with a lengthened or shortened�, the rejection slope at the upperfrequency can be enhanced. A 4th order 10% FBW Chebyshev filterwas designed at 5 GHz with additional rejection of 23 dB at 6 GHz.Since the overall filter resonator length remains unchanged, the im-provement in performance does not compromise on circuit size.

REFERENCES

1. S.B. Cohn, Parallel-coupled transmission-line-resonator filters, IRETrans Microwave Theory Tech MTT-6 (1958), 223-221.

2. M. Makimoto and S. Yamashita, Bandpass filters using parallel coupledstripline stepped impedance resonators, IEEE Trans Microwave TheoryTech MTT-28 (1980), 1413-1417.

3. J.-T. Kuo, S.-P. Chen, and M. Jiang, Parallel-coupled microstrip filterswith over-coupled end stages for suppression of spurious responses,IEEE Microwave Wireless Compon Lett 13 (2003), 440-442.

4. C.-Y. Chang and T. Itoh, A modified parallel-coupled filter structurethat improves the upper stopband rejection and response symmetry,IEEE Trans Microwave Theory Tech 39 (1991), 310-314.

5. J.S. Hong and M.J. Lancaster, Microstrip filters for RF/microwaveapplications, Wiley, New York, 2001.

6. M. Makimoto and S. Yamashita, Microwave resonators and filters forwireless communication, Springer, 2001.

7. D.M. Pozar, Microwave engineering, 3rd ed. New Wiley, Hoboken, NJ,2003.

© 2008 Wiley Periodicals, Inc.

BANDWIDTH-ENHANCED INTERNALPIFA WITH A COUPLING FEED FORQUAD-BAND OPERATION IN THEMOBILE PHONE

Kin-Lu Wong and Chih-Hong HuangDepartment of Electrical Engineering National Sun Yat-Sen UniversityKaohsiung 80424, Taiwan, Republic of China; Corresponding author:wongkl@ema.ee.nsysu.edu.tw

Received 11 August 2007

ABSTRACT: A novel bandwidth-enhanced internal planar inverted-Fantenna (PIFA) for quad-band operation in the mobile phone is pre-sented. The proposed PIFA is capacitively excited using a coupling feed,which leads to a dual-resonance excitation in the antenna’s lower bandat about 900 MHz and a wide-band resonant mode excitation at about1800 MHz. This makes the operating bandwidths of both the antenna’slower and upper bands larger than 20% (3:1 VSWR), thus easily cover-ing GSM850/900 and DCS/PCS operation. In addition, with quad-bandoperation obtained, the proposed PIFA occupies a small volume of 5 �6 � 60 mm3 or 1.8 cm3 only. Details of the proposed PIFA are described,and results of fabricated prototypes are presented and discussed. © 2008Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 683–687, 2008;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.23175

Key words: internal mobile phone antennas; mobile antennas; PIFA;coupling feed; quad-band operation

1. INTRODUCTION

Conventional PIFAs (planar inverted-F antennas) have beenwidely applied in the mobile phone as internal antennas for oper-ating in the 900 and 1800 MHz bands [1]. However, owing to thelimited available volume that can be allocated for the internalantenna inside the mobile phone, it is usually not an easy task toobtain multiband operation for the conventional PIFA, especiallyin the 900 MHz band to cover GSM850 (824–896 MHz) andGSM900 (880–960 MHz) operation. To achieve enhanced band-widths for the internal PIFA, the technique of adding parasitic ordirect-feed additional resonators to the main radiator to provideadditional resonant modes has been applied [2-5]. This techniqueis especially suitable for increasing the bandwidth of the 1800MHz band or the antenna’s upper band, because the requiredlength of the additional resonators is shorter for providing anadditional resonant mode near 1800 MHz.

The use of an external matching circuitry has also been applied[6-9]. This technique is promising to enhance the bandwidth ateither the 900 or 1800 MHz band or both the 900 and 1800 MHzbands. However, it will occupy some valuable board space on thesystem circuit board of the mobile phone and increase someinsertion loss also. In this article, we present a novel bandwidth-enhanced internal PIFA using a coupling feed, which can beconsidered as an internal matching circuitry and will not increasethe occupied volume of the antenna inside the mobile phone. Withthe coupling feed, the proposed PIFA can be excited to achieve adual-resonance at about 900 MHz and a wide-band resonance atabout 1800 MHz. This behavior makes the operating bandwidthsof the antenna’s lower and upper bands larger than 20% (3:1VSWR), allowing the antenna to cover GSM850/900 and DCS/PCS (1710–1880/1920–2170 MHz) bands for quad-band opera-tion. In addition, the proposed quad-band PIFA occupies a smallvolume of 1.8 cm3 only inside the mobile phone. Design consid-erations of the proposed PIFA are described in the article. Exper-

Figure 4 Momentum simulation results for � � 0°, 30°, 50°. Zoom in onrejection slope

TABLE 3 Summary of Measured and Simulated Rejection ofFilters at (f0 � 1) GHz

� Simulated Rejection (dBc) Measured Rejection (dBc)

0° 45.9 45.730° 55.1 55.650° 73.4 68.9

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 683

imental and simulation results for fabricated prototypes of theproposed PIFA are presented and discussed.

