Design and Simulation of Ultra Wide-bandAntenna for High Data Rate Applications

Ramaswamy Karthikeyan B1, Govind R.Kadambi21 M.Sc (Engg.) student, 2 Professor and Dean-Research,

Wireless and Optical CommunicationM.S.Ramaiah School of Advanced Studies, Bangalore

AbstractThe UWE radio application covering the frequency spectrum of 3.1-10.6 GHz offers a potentially attractive solution forwireless system applications. The associated benefits of UWE technology are bandwidth. cost. power consumption andconsistent high data rates. However for system implementation of UWE technology. design and development of antennasconstitute a critical role in determining the viability and engineeringfeasibility ofUWB.

One of the primary objectives of this paper is to formulate a design concept of utilization of planar monopole ofcompact size and relatively simpler structural configurations for UWB systems. This paper presents design and simulationstudies on UWB planar monopole configuration comprising the geometric profiles of canonical shapes such as rectangular.ellipse and circular. This paper also covers the novelties and the design advantages of a slot on the radiating structure ofplanar monopole for the design implementation to accomplish UWEperformance. The concept of tuning stubs located on theslot contours of the planar monopole has been illustrated to realize the ultra wide bandwidth of the antenna. This paper alsobrings out a relative performance comparison of various monopole profiles. The sizes of the radiating element as well as thatof the ground plane of the proposed design are significantly smaller than that presented in earlier reported designs. Theconcept of open slot on a planar monopole and the design of the tuning stubs are major contributions of this paper to antennaengineering.

With a view to place emphasis on the nature of the radiation characteristics of UWB antenna. thus paper also dealswith a typical study on the development of Vivaldi antenna. Vivaldi antenna has the distinct feature of exhibiting end-fireradiation pattern.

Key Words: UWB Antenna, Planar Monopole Antenna and Vivaldi Antenna

1. Introduction

UWB is an unique and new usage of recently legalizedfrequency spectrum. UWB radios can use frequencies from3.1 GHz to 10.6 GHz - a band of7.5 GHz wide. Each radio

channel of UWB system can have a bandwidth of more than500 MHz, depending on its center frequency [1-3].

The antenna design faces new challenges due to thedistribution of the wide span of UWB band with specialemission masks. The requirements for conventional antennasused for UWB or impulse systems include a broadimpedance bandwidth, a stable transmit-receive transferresponse, and high radiation efficiency.

In the past, many designers and researchers havestudied / analyzed such antennas in great detail. Transverseelectromagnetic mode horns and self-similar spiral antennasare typical broadband solutions but suffer from the severelyfrequency-dependent changes in their phase centers.Biconical and disk-conical antennas feature stable phasecenters across broad bandwidths by using resistive loads tosuppress the reflection occurring at their ends. However, theconventional broadband antennas mentioned above usuallyare too bulky to be suitable for portable UWB devices [4-5].The antenna structures like spiral, log periodic antennas aredifficult to be modeled using commercial EM softwares.Design optimization involving the significant dimensions isnot a straightforward proposition [6-7].

In the design of microstrip antenna proposed bySeok H. Choi et al. [8], the realized bandwidth was partiallydue to non existence of ground plane under a section ofmicrostrip antenna. The design presented by Takashi Aritaet. al. [9], was lacking the compactness as well as designversatility. In the analysis of Johnna Powell [10], the Horn,biconical and helical antennas are suitable for UWB but theyare 3D and non planar in structure. The planar geometrieslike bowtie, diamond dipole and rectangular loop antennasdo not have bandwidths broad enough for UWB.

In rectangular planar monopole design of GirishKumar et al. [II], the authors are not successful in realizingthe bandwidth sufficient for UWB applications. Further it isalso observed that the dimensions required for theachievement of bandwidth for UWB with certain

configurations are significantly large. The Vivaldi antennaproposed by Raviprakash Rajaraman [12] does not possesthe UWB bandwidth for its single element.

In view of the above observations there is a need

to look into the alternative design solutions for UWBantennas, which are characterized with desirable featureslike compactness, design versatility and provision forrelatively easy tuning. This paper attempts to undertake adesign methodology of UWB antennas, which overcomessome of the shortcomings of the existing solutions.

