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Miniaturized printed monopole antenna with applying the modified conductor-backed plane and three embedded strips based on CPW for multi-band telecommunication devices

Published online by Cambridge University Press:  10 December 2014

Mohammad Alibakhshi-Kenari
Mohammad Alibakhshi-Kenari*
Affiliation:
School of Electrical and Communication Engineering, Shahid Bahonar University, Kerman, Iran
*
Corresponding author: M. Alibakhshi-Kenari Email: Naeem.alibakhshi@yahoo.com, makenari@mtu.edu
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Abstract

In this article, a new construction of a small planar dual-band fed printed monopole antenna based on coplanar waveguide is suggested. Impedance matching for dual-band operations is obtained by embedding three vertical strips with different sizes in the U-shaped conductor-backed plane. The main problem of the designed antenna is the measuring of the specifications with the Agilent 8722ES Vector Network Analyzer, when the coaxial cable is connected to the antenna. Hence, in this paper a new method for decoupling the cable from the antenna is presented. This method is based on using the ferrite bead. The ferrite bead reduces the cable radiation, so that its position plays the important part in the antenna radiation characteristics. The fabricated antenna includes the benefits of the miniaturized size and dual-band operating specifications, so that the mentioned properties have been achieved without modifying the coplanar-waveguide-ground surface or radiator patch. The antenna has the small size of 15 × 15 × 0.8 mm3and bandwidths with S11 < −10 dB about 2.2 GHz (5.05–7.25 GHz) for WLAN-band or IEEE 802.11a-band and 5.2GHz (7.6–12.8 GHz) for X-band, which correspond to 36 and 51% practical bandwidths, respectively. The antenna measured peak gains are about 1.8 dBi at WLAN-band and 4.3 dBi at X-band.

Keywords

Dual-band fed printed monopole antennaIEEE 802.11a bandWLAN-bandX-bandThree embedded stripsCoplanar waveguideU-formed conductor-backed plane (CBP)Multi-band telecommunication systemsMobile devicesAgilent 8722ES Vector Network Analyzer

Information

Type
Online Only Papers
Information
International Journal of Microwave and Wireless Technologies , Volume 8 , Issue 8 , December 2016 , e2
Copyright
Copyright © Cambridge University Press and the European Microwave Association 2014 

I. INTRODUCTION

In recent years, use of the high data rate telecommunications have been increased rapidly making considerable demands on bandwidth and interoperability within the physical layer, which have been resulted in wireless devices expected to function under different multi-band performances [Reference Wu, Kang, Chen and Tarng1–Reference Yuan, Yin, Ding, Li and Xie3]. An exclusively impellent issue for these applications is the design and development of the multi-band antennas which are simple structures, and possess the multi-band functionalities. Furthermore miniaturization is particularly important for the antenna designer and arises out of the limited available volume of the wireless handset casing [Reference Alibakhshi Kenari4–Reference Alibakhshi Kenari5]. The printed monopole antennas represent the principal parameters in designing the multi-frequency antennas such as easy to fabrication, small size, low cost, and consistent radiation patterns throughout the designated operational bands [Reference Liu, Wu and Dai6–Reference Xu, Yan and Wang8].

Several designs in the papers concerning the printed monopole antenna with multi-band characteristics and large size or even small size have been reported recently [Reference Yoon and Lee9–Reference Singh and Mulgi12]. Those designs have been used the different kinds of slots, and parasitic elements in the radiator, the ground plane or even in the feeder for exciting the dual or multi-frequency modes and agreeable radiation characteristics [Reference Liao, Yang, Han and Li13–Reference Malekpoor and Jam15]. The disadvantages of these antennas contain the non-effective broadband matching, unacceptable gain and complexity to realize the required operating frequency bands.

In this paper, a novel-fed printed monopole (FPM) antenna based on coplanar waveguide (CPW) is proposed to cover the following operational bands: 5 GHz WLAN (5150–5350/5725–5825 MHz specified by IEEE 802.11a) and 8 GHz X-band (8–12 GHz). In here, the target is to propose a simple structure of patch and ground plane with a step-by-step design process to access the dual-band applications. By etching three strips on initial U-shaped conductor-backed plane (CBP), which is embedded on the other side of substrate, a dual-band property is created for the desired antenna. The impedance matching of the antenna has been more enhanced with regulating the dimensions of the inserted strips on CBP. The broader bandwidth is obtained on both frequency bands by applying the final structure of CBP, so that it prepares the wide utilizable fractional bandwidths of 36% for band 1 and 51% for band 2. Prototypes of the antennas have been validated by high-frequency structure simulator (HFSS) and after the optimization processes they are constructed.

II. STEP BY STEP DESIGN PROCEDURES OF THE DESIRED DUAL BAND FED PRINTED MONOPOLE (DBFPM) ANTENNA

Layout of the desired antenna structure with W sub × L sub dimension is indicated in Fig. 1.

