x 5 array, number of elements in respective of the axis need to be defined at each axis, along with its distance between the elements. With this option too, the amplitude and phase for each antenna elements can be

This research and development project is sponsored by Telekom Malaysia Berhad.

L

H

W

≈λ/4

defined individually thus amplitude tapering or pattern synthesis can be implemented.

Figure 2. Farfield calculation of antenna arrays by using radiation pattern of

a single element antenna.

The distance between the elements in X axis is fixed to ≈ λ while in the Z axis is fixed to ≈ λ/2. The overall size for the base plate is ≈ 4λ x 3λ as illustrated in figure 3.

Figure 3. Dimension of the proposed 4 x 5 antenna arrays.

Two options on the amplitude values were selected to compare the radiation pattern of equal power distribution (option 1) against concentrated power distribution (option 2) as shown in table 1. The “Zn” ratio control the horizontal pattern while the “Xn” ratio control the vertical pattern as shown in figure 4.

TABLE I. POWER RATIO MATRIX

Options Control Horizontal Control Vertical

ZC Z1 Z2 XC X1 1 1 1 1 1 1 2 4 2 1 4 1

Ratio normalize to value 1

Figure 4. Feed network configuration of the 4 x 5 antenna arrays.

The results is then verified by designing the complete antenna arrays like in figure 3 and then the amplitude for each element is defined by using the “Simultaneous excitation” option in the Transient Solver of Microwave Studio as shown in figure 5.

Figure 5. Simultaneous excitation when simulating the whole antenna arrays

without the power dividers.

The next step is to design the power divider that will represent the tapered amplitude of the antenna arrays. The two simulations above only predict the possible radiation pattern of the antenna array without simulating the VSWR of the whole network, since the feed network is not available yet. To realize the whole network, the amplitude for each element need to be determined first before the power divider can be designed as mentioned earlier.

D. Power Divider Based on the best feed network configuration and the

amplitude of the antenna elements which is option 6 from table 1, 5 types of 2-way Wilkinson power dividers is designed based on the design rules in [8]. The whole feed network consists of a total 19 2-way Wilkinson power dividers as shown in figure 6. The lengths of cables are adjusted so that the phase delay towards each element should be almost the same and the phase delay should include the delay at the power divider.

5 Ways PD

INPUT

ZC Z1 Z2 Z1 Z2

Z2 Z2 Z1 ZC Z1

4 Ways PD

XC

X1 XC

X1 4 Ways PD

XC

X1 XC

X1 4 Ways PD

XC

X1 XC

X1 4 Ways PD

XC

X1 XC

X1 4 Ways PD

XC

X1 XC

X1

4λ 3λ

Z axis X axis

Figure 6. Power divider arrangements.

E. Design Integration Once the antenna and the power divider are complete, the

designs will be integrated in the CST Design Studio as shown in figure 7. With this integration, the performance of the antenna arrays can be collected. The integration is last step in the simulation process before the fabrication work can be done.

Figure 7. Design integration in Design Studio

III. RESULTS AND DISCUSSIONS

A. Single Antenna The VSWR for the bended dipole antenna is as shown in

figure 8. The antenna achieved VSWR below 2 from 450 to 470 MHz. From figure 9 and 10, the gain for a single element is 6 dB. The horizontal and vertical beamwidth is respectively at 76.6° and 88.5°. The front-to-back ratio is 16.8 dB.

Figure 8. VSWR for single element bended dipole antenna.

Figure 9. Horizontal radiation pattern for single element bended dipole

antenna.

Figure 10. Vertical radiation pattern for single element bended dipole

antenna.

B. Antenna Array The results using single radiation pattern to emulate an

array pattern is not shown since the results using simultaneous excitation is almost close to the final result.

Port

Legend:

Power Divider

The summary of the simulation results is as shown in table 2. The first column is the specification requirement requested by the customer. The following columns show the overall results of the simulations.

TABLE II. SIMULATION RESULTS SUMMARY

Technical Specification

Simulation (Simultaneous

excitation)

Simulation (Simultaneous

excitation)

Simulation (Design Studio)

Option 1 2 2

Frequency Range (MHz) 450 – 470 450 – 470 450 – 470 450 – 470

VSWR ≤ 1.5 NA NA ≤ 1.8 Gain 15 dB 19.1 16.6 18 dB

Polarization Vertical Vertical Vertical Vertical

Horizontal Beam width

30 ° 22.9° 30.1° 26 °

Vertical Beam width

18 ° 14.3° 18.4° 15.9 °

Front-to-back ratio > 33 dB 25.9 dB 26.2 dB 25 dB

Side Lobe Level ≥ 20 dB 14 dB 26.2 dB 25.4 dB

As shown in figure 11 and 12, by tapering the amplitude, the side lobe level can be reduced along with its front-to-back ratio in both axes, horizontal or vertical.

Figure 11. Horizontal radiation pattern using simulateneuos excitation.

Figure 12. Vertical radiation pattern using simulateneous excitation.

From the simulation using simultaneous, option 2 is selected because of its performance closer to the required specification and because of its low side lobe level and its high front-to-back ratio. The power dividers are then designed to follow the power distribution required from the previous simulation. After the design and integration is done, the simulation ran in Design Studio.

The VSWR achieve for the whole antenna system is as shown in figure 13. It shows that the antenna almost achieve required VSWR specification of below 1.5.

Figure 13. VSWR of the antenna array system.

Figure 14 and 15 shows the radiation pattern for the whole system. It can be observed that the horizontal radiation pattern is skewed to left. This is mainly because of the power divider’s ratio distribution for the horizontal plane is not fully balance and the designed power divider did not achieve the required power ratio.

Option 1 Option 2

Option 1 Option 2

Figure 14. Horizontal radiation pattern for the complete antenna array.

Figure 15. Vertical radiation pattern for the complete antenna array.

IV. CONCLUSION A bended dipole antenna and antenna array which operates

at 460MHz was designed and simulated. Through multiple simulation steps, the 4 x 5 antenna array achieves the desired requirement specification. The simulated gain of the antenna array shows a promising result for future fabrication.

This paper proves that by bending the dipole arm, it still can operate at the desired frequency. Further improvements are needed to further lower the VSWR and increase the antenna gain individually. The bended dipole antenna’s application can be extended to broad range of functions such as to design base station antennas, indoor antennas and repeater type of antenna.

Further improvements are needed for the array design especially on the power divider. Considerations are being made to reduce the overall size of the antenna since currently the antenna size is too big which leads to higher wind loading and difficulties in antenna installation and transportation. The number of power divider, antenna element and the feed network configuration are items that will be change to further improve the results as well as reducing the complexity during assembly which can contribute to abnormality in the performance such as shown in figure 14.

REFERENCES [1] http://www.cdg.org [2] http://www.tm.com.my [3] http://www.qualcomm.com/common/documents/white_papers/Network

PlanningwithRepeaters.pdf [4] Niemelä, J. , Lähdekorpi, P. , Borkowski, J. , Lempiäinen, J.,

“Assessment of Repeaters for WCDMA UL and DL Performance in Capacity-Limited Environment,” 14th IST Mobile & Wireless Communication Summit. Dresden, June 2005.

[5] http://www.cst.com [6] C. A. Balanis, “Antenna Theory: Analysis and Design,” 3rd Edition. New

Jersey, 2005. [7] A. Asrokin, A. Abas, R. H. Basri, N. Jamlus, “Design of Bended Dipole

PCB Antenna for CDMA 450,” World Engineering Congress 2010. Sarawak, August 2010.

[8] D. M. Pozar, “Microwave Engineering,” 2nd Edition. USA, 1998.

Option 2

Option 2