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Study of CPW-fed circular disc monopole antenna forultra wideband applications
J. Liang, L. Guo, C.C. Chiau, X. Chen and C.G. Parini
Abstract: The paper presents a study of coplanar waveguide (CPW) fed circular disc monopoleantenna for ultra-wideband (UWB) applications. A circular disc monopole printed on a dielectricsubstrate and fed by a 50O CPW on the same layer can yield an ultra-wide 10dB return lossbandwidth with satisfactory radiation patterns. The performance and characteristics of the antennaare investigated in order to understand its operation. Good agreement has been obtained betweenthe simulation and experiment.
1 Introduction
Broadband monopole antennas have received considerableattention owing to their attractive merits, such as ultra-widefrequency band, good radiation properties, simple structureand ease of fabrication [1–3]. However, they are not planarstructures because they require a ground plane which isperpendicular to the radiator. Although the ground planecan be miniaturised significantly [4], they are still notsuitable for integration with a printed circuit board.
Recently, planar UWB monopoles have been realised byusing either a microstrip-line [5] or CPW feeds [6–11]. In thispaper, the CPW-fed circular disc monopole is investigatedwith an emphasis on the understanding of the mechanismwhich leads to the UWB characteristic. The designparameters for optimal operation of the antenna areanalysed extensively. The performance and characteristicsof the antenna are also studied both numerically andexperimentally. It will be demonstrated that the optimaldesign of this type of antenna can achieve an ultra widebandwidth with satisfactory radiation patterns.
2 Antenna design and performance
The CPW-fed disc monopole antenna studied in this paperhas a single layer metallic structure, as shown in Fig. 1. Acircular disc monopole with a radius of r and a 50O CPWare printed on the same side of a dielectric substrate. Wf isthe width of the metal strip and g is the gap between thestrip and the coplanar ground plane. W and L¼ 10mmdenote the width and the length of the ground plane,respectively, h is the feed gap between the disc and theground plane. In this study, a dielectric substrate with athickness of H¼ 1.6mm and a relative permittivity of er¼ 3is chosen, so Wf and g are fixed at 4mm and 0.33mm,respectively, in order to achieve 50O impedance.
The simulations were performed using the CST Micro-
wave Studiot package, which utilises the finite integration
technique for electromagnetic computation [12]. Thecomplete configuration of the antenna, including a 50OSMA feeding port, was simulated using this package, butthis does lead to a substantial computing overhead.
A prototype of the proposed circular disc monopoleantenna with optimal design, i.e. r¼ 12.5mm, h¼ 0.3mmand W¼ 47mm, as shown in Fig. 1, was built and tested inthe Antenna Measurement Laboratory at Queen Mary,University of London (QMUL). The return losses weremeasured in an anechoic chamber by using a HP 8720ESnetwork analyser.
Figure 2 illustrates the simulated and the measured returnloss curves. The measured return loss curve agrees very wellwith the simulated one in most of the frequency band rangeexcept between 7GHz and 10GHz. It is shown that thethird resonance occurs at around 7.8GHz in the simulation;this resonance also appears in the measurement, but it is notapparent, this could be due to the effect of the SMA port.For the other three resonances (at around 3.2GHz,5.8GHz and 11.1GHz), the measured ones are very closeto those obtained in the simulation with differences less than5%. Generally speaking, the 10dB bandwidth spans anextremely wide frequency range in both simulation andmeasurement. The simulated bandwidth ranges from2.64GHz to more than 12GHz. This UWB characteristicof the proposed CPW-fed circular disc monopole antenna isconfirmed in the measurement, with only a slight shift of thelower frequency to 2.73GHz.
3 Effects of design parameters
It has been shown in the simulation that the operatingbandwidth of the CPW-fed disc monopole is criticallydependent on the feed gap h, the width of the ground planeW and the radius of the disc r. So these parameters shouldbe optimised for maximum bandwidth.
In this Section, the 50O SMA feeding port is not takeninto account in all of the simulations so as to ease thecomputational requirements. It is noticed that this SMAport mainly affects the third and fourth resonances byshifting their resonant frequencies.
3.1 The effect of feed gap hFigure 3 plots the simulated return loss curves with differentfeed gaps (h¼ 0.3, 0.7, 1 and 1.5mm) when W is fixed at47mm and r at 12.5mm, respectively; their corresponding
The authors are with the Department of Electronic Engineering, Queen Mary,University of London, London, E1 4NS, UK
E-mail: [email protected]
r IEE, 2005
IEE Proceedings online no. 20045179
doi:10.1049/ip-map:20045179
Paper first received 20th December 2004 and in revised form 29th March 2005
520 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005
input impedance curves are plotted in Fig. 4. It can be seenin Fig. 3 that the return loss curves have similar shape forthe four different feed gaps, but the 10dB bandwidth ofthe antenna changes significantly with the variation of h.From what we have learned in [3–5], the ground plane,
serving as an impedance matching circuit, tunes the inputimpedance and hence the operating bandwidth while thefeed gap is varied, as shown in Fig. 4.