2. DESIGN CONSIDERATIONS OF PROPOSED ANTENNA

Figure 1(a) shows the geometry of the proposed PIFA with acoupling feed for mobile phone application. The proposed PIFAoccupies a small volume of 5 � 6 � 60 mm3 (1.8 cm3) and ismounted at the top no-ground portion (size 6 � 60 mm2) of thesystem circuit board of the mobile phone. In the study, a 0.8-mmthick FR4 substrate is used as the system circuit board; on its backsurface, there is a printed system ground plane of length 100 andwidth 60 mm. The dimensions of the system circuit board andground plane considered here are practical for general smartphones or personal digital assistant (PDA) phones available in themarket. In the experiment, the proposed PIFA is fed at point A (thefeeding point) using a 50-� microstrip line printed on the frontsurface of the system circuit board.

The dimensions of the proposed PIFA in its planar structure areshown in Figure 1(b). Similar to the conventional PIFA, theproposed PIFA mainly comprises a feeding portion, a shortingportion, and a radiating portion. However, instead of using asimple direct feed for the conventional PIFA, the feeding portionhere consists of a coupling strip CD of length (a) 8.5 mm andwidth (b) 0.5 mm. This coupling strip, connected to the 50-�

microstrip feedline through a short strip of length 3.25 mm, ca-pacitively excites the proposed PIFA. This coupling feed incorpo-rating the 5-mm long shorting strip BD forms as an LC internalmatching circuitry (from the strip CD then through FE to BE pointB is further connected to the top edge of the ground plane on theback surface of the circuit board (the via-hole through the circuitboard not shown in the figure). By varying the parameters a and bof the coupling strip, the input impedance of the antenna can beadjusted. In the proposed PIFA here, the input reactance over thefrequency range of interest (at about 900 MHz) can be decreasedto a lower value, and an additional resonance (zero input reac-tance) can also be obtained. This can lead to a dual-resonanceexcitation at about 900 MHz for the antenna’s lower band. At thesame time, a wide-band resonance excitation at about 1800 MHzfor the antenna’s upper band can also be achieved. This behaviorallows the proposed PIFA to easily cover GSM850/900/DCS/PCSquad-band operation. More detailed effects of the coupling feedwill be discussed in the next section with the aid of Figures 3–5.

Also note that the proposed PIFA in the study is printed on a0.4-mm thick FR4 substrate and then bent into an inverted-U shapeto achieve a compact configuration. For practical applications, theproposed PIFA can be printed on a flexible printed circuit board(FPCB). In this case, smooth bending of the proposed PIFA can beobtained, which will make it more flexible to fit in inside thesmooth casing of the modern mobile phones.

The radiating portion of the proposed PIFA is composed of tworadiating strips of different lengths. Strip 1 starting from point E hasa length of about 90 mm, close to about 0.25 wavelength at 900 MHz.This allows the excitation of a 0.25 wavelength mode at about 900MHz for the proposed PIFA, which is further tuned to become adual-resonance mode by using the proposed coupling feed. The dual-resonance excitation leads to bandwidth enhancement for the anten-na’s lower band to cover GSM850/900 operation. On the other hand,strip 2 starting from point F has a length of about 40 mm, which isclose to about 0.25 wavelength at about 1800 MHz and allows theexcitation of a wide-band resonance mode for the antenna’s upperband to cover DCS/PCS operation. Note that both open-end portionsof strips 1 and 2 are widened to have a width of 5 mm, which can leadto smooth excited surface current distributions at the strip’s open endsand is helpful for achieving bandwidth enhancement for the excitedresonant modes [10, 11].

3. RESULTS AND DISCUSSION

The proposed PIFA was fabricated and studied. Figure 2 showsthe measured and simulated return loss for the proposed PIFA.

