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7/27/2019 Miniature Microstrip-fed Ultra-wideband Printed Monopole Antenna With a Partial Ground Plane Structure

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Published in IET Microwaves, Antennas & Propagation

Received on 9th September 2010

Revised on 11th April 2011

doi: 10.1049/iet-map.2010.0450

ISSN 1751-8725

Miniature microstrip-fed ultra-widebandprinted monopole antenna with a partial groundplane structureM. Koohestani1 M.N. Moghadasi1 B.S. Virdee2

1Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran2Faculty of Computing, Centre for Communications Technology, London Metropolitan University, 166-220 Holloway Road,

London N7 8DB, UKE-mail: [email protected]

Abstract: A new microstrip-fed planar monopole antenna is presented for ultra-wideband (UWB) communication systems. Theantenna structure consists of a dome-topped, bowl-shaped patch with a truncated ground plane structure. The ground plane is

predominately tapered and includes a notch below the feed-line in the vicinity of the patch. The effects of dimensionalparameters on the performance of the antenna have been investigated through a parametric study. Current density distributionon the antenna was also computed to gain a better insight of its behaviour. The antenna performance is validated throughmeasurement, including its radiation patterns. The measured impedance bandwidth of the proposed antenna for |S11| 2 10 dB is 10.35 GHz (2.6513.0 GHz), constituting 132% impedance bandwidth. The fabricated antenna has a compact sizeof 18 20 1.6 mm3. Additional features that make the antenna a suitable candidate for UWB systems are its simpleconfiguration, compactness and low fabrication cost.

1 Introduction

Tremendous effort has been expended to date on thedevelopment of ultra-wideband (UWB) antennas since theUS Federal Communication Commission (FCC) allocatedthe frequency band (3.1 10.6 GHz) for communicationsystem applications. This is because the design of suchantennas is one of the challenging tasks in these systems. Inorder to make such systems coexist with and overlayexisting narrow band radio services, the maximum

permissible effective isotropic radiated power (EIRP)density is restricted to 41.3 dBm/MHz. Hence, theemission limit of the radiated power is a criticalconsideration in a UWB system, especially for the antennadesign [1]. In addition, many systems now operate inmultiple frequency bands, requiring dual- or triple-bandoperation of fundamentally narrowband antennas. Theseinclude satellite navigation systems, cellular systems,wireless LANs and combinations of these systems.Advances in software-defined and reconfigurable radionetworks necessitate their operation over a wide range offrequencies or operation in a multi-band manner. Hence, tocover more wireless communication services, antennasoperating at a broadband range are in high demand. Asuitable UWB antenna should possess properties of low

return loss performance with satisfactory radiation patternproperties over the entire UWB frequency range. Variousantennas have been investigated and implemented for UWBsystems, such as the waveguide horn [2], log periodic [3]

and biconical [4]. These antennas usually radiate differentfrequency components from different parts of the antenna,which tend to be dispersive. In addition, several broadbandmonopole configurations, such as circular, square, ellipticaland pentagonal, have been proposed for UWB applications[58]. These broadband monopoles are not planarstructures because their ground planes are perpendicular tothe radiators and therefore are not suitable for integrationwith a printed circuit board (PCB) technology. Recentantenna investigations for UWB applications includedouble-sided microstrip antennas with a modified ground

plane. For example in [9] to enhance the impedancebandwidth, antenna parameters are optimised and theground plane is modified by cutting slots on the top edge toform a symmetrical sawtooth shape. Another double-sided

printed omni-directional UWB antenna operating within aband from 3.2 to 12.2 GHz is presented in [10]. In thiscase, the antenna comprises a rectangular patch withinverted L-shaped slits cut out in the ground to control theantennas resonant frequency and bandwidth.

Microstrip antennas because of their ability to be integratedeasily on a PCB are popular. In addition, this technologyoffers antennas with the attributes of low profile, lightweight, low cost and ease of fabrication. These featuresmake microstrip antennas very attractive for wireless

communications systems [1121]. To realise ultra-wideimpedance bandwidth from microstrip antennas severaltechniques have been explored including the monopoleantenna [1113], backed conduct resonator [14] and partial

IET Microw. Antennas Propag., 2011, Vol. 5, Iss. 14, pp. 16831689 1683

doi: 10.1049/iet-map.2010.0450 & The Institution of Engineering and Technology 2011

