A New Dual Feed PIFA Diversity Antenna - IEEE Xplore

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Department of Electrical Engineering and Electronics. University of Liverpool, Liverpool, L69 3GJ, UK. S.Al-jaafreh@liverpool.ac.uk; [email protected]; Lei.
The 8th European Conference on Antennas and Propagation (EuCAP 2014)

A New Dual Feed PIFA Diversity Antenna Saqer S. Alja’afreh1, Yi Huang2, Lei Xing3 Department of Electrical Engineering and Electronics University of Liverpool, Liverpool, L69 3GJ, UK [email protected]; [email protected]; [email protected] Abstract— A new wideband, very low profile (3 mm), PIFA antenna is proposed. It utilizes two collinear feeds, ground plane slots and modified top radiating plate to reduce the mutual coupling between the feeds and improve the impedance matching. The proposed antenna has a 10-dB impedance bandwidth from 2.25 to 3.2 GHz or a fractional bandwidth of 35 %. It covers several important wireless applications such as 2.4 GHz WLAN, bluetooth, four LTE bands (bands 7, 38, 40 and 41) and S-DMB band 2.63-2.655 GHz. Both simulated and measured results are presented to quantify and validate the antenna performance. It is demonstrated that this new antenna is an excellent candidate for mobile diversity/MIMO applications Index Terms— Diversity antenna, multiple input multiple output (MIMO) antenna, wideband antenna, impedance matching and PIFA antenna.

I.

INTRODUCTION

Nowadays, there is continuous demand for a higher data rate and better reliability in wireless communication systems. Thus multiple input and multiple output (MIMO) technology [1] has become a core of many new wireless communication systems; this is related to its ability in increasing the channel capacity linearly with the number of antennas at both communication ends without using the conventional techniques like increasing the transmitted power or using additional spectrum [2]. Also with multiple antenna schemes, antenna diversity plays an important role in increasing the reliability of wireless systems. MIMO antenna design can be done using two approaches. The first one is called multiple element antennas (MEA), it has been shown that MEA arranged in pattern or polarization diversity configuration gain with sufficient antenna spacing can provide a large diversity [3]. However, recent trends in mobile market require small, slim handset terminals with multisystem use; these increase the demands for low profile, small size, and multiband/wideband antennas. The design of MIMO antenna system for small mobile terminal becomes challenging because of saving space means increasing in the level of mutual coupling that will degrade MIMO and diversity systems performance. In order to achieve the compactness of MIMO antenna system, a new design approach was introduced to save space in small devices; it is called isolated mode antenna technology (IMAT) [4] in which a single antenna element with more than one feed can be used as a compact MIMO antenna. However, the compactness can be achieved

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with the expense of a large mutual coupling level that requires isolation techniques. The isolation between MIMO antenna elements or ports is a critical design parameter. Strongly coupled antennas have very poor radiation efficiencies and MIMO system performance because each antenna element or port will act as a load for the others [5]. At present, several decoupling techniques have already been proposed in literature to reduce antenna mutual coupling in small portable devices: changing the spacing and orientation of antennas is one of the earliest techniques used for decreasing mutual coupling level [5]. Matching networks have been used as decoupling networks; they applied for several antenna structures like monopoles [6] and planar inverted-F antennas (PIFA) [9]. Furthermore, decoupling current path known as neutralization line has been proposed between PIFA antennas [10-11]. Also, defect ground plane structure [12] is one of popular isolation techniques, and it has been widely used for closely spaced and coupled antennas. Among various antenna structures like monopoles, patches, and slots: the PIFA is the most widely used in portable handheld wireless devices due to its excellent features (low profile, ease to design, low cost and reliable performance) and has received a lot of attentions in designing MIMO antenna. Recently, few designs for compact dual-feed diversity PIFA antenna are proposed for MIMO applications [13-15]. Although these published designs are novel, they have several drawbacks such as the ground plane between the feeds of PIFA is totally removed to provide the required isolation level, this adds a difficulty in integrating the upper feed to other equipment on PCB (RF module, transceivers chips, speaker) via the feeding line. Other drawbacks are related to the high antenna profile and narrow bandwidth. In this paper, a new and very low profile dual feed PIFA antenna is presented. Unlike the antennas in [13-15], different feeding structures and isolation techniques are employed. The proposed antenna is designed, optimized, fabricated and tested, and very good results are obtained.

II.