It is also noticed that the lower edge of the 10dBbandwidth increases when h gets smaller. The optimisedfeed gap is found to be at h¼ 0.3mm.
3.2 The effect of the width of the groundplane WSimulations have shown that when the length L of theground plane is more than 4mm, the performance of theantenna is almost independent of L. The simulated returnloss curves with r¼ 12.5mm and optimal feed gap h of
substrateH r
r
50Ω coplanarwaveguide ground plane
L
h
W
z
y
g gwf
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y
Fig. 1 Geometry of the CPW-fed circular disc monopole
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retu
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Fig. 2 Simulated and measured return loss curves with r¼ 12.5mm,W¼ 47mm and h¼ 0.3 mm
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retu
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Fig. 3 Simulated return loss curves for different feed gaps withW¼ 47 mm and r¼ 12.5 mm
0
25
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h = 0.3mm h = 0.7mm
h =1mm h =1.5mm
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Fig. 4 Simulated input impedance for different feed gaps withW¼ 47 mm and r¼ 12.5 mma Resistance Rb Reactance X
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w = 52mm w =60mm
Fig. 5 Simulated return loss curves for different widths of theground plane with h ¼ 0.3 mm and r¼ 12.5 mm
IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 521
0.3mm for different widths W of the ground planes arepresented in Fig. 5. It is observed in Fig. 5 that the returnloss curves vary substantially and no longer have similarshapes for the four different W, unlike those for the fourdifferent h, as shown in Fig. 3. Again, this can be readilyunderstood while the ground plane is treated as animpedance matching circuit. The intrinsic impedance ofthe ground plane seems to be mostly influenced by its widthW in this case. When W is changed, the first resonantfrequency does not change much, however the higherresonant frequencies vary substantially, leading to thevariations of the operating bandwidth of the antenna, asshown in Fig. 5.
It is also seen in Fig. 5 that, when W is equal to 47mm,the10dB bandwidth covers an ultra wide frequency band,from 2.27GHz to more than 12GHz (up to 20GHz in thesimulation); when W rises to 52mm and 60mm, the loweredge of the bandwidth decreases tardily to 2.19GHz and2.08GHz, respectively. However, the upper edge is reduceddramatically to 4.03GHz and 3.47GHz, respectively, leadingto a remarkable narrowing of the bandwidth; whenW¼ 40mm, the bandwidth ranges from 2.54GHz to6.72GHz. The optimal width of the ground plane is foundto be at W¼ 47mm.
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Fig. 6 Simulated return loss curves for different dimensions of thecircular disc with the optimal designsr¼ 7.5mm with h¼ 0.1mm and W¼ 28mm; r¼ 12.5mm withh¼ 0.3mm and W¼ 47mm; r¼ 15mm with h¼ 0.3mm andW¼ 56mm; r¼ 25mm with h¼ 0.5mm and W¼ 90mm
Table 1: The relationships between the diameters and thefirst resonances
Diameterr (mm)
First resonancef1 (GHz)
Wavelengthl at f1 (mm)
2r/l
25 1.52 197.4 0.25
15 2.57 116.7 0.26
12.5 3.01 99.7 0.25
7.5 5.09 58.9 0.25
5
A / m
0
5
A / m
0
5
A / m
0
5
A / m
0
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y
z
y
z
y
z
y
a b
c d
Fig. 8 Simulated current distributions of the disc monopole with r¼ 12.5 mm, h¼ 0.3 mm and W¼ 47 mma At 3GHzb At 5.6GHzc At 7.8GHzd At 11GHz
frequencyf1 f2 f3 f4
Fig. 7 Overlapping of the multiple resonance modes
522 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005
3.3 The effect of the dimension of the discThere is an important phenomenon in Figs. 3 and 5 that thefirst resonance f1 always occurs at around 3GHz fordifferent h and W when r equals 12.5mm. In fact, thequarter wavelength at this first resonant frequency (25mm)just equals to the diameter of the disc. This implies that thisresonant frequency is mostly determined by the circular discand not much detuned by the ground plane.
To confirm this, Fig. 6 shows the simulated return losscurves for different dimensions of the circular disc with theirrespective optimal designs (r¼ 7.5mm with h¼ 0.1mm andW¼ 28mm; r¼ 12.5mm with h¼ 0.3mm and W¼ 47mm;r¼ 15mm with h¼ 0.3mm and W¼ 56mm; r¼ 25mmwith h¼ 0.5mm and W¼ 90mm). It is observed fromFig. 6 that the first resonant frequency decreases with theincrease of the dimension of the disc. The relationshipsbetween the diameters and the first resonances are given inTable 1.