Fig. 1.

Layout of the desired antenna (dimensions in terms of mm).

The suggested antenna with miniaturized dimension of 15 × 15 mm2 has been designed, tooled, and constructed on the FR4 substrate with thickness of 0.8 mm, loss tangent of 0.022 and relative dielectric constant of ετ = 4.4. The antenna is fed by a 50 Ω CPW. The main radiator in form of the square shaped patch with the dimension of 7.5 × 7.5 mm2 along with a partial CPW ground plane have been made up on the top layer of substrate. The bottom layer includes the modified U-shaped CBP. For the impedance matching, the distance between the radiator and the CPW ground plane is indicated by a gap, which provides the competent control between the lower edge of patch and the CPW ground plane. The optimized gap between the patch and the CPW ground plane is 2.2 mm. To improve the desired antenna performances into two operating bands of WLAN (5.15–5.825 GHz) and X (8–12 GHz), the CBP is modeled and designed in the manner shown in Fig. 1. According to this figure, the conductor-back plane (CBP) is modeled and designed in two steps:

  1. Step (1) Build a simple U-shaped CBP (as shown in Fig. 2(a)).

  2. Step (2) Insert three strips to step. 1 (so that is shown in Fig. 2(b)).

Fig. 2.

Layouts of the CBPs.

The proposed structures based on CBPs are shown in Fig. 2. The U-shaped CBP and the modified U-shaped CBP along with three embedded strips are responsible for making the first band centered at 5.5 GHz and the second band centered at 10 GHz, respectively. The CBP dimensions are regulated and optimized by parametric study to achieve the enhanced impedance matching over the frequency bands. The optimization of the structure is obtained using the HFSS. The optimal parameters of the realized antenna are organized in Table 1.

Table 1.

Optimal parameters of the DBFPM antenna.

The other optimized dimensions of the antenna are indicated in Fig. 1.

III. DBFPM ANTENNA DATA

The simulation procedures have been accomplished by applying the electromagnetic software Ansoft HFSS. The antennas were tooled, fabricated and tested at the facility and workshop. The impedance bandwidth of the constructed antenna with proposed novel CBP structure was measured by using an Agilent 8722ES Vector Network Analyzer (VNA). The main problem of the designed antenna is the measuring of its specifications with the VNA, when the coaxial cable is connected to the antenna. If the measurement cable does not decoupled properly, it becomes part of the radiating structure, and thus, changes the input impedance of the antenna. Moreover, the cable distorts the far-field radiation pattern of the antenna by reradiating part of the signal that leaks into the cable. Hence, in this paper a new method for decoupling the cable from the antenna is presented. This method is based on using the ferrite bead.

A common method for minimizing of the cable radiation is employing the ferrite bead on the coaxial cable as close as possible to the feed point of the antenna.

In investigation, a simple monopole element is located at end of the coaxial feed with a ferrite bead positioned on the feed cable. The bead provides a discontinuity in the propagation coefficient for current on the U-shaped CBP, which results in the significant reflection. In the absence of the ferrite bead, the radiation from the cable is directed to below of the U-shaped CBP. When the ferrite bead is positioned on the coaxial cable, the currents in the U-shaped CBP of the coaxial cable are confined to the part above of top of the ferrite bead. It should be noted that, maximum of the radiation efficiency occurs when the bead is λ/4 from the U-shaped CBP. Clearly, the quite minor changes in the position of the ferrite bead can dramatically effect on the radiation efficiency of the antenna. The ferrite bead appears to be a much simpler method of construction for this type of antenna.

To reduction the effect of the cable on the antenna, high-impedance ferrite bead is utilized along the cable near to its connection to absorb the induced power. The measured input impedance curves of the antenna at the operating bands are plotted in Fig. 3.

Fig. 3.

Curves of the measured input impedance versus frequency.

In the proposed antenna with introducing the initial U-formed CBP in the manner indicated in Fig. 2(a), the first operating band can be obtained as is shown in Fig. 4. Also, by etching three embedded rectangular strips on the CBP in the manner shown in Fig. 2(b), the second operational band can be achieved as is illustrated in Fig. 4.

Fig. 4.

Measured and simulated S 11 of the proposed antenna with dual-band operations.

This figure depicts that there is a good agreement between the simulated and measured results. It is noticed from Fig. 4 that, by embedding the strips in the manner shown in Fig. 2(b), a suitable control on the dual-band operations can be obtained. The effects of the structural element of strips (L S 1) on the antenna's performances are investigated in the Fig. 5. It can be observed that using the structure of Fig. 2(b) and choosing a proper length for the strips (Ls) the antenna's performances have been significantly improved at upper band and the impedance bandwidth of lower band has been enhanced.

Fig. 5.

Simulated S 11 for different values of L S 1 when W S 1 is 0.5 mm.