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ground plane [15]. Furthermore, miniaturisation of antennasis also a highly desired attribute, and it represents anotherchallenge in the design of such antennas. Hence, the aim ofthis paper is to present an UWB microstrip antenna with acompact size and that operates across the entire ultra-wide spectrum defined by FCC. This paper presents anew compact UWB monopole antenna having a partiallymodified ground plane structure. The characteristics of the

proposed antenna are investigated through a parametricstudy. The measured results of impedance bandwidth andradiation pattern of the proposed antenna are presented tovalidate the design. The results show ultra-wide bandwidth

performance and stable omni-directional radiation patterns.Furthermore, the physical size of the proposed antenna issubstantially smaller than recently developed UWB antennasin [9] by 45.5%, in [10] by 85.6% and in [12] by 40%.

2 Antenna structure and design

A new compact UWB antenna structure is proposed herewhich is based on the recent work in UWB antenna design[12]. The geometry of the proposed antenna comprises adome topped, bowl-shaped patch with a truncated ground

plane structure, as shown in Fig. 1 along with itsdimensional parameters. The antenna feed structure consistsof a microstrip line connected to the radiating patch. In thisstudy, a modified ground plane structure was employed toachieve the desired ultra-wide bandwidth operation. Thetruncated ground plane comprises a rectangular notchcentred under the feed-line in the vicinity of the patch, andits upper side is tapered as shown in Fig. 1. The antenna islocated in the xy plane and the normal direction is z-axis.As shown in Fig. 1, parameters a a n d b denote thewidth and the length of the dielectric substrate, respectively.The combination of two semicircles with difference radii

forms the radiator element. The larger and smallersemicircles have radii of r1 and r2, respectively. On theopposite side of the substrate, the conducting ground planewith a length of f 5 mm only covers the section of themicrostrip feed-line. Parameter k is the height of the feed

gap between the radiator and the ground plane. The antennastructure was fabricated on FR4 microwave substrate with arelative permittivity of 4.4, thickness of 1.6 mm and metalthickness of 35 mm. The rectangular notch section on theground plane (c d), the tapered sides of the ground

plane, the feed gap (distance between the radiator and theground plane) and the radii of the semicircles stronglyaffect the antenna impedance bandwidth. The proposedantenna was fabricated using the optimised parametervalues given in Table 1. The photograph of the antenna isdepicted in Fig. 2, which has a surface area of 18 20 mm2. Although the performance of the monopoleantenna presented here and slot antenna in [12] is differentfrom each other, it should be noted that the total area of the

proposed antenna herein is significantly smaller (40%)than the antenna presented in [12]. Compared with recentUWB antennas in [9] and [10] the antenna is substantially

smaller by 45.5 and 85.6%, respectively.

3 Simulation and measurement results

To validate the proposed design, the fabricated antenna wasmeasured using an Agilent E8363 network analyser (1040 GHz). Fig. 3 presents the simulated and measured returnloss responses. The measured 10 dB return loss bandwidthis from 2.65 to than 13.0 GHz, whereas its simulationreturn loss bandwidth is from 2.6 to 12.7 GHz.The measurement confirms the UWB characteristic of the

proposed antenna, as predicted in the simulation. Themeasured input return loss reasonably agrees withthe simulated results. The discrepancy between tworesponses is attributed to factors such as imperfect solder

joints of the SubMiniature version A connector tomicrostrip feed-line, and manufacturing tolerance. To gainan insight of the effects associated with the antenna

parameters, and the relation between radiation pattern andcurrent distribution over the frequency range of interest, thecurrent distributions of the proposed antenna werecomputed. Fig. 4 shows the current distribution on theradiator and the ground plane of the antenna at 3.4, 8 and11.8 GHz. Fig. 4a shows the electric current distributionFig. 1 Antenna geometry and its design parameters

Table 1 Parameter values of the fabricated antenna

Parameter a b c d e f i k w r 1 r2 h

value, mm 18 20 4 2 2.5 5 1.25 2.5 1.7 8.5 4 1.6

Fig. 2 Photograph of the proposed antenna (front and back views)

1684 IET Microw. Antennas Propag., 2011, Vol. 5, Iss. 14, pp. 16831689

& The Institution of Engineering and Technology 2011 doi: 10.1049/iet-map.2010.0450