ANTENNA CONFIGURATION

A dual collinear-feed PIFA design is derived from the reference antenna in [14], the geometry of the proposed antenna is shown in Fig. 1. The top radiating plate with dimensions W × L is placed at height h over the PCB grounded substrate with dimensions Lg × Wg. In order to improve impedance matching for feed 2, the top plate is

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The 8th European Conference on Antennas and Propagation (EuCAP 2014)

modified by creating a small strip with width Wt , length Lt, xposition Xt and y-position Yt. The ground plane has a full FR-4 substrate with thickness t = 1.5 mm and permittivity r = 4.4. The radiating plate has a three legs two of them are the feeds, the dimensions of the Feed1 and Feed2 plates are Wf1 × h and Wf2 ×h, respectively. The third leg is the shorted pin with dimensions Ws × (h + t). The horizontal distance of shorted pin from ground plane side edge is Xsh and the vertical distance from top edge is Ysh, while the horizontal distances of Feed 1 and Feed 2 from side edge are dxf1 and dxf2, respectively. Both feeds have no vertical distances from top edge of the ground plane; they are located collinearly along the top edge of the ground plane for the purpose of strong capacitive coupling with a low Q chassis mode.

3 mm, Xt = 18 mm, Xs1 = 18 mm, Ws1 = 5.5 mm, Ls1 = 19 mm, Ws2 = 7 mm, Ls2 = 13.5 mm, Ys2 = 12 mm, Ls3 = 18 mm, Ws3 = 6 mm and Ys3 = 13 mm. The fabricated prototype of the proposed antenna is shown in Fig. 2. It can be seen from Fig. 3 that simulated and measured results of S-parameters are in a good agreement. The resulted bandwidth with Sii < -10 dB and Sij around -14 dB is from 2.25 to 3.2 GHz, it covers the following wireless applications; 2.4 GHz WLAN, three LTE bands (band 7, 38, 40 and 41) and S-DMB band 2.63-2.655 GHz.

(a)

Fig. 2. Prototype of proposed antenna (a) Top view and (b) Back view

(b) 0

Fig. 1. Geometry of proposed antenna (a) Antenna strucure and (b) modified ground plane. Reflection coeffcients (dB)

Fig. 1(b) represents the modified ground plane. Two kinds of slots are created, slot1 (L-shaped slot) represents bandstop resonator that provides isolation between antenna feeds, while slot 2 is created to enhance the radiation of Feed 1. The geometry and parameters of these slots are also shown in Fig. 1(b).

-5 -10 -15 -20 -25 -30 S11 S11 S22 S22

-35 -40

III.

-45 2

ANTENNA CHARACTRIZATION

The antenna is optimized using CST Microwave studio via a parametric approach. The optimized antenna has the following main dimensions: Lg = 100 mm, Wg = 40 mm, L = 20 mm, W = 40 mm, Xf1 = 7 mm, Xf2 = 38 mm, Wf1 = 5 mm, Wf2 = 2 mm, df = 26 mm, Xsh = 40 mm, Ysh = 16.5 mm, Wsh = 0.5 mm, h =3 mm, t = 1.5 mm, Lt = 9 mm, Wt = 3.5 mm, Yt =

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2.5

3 Frequency (GHz)

(a)

3.5

measured simulated measured simulated 4

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

envelope correlation coefficient and pattern diversity between antennas feeds pattern.

0 S12 & S21 measured S12 & S21 simulated

S12 & S21 (dB)

-5

For diversity applications, two important parameters should be satisfied: the first one is the envelope correlation coefficient (ȡe) and the second one is mean effective gain ratio (K). The critical values for these two conditions are shown in (1):

-10 -15 -20

ρ e = 0.5 , K =

-25 -30 2

2.5

3 Frequency (GHz)

3.5

4

(b) Fig. 3. Simulated and measured S-parameters (a) Reflection parameters and (b) Transmission parameters

MEG1 ≅ 0dB MEG2

(1)

Simulation results are available for these two parameters; they are calculated using CST package from far field results. MEG ratio in isotropic environment is equal to 0 dB and the calculated envelope correlation is shown in Fig. 6. From both results it can be concluded that the proposed antenna satisfied diversity conditions. 0.09

Envelope Correlation Coefficient

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01

Fig. 4. Simulated radiation patterns at 2.4 GHz (a) XOZ plane (ĭ = 0 (b) YOZ plane (ĭ = 90 )

) and

0 2

2.5

3 Frequency (GHz)

3.5

4

Fig. 6. Envelope correlation coeffcient calculated from far field results

Finally, to figure out how well this antenna will cope with channel problems like fading, the diversity gain is calculated from the simulation. The calculated value of diversity gain based on the correlation effeciency is about 9.95 dB which is high enough for MIMO and diversiy applications in mobile systems. ( a)

(b)

Fig. 5. Measured radiation patterns at 2.4 GHz (a) XOZ plane (ĭ = 0 and (b) YOZ plane (ĭ = 90 )