The simulated and measured peak gain from 4.5 to 6.5 GHz and 8 to 12 GHz for W-LAN and X bands are plot in Fig. 6. In this figure, the antenna gain is proliferated dramatically from about 2 to 4.2 dBi for X-band, whereas the WLAN-band has indicated a gain variation of less than 1 dB between 1 and 1.9 dBi.

Fig. 6.

Simulated and measured peak gain for the proposed antenna with dual-band operations.

The measured radiation patterns for four different spot frequencies of 5.2 and 5.8 GHz for WLAN-band and 9 and 11 GHz for X-band in H-plane and E-plane are shown in Fig. 7.

Fig. 7.

Radiation patterns of the proposed antenna at the different operational frequencies of 5.2/5.8 and 9/11 GHz for WLAN and X bands, respectively.

Obviously it is understand from Fig. 7 that, this figure approximately exhibits the omni-directional radiation patterns in H-plane and the dipole-like radiation patterns in the E-plane. From an overall view of these radiation patterns, it can know the proposed antenna behaves quite similar to the typical printed monopoles in the operating bands.

To validate of the design processes, the proposed antennas have been compared with conventional antennas and their characteristics are summarized in Table 2.

Table 2.

Characteristics of the antennas.

IV. CONCLUSION

This article describes a novel prototype of a FPM antenna based on CPW that depicts the dual-band operations of the WLAN-band and X-band. The proposed antenna consists of a simple rectangular patch as the main radiator, the modified U-formed conductor back plane (CBP), and the partial rectangular CPW-ground surface. As well as in this structure has been used of the ferrite bead on the coaxial cable for minimizing of the cable radiation. So that the ferrite bead on the U-shaped CBP of a coaxial cable reflect the currents which flow on the U-shaped CBP. The position of the ferrite bead plays the important part in the radiation characteristics of the antenna.

Also in this article, the dual-band performances are obtained by embedding and adjusting of dimensions of three strips on U-formed CBP. The experimental results indicate that the fabricated antenna includes the advantages such as the miniaturized dimensions and the good radiation characteristics along with the reasonable measured gains. As a result, the designed antenna can be a good candidate for use in the various applications of the future multi-band telecommunication systems and the mobile devices.

ACKNOWLEDGEMENTS

The designer would like to express his sincere thanks to the Iran Telecommunication Research Center (ITRC) with grant number of 6987/500/T, the microwave and millimeter wave laboratory of the Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran, and the antenna laboratory of the K. N. Toosi University of Technology, Tehran, Iran.

Mohammad Alibakhshi-kenari was born in 24 February 1988 in Iran, Mazandaran State, Feredounkenar City. He received his B.S. and M.S. degrees in the Electrical Engineering, field of Telecommunication from the Islamic Azad University, Najafabad Branch of Esfahan, Iran on February 2010 and the Islamic Republic of Iran, Shahid Bahonar University of Kerman on February 2013, respectively. His main areas of interest in research are microwave and millimeter wave circuits, radars, antennas and waves propagation, composite right/left-handed transmission lines (CRLH-TLs), metamaterial (MTM) applications, fractals, integrated RF technologies, embedded systems, electromagnetic waves applications, and wireless telecommunication systems. He is now “Editor-in-Chief” in Journal Club for Electronic and Communication Engineering (JCECE) and also works as a reviewer in the several ISI journals such as Elsevier, Taylor and Francis, Wiley, ACES, etc. M.A-kenari has served as a Member of the Technical Program Committee (M-TPC) of some of the international conferences, such as APACE 2014, APPEIC 2014, MobiWIS 2014, ICEPIT 2014, CICN 2014, ADVCIT 2014, DPNoC 2015, MobiApps 2015, etc. So far, he has been published several papers. Also, the Master Thesis of M.A-kenari entitled “Designing and Fabricating the Ultra Compact and UWB Antennas based on Metamaterial Transmission Lines with Application in Wireless Radio Transceivers” was approved and granted by Iran Telecommunication Research Center (ITRC) on December 2012 with grant number of 6987/500/T.

References

REFERENCES

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Figure 0

Fig. 1. Layout of the desired antenna (dimensions in terms of mm).

Figure 1

Fig. 2. Layouts of the CBPs.

Figure 2

Table 1. Optimal parameters of the DBFPM antenna.

Figure 3

Fig. 3. Curves of the measured input impedance versus frequency.

Figure 4

Fig. 4. Measured and simulated S11 of the proposed antenna with dual-band operations.

Figure 5

Fig. 5. Simulated S11 for different values of LS1 when WS1 is 0.5 mm.

Figure 6

Fig. 6. Simulated and measured peak gain for the proposed antenna with dual-band operations.

Figure 7

Fig. 7. Radiation patterns of the proposed antenna at the different operational frequencies of 5.2/5.8 and 9/11 GHz for WLAN and X bands, respectively.

Figure 8

Table 2. Characteristics of the antennas.