Actually, as shown in Fig. 6, the circular disc is capableof supporting multiple resonance modes, the higher ordermodes (f2, f3,y, fn) being the harmonics of the funda-mental mode of the disc. So the wavelengths of the higherorder modes satisfy 2r¼ n ln/4¼ l1/4, where n is the modenumber. Figure 6 also indicates that these higher ordermodes are closely spaced. Hence, the overlapping of thesehigher order modes leads to the UWB characteristic, asillustrated in Fig. 7.
The simulated current distributions at different frequen-cies for the optimal design with r¼ 12.5mm, h¼ 0.3mmand W¼ 47mm are presented in Fig. 8. Figure 8a showsthe current pattern near the first resonance at 3GHz. Thecurrent pattern near the second resonance at 5.6GHz isgiven in Fig. 8b, indicating approximately a second orderharmonic. As mentioned in Section 2, the third and fourthresonances are shifted to 7.8GHz and 11GHz in themeasurement owing to the presence of the SMA port. It hasalso been demonstrated that the simulated current distribu-tions at these two frequencies will not change if the SMAport is removed in the simulation. So Fig. 8c and 8dillustrate two more complicated current patterns at 7.8GHzand 11GHz, corresponding to the third and fourth orderharmonics, respectively. The current distributions also verifythat the UWB characteristic of the antenna is attributed tothe overlapping of this sequence of resonance harmonics.
As shown in Fig. 8, the current is distributed mainlyalong the edge of the disc. This is the reason why the firstresonant frequency is associated with the diameter of thecircular disc. On the ground plane, the current is distributedmainly on the upper edge along the y-direction, whichexplains why the performance of the antenna is criticallydependent on the width of the ground plane W.
4 Radiation patterns
The measured and simulated radiation patterns at 3GHz, 5.6GHz, 7.8GHz and 11GHz are plotted in Figs. 9–12,respectively. The measured co-polarisation patterns are veryclose to those obtained in the simulation.
In the z–y plane (Figs. 9a–12a), the co-polarisationpatterns have large back lobes at lower frequencies. Withthe increase of the frequency, the back lobes becomesmaller, splitting into many minor ones, while the frontlobes start to form notches and get more directional. Theco-polarisation patterns correspond well to the currentdistributions, as shown in Fig. 8. The cross-polarisationpattern is lower than 15dB in most of the directions at3GHz; with the increase of frequency, it is getting high, butstill lower than 10dB at 11GHz. Additionally, there is a
significant discrepancy between the simulated and measuredcross-polarisation patterns. This is because in cross-polarisation the signal is weak. Hence, the effects of thenoise in the chamber and the SMA port become morenotable in the measurement.
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y
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Fig. 9 Simulated (solid line) and measured (dotted line)co-polarisation (thick line) and cross-polarisation (thin line) radiationpatterns with r¼ 12.5 mm, W¼ 47 mm and h¼ 0.3 mm at 3 GHza In z–y planeb In x–y planec In z–x plane
IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 523
It is noticed that in the x–y plane (Figs. 9b–12b), the co-polarisation pattern is omni-directional at lower frequencies(3GHz and 5.6GHz) and only distorted slightly at higherfrequencies (the gain relative to the peak radiated signal
direction being reduced less than 10dB in the x-direction at11GHz). So the patterns are generally omni-directionalover the entire bandwidth, like a conventional monopoleantenna. The cross-polarisation pattern is getting stronger
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Fig. 10 Simulated (solid line) and measured (dotted line)co-polarisation (thick line) and cross-polarisation (thin line) radiationpatterns with r¼ 12.5 mm, W¼ 47 mm and h¼ 0.3 mm at 5.6 GHza In z–y planeb In x–y planec In z–x plane
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Fig. 11 Simulated (solid line) and measured (dotted line)co-polarisation (thick line) and cross-polarisation (thin line) radiationpatterns with r¼ 12.5 mm, W¼ 47 mm and h¼ 0.3 mm at 7.8 GHza In z–y planeb In x–y planec In z–x plane
524 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005
with the increase of frequency, and is lower than 8dBin most of the directions at 11GHz, compared to theco-polarisation one.
In the z–x plane (Fig. 9c–12c), the co-polarisationpatterns are similar to those in the z–y plane. The simulatedcross-polarisation pattern curves are not visible in the
Figures. This is because the disc monopole performs like aconventional wire monopole in the z–x plane. Conse-quently, the cross-polarisation patterns are extremely low(less than 100dB in all of the directions at all of the fourfrequencies). However, the measured ones are prominentowing to the noise and the presence of the SMA port.