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near the first resonance frequency at 3.4 GHz. As shown inFig. 4a, the electric current is concentrated around themiddle of the ground plane and in the lower portion of theradiator patch, which confirms that the feed gap affects theantenna performance at the lower operating frequencies. Onthe other hand, the current distribution on the radiator patchsuggests that the radiator patch has a significant effect on theantenna impedance bandwidth. It is observed in Fig. 4b thatthe current distributions are more complicated than those inFig. 4a. It is mostly concentrated around the lower portionalong the edge of the bowl-shaped patch radiator andvirtually all over the ground plane. That means the ground

plane structure and the feed gap affect the impedance

matching. Current distribution, shown in Fig. 4c, illustratesthat the current is distributed everywhere except for theouter left- and right-hand sides of the ground planestructure, and along the lower curved portions and the uppersides of the bowl-shaped part of the radiator, as well as thesides of the dome part constituting the patch radiator. It can

be elucidated from this that the higher end frequency of the10 dB return loss bandwidth of the antenna is related tothe dimension of the larger semicircle radius. As a result, theimpedance matching at high frequencies is more sensitive tothe larger semicircle radius than the smaller semicircleradius because the electric currents are more intensive thanthose on the upper radiating element. It also can be observedthat the majority of the electric currents are concentrated onthe antennas ground plane at all operating frequencies,which confirms that the ground plane greatly affects theantenna performance. This suggests that the radio frequency(RF) circuitry section of a transceiver cannot be located tooclose to antenna when it is integrated in a system.

The fabricated antennas radiation pattern was measured inan anechoic chamber in the principal orthogonal planes at3.4, 8 and 11.8 GHz, which are shown in Fig. 5. At higherfrequencies as the antenna operates in hybrid modes oftravelling and standing waves [22], the current distributionsare more complicated and this affects the antenna radiation

patterns. The normalised measured results clearly showthe stable omni-directional behaviour in the xz plane

(H-plane) and unsymmetrical bidirectional in the yzplane (E-plane). It is obvious from these results that theomni-directional radiation patterns are acceptable over theUWB bandwidth.

Fig. 3 Measured and simulated return loss of the antenna

Fig. 4 Current distributions of the antenna at

a 3.4 GHzb 8 GHzc 11.8 GHz



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4 Effects of key parameters

To obtain the optimum performance from the antenna, resultsof a parametric study were necessary and are reported in thissection. As will be shown below, the performance of the

proposed antenna is mainly affected by its geometricalparameters, that is, the radiator shape, ground planestructure and feed gap. The proposed structure wasoptimised using Ansoft high-frequency structure simulationelectromagnetic solver (HFSSTM) [23].

4.1 Effect of the larger semicircle radius (r1)

In this study, parameter r1 represents the larger semicircleradius. The current distribution suggests that the larger

semicircle radius of the radiating patch provides thenecessary curvature to the radiator that influences theimpedance bandwidth of the antenna. This is evident inFig. 6, which shows the effect of parameter r1 on theantennas return loss performance when other parametersare fixed as given in Table 1. As shown in Fig. 6, bydecreasing r1 the upper edge of 10 dB frequency

bandwidth shifts down from 12.7 to about 10 GHz,effectively decreasing the antennas impedance bandwidth.

4.2 Effect of the smaller semicircle radius (r2)

As defined in Fig. 1, parameter r2 represents the smallersemicircle radius. As this element is a part of the radiator, itsdimensions have direct effect on frequency characteristics.Fig. 7 shows the results of the variation in r2 when other

parameters are fixed as given in Table 1. The optimum valueof the r2 to achieve maximum bandwidth is 4 mm. Theresults demonstrate that by decreasing r2 the upper andlower edges of 10 dB frequency bandwidth will shift slightlydown and up, respectively. Hence, the antennas impedance

bandwidth decreases marginally.

4.3 Effect of the feed gap (k)

The electric current, which is distributed around the feed gap,as shown in Fig. 4, indicates that it affects the return loss

performance of the antenna. As shown in Fig. 1, the feedgap represented by k is the distance between the radiatingelement and the ground plane. The simulated return loss fordifferent values k is exhibited in Fig. 8. The results showthat by decreasing k, the return loss response improves(between 4.0 6.5 GHz and between 9.5 and 11.5 GHz);however, the impedance bandwidth of the antenna

Fig. 6 Simulated return loss performance as a function of antenna parameter r1

Fig. 5 Normalised measured radiation pattern of the antenna at

a H-planeb E-plane



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decreases. To have a wider impedance bandwidth, the feedgap needs to be optimised. It is noticeable that the values ofk in Fig. 8 cover the UWB spectrum defined by the FCC.