Figure 1 (a) Geometry of the proposed bandwidth-enhanced internalPIFA with a coupling feed for mobile phone application. (b) Dimensions ofthe proposed PIFA in its planar structure. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com]

Figure 2 Measured and simulated return loss for the proposed PIFA.[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com]

684 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 DOI 10.1002/mop

The simulated results are obtained using Ansoft simulationsoftware high frequency structure simulator (HFSS) [12], andagreement between the simulation and measurement is ob-tained. Dual-resonance excitation at about 900 MHz is clearlyseen, and a wide impedance bandwidth, defined by 3:1 VSWRor 6-dB return loss, of 190 MHz (800 –990 MHz) is achieved.This wide bandwidth reaches about 21% with respect to 900MHz and allows the antenna’s lower band to easily coverGSM850/900 operation. For the antenna’s upper band, a wideresonant mode centered at about 1800 MHz is excited. Theobtained bandwidth reaches 415 MHz (1665–2080 MHz) orabout 23% with respect to 1800 MHz, allowing the antenna tocover DCS/PCS operation.

To show the effect of the coupling feed used here, Figure 3shows the comparison of the simulated return loss of the proposedPIFA and the reference antenna (the corresponding conventionalPIFA with a simple feeding strip). The dimensions of the referenceantenna are shown in the inset in Figure 3, which uses a direct feedand has the same antenna volume as that of the proposed PIFA.Note that the dimensions of the reference antenna are adjusted toachieve optimal impedance bandwidths of the lower and upperbands. From the obtained results as shown in the figure, the lowerband of the reference antenna can cover GSM 900 operation only.Conversely, the proposed PIFA has a bandwidth-enhanced lowerband, allowing it to cover GSM850/900 operation. For the upperband, both the reference antenna and the proposed PIFA have awide bandwidth to cover DCS/PCS operation.

Figure 4 shows the simulated input impedance versus fre-quency and the simulated input impedance on the Smith chart forthe reference antenna in Figure 3 and the proposed PIFA. In thefigure, only the results for the antenna’s lower band are shown. InFigure 4(a), the input reactance of the proposed PIFA is smaller

than that of the reference antenna. The variations of the inputreactance are also seen to be much smaller for the proposed PIFAthan for the reference antenna. In addition, an additional resonance(zero input reactance) is occurred at about 840 MHz, which leadsto a dual-resonance excitation for the antenna’s lower band as seenin Figure 2. In Figure 4(b), the impedance curves for the frequencyrange of 700–1100 MHz are shown, and the frequency intervalsbetween markers of the impedance curves are 60 MHz. It is seenthat with the presence of the coupling feed (the proposed PIFA),the loop of the impedance curve is shifted inside the 3:1 VSWRcircle, making the bandwidth enhanced from 70 MHz for thereference antenna to be 190 MHz for the proposed PIFA.

Effects of the parameters a and b of the coupling strip on theantenna performances are studied in Figure 5. Figure 5(a) showsthe simulated return loss for the length a varied from 6.5 to 10.5mm, while the width b is fixed to be 0.5 mm. Results indicate thatdual-resonance excitation for the antenna’s lower band can beobtained for different values of a. The antenna’s upper band willalso be affected by the variation in the length a. A longer length acan shift the upper band to lower frequencies. Figure 5(b) showsthe simulated return loss for the width b varied from 0.3 to 0.7 mm,with the length a fixed as 8.5 mm. Similar effects as seen in Figure5(a) are obtained. A larger width b will also shift the upper bandto lower frequencies. Hence, by properly selecting the length a andwidth b of the coupling strip, dual-resonance excitation at about900 MHz can be obtained for GSM850/900 operation, and awide-band resonant mode can also be adjusted to occur at about1800 MHz for covering DCS/PCS operation.

Radiation patterns of the proposed PIFA are also studied. Figures6–9 plot the measured radiation patterns at 860, 925, 1795, and 1920MHz, respectively. These frequencies are the central frequencies of

Figure 3 Comparison of the simulated return loss of the proposed PIFAand the reference antenna (the corresponding conventional PIFA with asimple feeding strip). [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

180170160150140

130120

110100 90 80 70

6050

4030

2010

0-10

-20-30

-40

-50-60

-70-80-90-100-110

-120-130

-140-150-160-170

Figure 4 (a) Simulated input impedance versus frequency and (b) sim-ulated input impedance on the Smith chart for the reference antenna inFigure 3 and the proposed PIFA. Only the results for the lower band of theantenna are shown. [Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 685

GSM850, GSM900, DCS, and PCS bands. Results at other frequen-cies were also measured. Over each operating band, very similarmeasured radiation patterns as shown here are obtained, indicating

that stable radiation patterns are achieved over GSM850, GSM900,DCS, and PCS bands. The radiation patterns shown in Figures 6 and7 are also very similar to each other, and omnidirectional radiation inthe x-y plane (azimuthal plane) is obtained, which is similar to that ofthe conventional PIFA [1]. The radiation patterns at 1795 and 1920MHz are also similar to each other, which is expected since theantenna’s upper band is formed by a single wide-band resonant mode.The obtained radiation patterns also show no special distinctionscompared to those of the conventional PIFA for DCS or PCS oper-ation [1]. The measured antenna gain and simulated radiation effi-ciency for the proposed PIFA are presented in Figure 10. Over theGSM850/900 operating bands shown in Figure 10(a), the antennagain is varied from about 1.0 to 1.9 dBi, and the radiation efficiencyis all larger than 65%. For the DCS/PCS operating bands, the resultsin Figure 10(b) indicate that the antenna gain is varied from 1.4 to 2.0dBi, and the radiation efficiency is also larger than 65%. The obtainedradiation characteristics are good for practical applications in themobile phones.