Figure 13 illustrates the simulated peak gain of theproposed antenna with r¼ 12.5mm, h¼ 0.3mm andW¼ 47mm. It is shown that when the frequency increasesfrom 3GHz to 7GHz, the gain rises from 0.88dBi to5.76dBi; with the further increase of frequency from 7GHzto 11GHz, the gain does not change much and is fixed ataround 6dBi. This is because of the more directionalradiation properties at higher frequencies, as shown inFigs. 9–12.
5 Conclusion
This paper has provided further insights into the operationof the CPW-fed circular disc monopole antenna. It has beenshown that the feed gap h, the width of the ground plane W,and the dimension of the CPW-fed circular disc monopoleantenna are the most important parameters that determinethe performance of the antenna. The ground plane, servingas an impedance matching circuit, tunes the inputimpedance and hence the operating bandwidth by changingh and W. The first resonant frequency is determined directlyby the dimension of the circular disc because the currentis distributed mainly along the edge of the disc. Theoverlapping of multiple resonant harmonics leads to theUWB characteristic. Both simulation and measurementhave demonstrated that the CPW-fed circular disc mono-pole can achieve an ultra wide bandwidth, covering theFCC defined UWB frequency band. It is also observed thatthe radiation patterns are nearly omni-directional over theentire operating bandwidth. The results have proved thatthis antenna is very suitable for future UWB applications.
6 Acknowledgments
The authors would like to thank Mr. John Dupuy of theDepartment of Electronic Engineering, QMUL for his helpin the fabrication and measurement of the antenna. Theauthors would like to acknowledge Computer SimulationTechnology (CST), Germany, for the complimentary license
of the Microwave Studiot package. One of the authors(J. Liang) would also like to acknowledge the financialsupport provided by the K.C. Wong Education Founda-tion.
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Fig. 12 Simulated (solid line) and measured (dotted line)co-polarisation (thick line) and cross-polarisation (thin line) radiationpatterns with r¼ 12.5 mm, W¼ 47 mm and h¼ 0.3 mm at 11 GHza In z–y planeb In x–y planec In z–x plane
0
1
2
3
4
5
6
7
3 4 5 6 7 8 9 10 11
frequency, GHz
gain
, dB
i
Fig. 13 Simulated peak gain with r¼ 12.5 mm, W¼ 47 mm andh¼ 0.3 mm
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7 References
1 Ammann, M.J., and Chen, Z.N.: ‘Wideband monopole antennas formulti-band wireless systems’, IEEE Antennas Propag. Mag., 2003, 45,(2), pp. 146–150
2 Agrawall, N.P., Kumar, G., and Ray, K.P.: ‘Wide-band planarmonopole antennas’, IEEE Trans. Antennas Propag., 1998, 46, (2),pp. 294–295
3 Liang, J., Chiau, C.C., Chen, X., and Yu, J.: ‘Study of a circular discmonopole antenna for ultra wideband applications’. 2004 Int. Symp.Antennas Propag., Sendai, Japan, 17–21 Aug. 2004
4 Liang, J., Chiau, C.C., Chen, X., and Parini, C.G.: ‘Analysis anddesign of UWB disc monopole antennas’. IEE Seminar on UltraWideband Commun. Technol. Syst. Design, Queen Mary, Universityof London, 8 July 2004, pp. 103–106
5 Liang, J., Chiau, C.C., Chen, X., and Parini, C.G.: ‘Printed circulardisc monopole antenna for ultra wideband applications’, Electron.Lett., 2004, 40, (20), pp. 1246–1248
6 Kim, Y., and Kwon, D.-H.: ‘CPW-fed planar ultra wideband antennahaving a frequency band notch function’, Electron. Lett., 2004, 40,(7), pp. 403–405
7 Wang, W., Zhong, S.S., and Chen, S.-B.: ‘A novel wideband coplanar-fedmonopole antenna’, Microw. Opt. Technol. Lett., 2004, 43, (1), pp. 50–52
8 Suh, S.-Y., Shutzman, W., Davis, W., Waltho, A., and Schiffer, J.: ‘Anovel CPW-fed disc antenna’. IEEE Antennas Propag. Soc. Symp.,20–25 June 2004, Vol. 3, pp. 2919–2922
9 Yang, T., and Davis, W.A.: ‘Planar half-disk antenna structures forUWB communications’. IEEE Antennas Propag. Soc. Symp., 20–25June 2004, Vol. 3, pp. 2508–2511
10 Yoon, H., Kim, H., Chang, K., Yoon, Y.J., and Kim, Y.-H.: ‘A studyon the UWB antenna with band-rejection characteristic’. IEEEAntennas Propag. Soc. Symp., 20–25 June 2004, Vol. 2, pp. 1784–1787
11 Chung, K., Yun, T., and Choi, J.: ‘Wideband CPW-fed monopoleantenna with parasitic elements and slots’, Electron. Lett., 2004, 40,(17), pp. 1038–1040
12 CST-Microwave Studio, User’s Manual, 4, 2002
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