4.4 Effect of the ground plane shape

Since the ground plane acts as a radiator, its dimensionsinfluence the antennas return loss characteristic. Thecurrent distribution indicates that the notch is an important

part of the antenna ground plane structure. Thecharacteristics features of a rectangular notch, a triangularnotch and the notchless case are compared to understandthe function of the notch in the antenna design. In order toexamine the effect of the ground plane notch shape on theantennas impedance matching performance, the notch inFig. 1 was removed while keeping all other antennadimensions constant. Fig. 9 illustrates the results of thesimulation. It can be seen that the antenna without a notchis only able to achieve an impedance bandwidth with a

lower and upper edge frequency of 2.9 and 9.4 GHz,respectively. It is also observed that using the rectangular ora triangular notch cannot cover the UWB bandwidthdefined by FCC. It is therefore clear that the best

performance is achieved by the notch that is proposed inFig. 1 with a side ratio of 2:1.

4.5 Effect of the truncated ground planerectangular notch (c d)

Fig. 4 shows that the RF current is mostly distributed on thetop edges and middle of the ground plane. Accordingly, bytuning the dimensions of the rectangular notch, we cancontrol the antenna impedance bandwidth behaviour.Parameter d is made constant (d 2 mm) and c changed and then vice versa. Fig. 10 shows the return lossresponse of the antenna through the variation of these two

parameters. It can be seen that when c and d arechanged they both can significantly increase or decrease theimpedance bandwidth. The optimum parameter valuesextracted are c 4 mm and d 2 mm.

4.6 Effect of the truncated triangles dimensions

To achieve a wider impedance bandwidth the truncatedground plane needs to be tapered. The current distributionof the antenna in Fig. 4 shows that this portion of the

Fig. 7 Simulated return loss performance as a function of antenna parameter r2

Fig. 8 Simulated returns loss performance as a function of feed gap k



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Fig. 9 Effect of the ground plane notch on the antenna impedance bandwidth

Fig. 10 Simulated return loss performance as a function of ground plane rectangular notch dimensions (c d)

Fig. 11 Simulated return loss performance as a function of ground plane dimensions (e i)



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ground plane has an effect on the 10 dB impedancebandwidth. In this part of the parametric study, thedimensions e and i were changed to show the effect ofthis variation on the antennas performance. First the

parameter i was kept constant (i 1.25 mm) and e changed and then vice versa. Fig. 11 shows the simulationresults for various values of the parameters e and i . Itcan be seen that by increasing e the lower and upper edge

of 10 dB impedance bandwidth improves, and decreasing itcauses the upper edge of 10 dB to shift slightly down. Alsoit is observed that by increasing i the return loss degrades

between 5 and 10.5 GHz and with the decrease of i theupper edge of 10 dB frequency bandwidth shifts down.Although the variation in values of i is small, it can beseen that the 10 dB impedance bandwidth is reduced.

5 Conclusion

A new small monopole antenna, fed by a microstrip line, ispresented for UWB applications. The antenna, whichoccupies an area of 18 20 mm

2, provides an ultra-wide

bandwidth corresponding to an impedance bandwidth of10.35 GHz (2.6513.0 GHz), that is, 132%, exceeding theUWB frequency band defined by FCC. The radiation

patterns of the antenna were measured and presented. It hasbeen observed that the proposed monopole antenna has anomni-directional radiation pattern in the H-plane across amajor portion of its bandwidth. The effect of the salientantenna parameters was investigated to achieve an optimaldesign. In addition, the antenna electric current distributionwas studied to gain a deeper insight of its operation. It has

been shown that the performance of the antenna is mostlydependent on the feed gap, the ground plane structure andthe dimension of the radiator patch. The ultra-wide

bandwidth, small size and low cost make the antennasuitable for the next generation of UWB communicationsystems.

6 References

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21 Koohestani, M., Golpour, M.: U-shaped microstrip patch antenna withnovel parasitic tuning stubs for ultra wideband applications, IETMicrow. Antennas Propag., 2010, 4, (7), pp. 938946

22 Allen, B., Dohler, M., Okon, E.E., Malik, W.Q., Brown, A.K., Edwards,D.J.: Ultra-wideband antennas and propagation for communicationsradar and imaging (John Wiley & Sons, 2007)

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