IV. )

Fig. 4 displays 2-D simulated plots for the far field patterns of proposed antenna at 2.4 GHz, while the measured patterns are shown in Fig. 5 (measurement procedure was done by exciting the feed under test using a VNA and the other one is terminated with 50 Ohms load). It can be seen from both figures that there is a small differences between measured and simulated results and this is expected due to the using of large connectors in antenna prototype. The good thing in both figures is demonstrated in the discrepancy between feeds patterns in each plane, this is an evident about the very low

CONCLUSION

In this paper, a new wideband, very low profile (height = 3 mm) PIFA antenna has been presented as a diversity and MIMO antenna for handset applications over the frequency band 2.25-3.2 GHz. The antenna utilizes a new feed arrangement that produces lower level of mutual coupling. The low profile design was achieved with the aid of ground plane slots and a modified top plate radiator. The isolation has been achieved via band stop filter formed by an L-shaped ground plane slot. It has been investigated using a numerical simulation tool and measurement validation. The results have shown that this novel antenna is an excellent candidate for mobile hand-portable applications.

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The 8th European Conference on Antennas and Propagation (EuCAP 2014)

REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

G. J. F and. M. J. Gans, “On limits of wireless communications in a fading environment when using multiple antennas,” Bell Labs Technical Journal, vol. vol. 1, no.2, pp. 41-59, 1996 S. Zhang, B. K. Lau, Y. Tan et al., “Mutual coupling reduction of two PIFAs with a T-Shape slot impedance transformer for MIMO mobile terminals,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 3, pp. 1521-1531, 2012 C. B. Dietrich, Jr., K. Dietze, J. R. Nealy et al., “Spatial, polarization, and pattern diversity for wireless handheld terminals,” IEEE Transactions on Antennas and Propagation,, vol. 49, no. 9, pp. 12711281, 2001. T. Taga, “Analysis for mean effective gain of mobile antennas in land mobile radio environments,” IEEE Transactions on Vehicular Technology, vol. 39, no. 2, pp. 117-131, 1990. R. G. Vaughan, and J. B. Andersen, “Antenna diversity in mobile communications,” IEEE Transactions on Vehicular Technology, vol. 36, no. 4, pp. 149-172, 1987. J. Andersen, and H. Rasmussen, “Decoupling and descattering networks for antennas,” IEEE Transactions on Antennas and Propagation,, vol. 24, no. 6, pp. 841-846, 1976. J. W. Wallace, and M. A. Jensen, “Mutual coupling in MIMO wireless systems: a rigorous network theory analysis,” IEEE Transactions on Wireless Communications, vol. 3, no. 4, pp. 1317-1325, 2004. S. Dossche, J. Rodriguez, L. Jofre et al., “Decoupling of a two-element switched dual band patch antenna for optimum MIMO capacity, ” Antennas and Propagation Society International Symposium 2006, pp. 325-328, 2006. C. Volmer, J. Weber, R. Stephan et al., “Mutual coupling in multiantenna systems: Figures-of-merit and practical verification,” EuCAP

[10]

[11]

[12]

[13]

[14]

[15]

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2009. 3rd European Conference on Antennas and Propagation, pp. 1114-1118, 2009 A. Diallo, C. Luxey, P. Le Thuc et al., “Enhanced two-antenna structures for universal mobile telecommunications system diversity terminals,” Microwaves, Antennas & Propagation, IET, vol. 2, no. 1, pp. 93-101, 2008. A. Diallo, C. Luxey et al, “Study and Reduction of the Mutual Coupling between Two Mobile Phone PIFAs operating in the DCS 1800 and UMTS Bands, ” IEEE Transactions on Antennas and Propagation, vol. 54, no. 11, pp. 3063-3073, 2006. F. Zhu, J. Xu, and Q. Xu, “Reduction of mutual coupling between closely-packed antenna elements using defected ground structure, ” 3rd IEEE International Symposium on Microwave, Antenna Propagation and EMC Technologies for Wireless Communications, pp. 1-4, 2009 H. T. Chattha, Y. Huang, S. J. Boyes et al., “Polarization and pattern diversity-based dual-feed planar inverted-F antenna,” Antennas IEEE Transactions on and Propagation, vol. 60, no. 3, pp. 1532-1539, 2012. H. T. Chattha, Y. Huang “Low profile dual-feed Planar Inverted-F Antenna for wireless LAN applications,” Microw. Opt. Technol. Lett., vol. 53, pp. 1382–1386, 2011. H. T. Chattha, Y. Huang, X. Zhu et al., “Dual-feed PIFA diversity antenna for wireless applications,” Electronics Letters, vol. 46, no. 3, pp. 189-190, 2010.