Figure 5 Simulated return loss as a function of (a) the length a and (b) thewidth b of the coupling strip in the coupling feed. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com]

Figure 6 Measured radiation patterns at 860 MHz for the proposedPIFA. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

Figure 7 Measured radiation patterns at 925 MHz for the proposedPIFA. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

Figure 8 Measured radiation patterns at 1795 MHz for the proposedPIFA. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

686 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 DOI 10.1002/mop

4. CONCLUSION

A novel internal PIFA with a coupling feed for quad-band oper-ation in the mobile phone has been proposed. Compared to the

conventional PIFA with a simple direct feed, the coupling feedused in the proposed PIFA does not increase the volume of theantenna. This novel coupling feed can lead to a dual-resonanceexcitation at about 900 MHz for the antenna’s lower band and awide-band resonant mode excitation at about 1800 MHz for theantenna’s upper band. Both the obtained bandwidths for the lowerand upper bands of the proposed PIFA are larger than 20%, whichmakes it easily cover GSM850/900/DCS/PCS quad-band opera-tion. Good radiation characteristics over the operating bands havealso been obtained. Since the occupied volume of the proposedquad-band PIFA is 1.8 cm3 only and it is also easy to be fabricatedon a dielectric substrate, especially on a flexible printed circuitboard, at low cost, the proposed PIFA is very promising forpractical applications as a compact quad-band internal mobilephone antenna.

REFERENCES

1. K.L. Wong, Planar antennas for wireless communications, Wiley, NewYork, 2003.

2. J. Ollikainen and A. Lehtola, Internal multi-band antenna with im-proved radiation efficiency, U.S. Patent No. 6,552,686 B2, April 22,2003.

3. K.L. Wong, L.C. Chou, and C.M. Su, Dual-band flat-plate antennawith a shorted parasitic element for laptop applications, IEEE TransAntennas Propag 53 (2005), 539-544.

4. Y.X. Guo, M.Y.W. Chia, and Z.N. Chen, Miniature built-in multibandantennas for mobile handsets, IEEE Trans Antennas Propag 52 (2004),1936-1944.

5. K.L. Wong and Y.C. Lin, Thin internal planar antenna for GSM/DCS/PCS/ UMTS operation in a PDA phone, Microwave Opt Technol Lett47 (2005), 429-432.

6. J. Ollikainen, O. Kivekas, C. Ichein, and P. Vainikainen, Internalmultiband handset antenna realized with an integrated matching cir-cuit, In: Proceedings of the 12th International Conference on Antennasand Prpagation, Vol. 2, 2003, pp. 629–632.

7. K.R. Boyle, M. Udink, A. de Graauw, and L.P. Ligthart, A noveldual-fed, self-diplexing PIFA and RF front-end (PIN-DF2-PIFA), In:IEEE Antennas and Propagation International Symposium Digest,Monterey, CA, Vol. 2, 2004, pp. 1935–1938.

8. M. Tzortzakakis and R.J. Langley, Quad-band internal mobile phoneantenna, IEEE Trans Antennas Propag 55 (2007), 2097-2103.

9. J. Villanen, C. Icheln, and P. Vainikainen, A coupling element-basedquad-band antenna element structure for mobile terminals, MicrowaveOpt Technol Lett 49 (2007), 1277-1282.

10. Y.L. Kuo and K.L. Wong, Printed double-T monopole antenna for2.4/5.2 GHz dual-band WLAN operations, IEEE Trans AntennasPropag 51 (2003), 2187-2192.

11. K.L. Wong, Y.C. Lin, and T. C. Tseng, Thin internal GSM/DCS patchantenna for a portable mobile terminal, IEEE Trans Antennas Propag54 (2006), 238-242.

12. Ansoft Corporation HFSS. http://www.ansoft.com/products/hf/hfss/.

© 2008 Wiley Periodicals, Inc.

Figure 9 Measured radiation patterns at 1920 MHz for the proposedPIFA. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

1.6 1.7 1.8 1.9 2 2.1

Figure 10 Measured antenna gain and simulated radiation efficiency forthe proposed PIFA. (a) The lower band for GSM850/900 operation. (b) Theupper band for DCS/PCS operation. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 687

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