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2. GEOMETRIC MODEL

Planar monopole antenna can be designed in manyradiator geometric profiles like Rectangular, Circular,Elliptical, Triangular and unsymmetrical shapes. Since theantenna is designed for UWB system, its compactness andmobility are of major concern while designing the antenna.The radiator is modified with the different canonical shapeslike Rectangular, Circular, Elliptical Monopoles and thelength of the ground plane is fixed at 25 mm. However thesurface area of radiator is identical for all the geometricshapes. The Dielectric substrate is chosen as FR-4 ofDielectric constant 1:,=4.3. The thickness of the metallizationover the substrate is 0.02mm.

2.1 Rectangular Monopole

The rectangular monopole is designed with radiatordimension of 20 x 21 mm. The radiator is modified by theintroduction of slots and the feed point is varied for itsoptimum impedance matching, which is explained in detaillater.

T1I1linc Stub.

Substrate

Radiator

L1IIUped Pori at

OptUnwn Point

--_. Ground plane

Fig I: Rectangular Monopole

The optimized design geometry of the rectangularmonopole is shown in the Fig 1. The rectangular monopoleantenna shows a return loss better than -10 dB across the

UWB spectrum of 3.1-10.6 GHz.

2.2 Circular Monopole

The circular monopole antenna has been optimizedfor the radiator dimension of radius 11.4mm. The

introduction of slots and matching stubs are important tocontrol the bandwidth and the resonance. The Fig 2 showsthe optimized design of circular monopole for UWBapplications.

Slots

Tuning Stubs

Radiator

Substrate

Strip Connector to thebase of radiator

Lumped Port atOptinnun Point

•• Ground plane

Fig 2:Circular Monopole

2.3 Elliptical Monopole

The design of the elliptical monopole has beenoptimized with the radiator dimensions of major axis 16mmand minor axis 10.4mm. The elliptical slots are introducedfor better impedance bandwidth. The effects of slots, tuningstubs and change in the feed point are also described indetail in later sections.

Slots

- Tuning Stubs (I1 and T2)

Radiator

Strip Connector to thebase of radiator

- Substrate

Lumped Port atOptimum Point

Ground plane

Fig 3: Elliptical Monopole

2.4 Vivaldi Antenna

The Vivaldi antenna illustrated in Fig 4 consists oftapered slot etched on one side of a double-sided PCB. Theslot is narrower towards one end. The traveling wave, whichpropagates along the tapered slot, radiates in the end-firedirection. It has a very wide pattern bandwidth and has thecapacity to generate a symmetric main beam despite itsplanar geometry.

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The very important aspect of the Vivaldi antenna is that theradiator is the etched part of the ground plane. Theoreticallythe bandwidth of the Vivaldi antenna can be infinite, but islimited by its physical size and the fabrication capabilities

141mm

j

Dielectric Substrate

Top Metallization

Tapered Slot

Slot line Transition

Feeding Stub

Microstrip Port

There are many techniques to optimize the planar antennaperformance and some of the designs have been reviewed.Here the experimentation is carried out with the variation offeed point, introduction of slots with open end and tuningstubs on the slot contour.

3.1 Feed point Variation

The determination of the feed point is an importantparameter in the antenna design. In the initial designsimulation, the feed point is assumed to be at the mid pointalong the width of the planar monopole. In subsequentdesign iterations, the feed point is moved incrementallyalong the width of the planar monopole to determine theoptimum impedance match.---36.6mm-

3. SIMULA nON OF ANTENNA DESIGNS

The basic design dimensions of the antenna are arrived atfrom theoretical as well as empirical basis and drawn in theGanymede Graphical editor, which comes along with theEmpire 3D Electromagnetic field solver.

3.1.2 Feed point variation in Circular monopole

The feed point of the circular monopole cannot be changedin view of its circular shape. So a small of strip of height0.25 mrn is connected at the base of the radiator and the

lumped feed port is connected to the strip shown in Fig 2.The feed port can be moved along the strip to locate theoptimum feed point and it is placed at 1.5mm towards rightfrom the center of the radiator.

3.2 Effect of slot to shift the resonance frequencyWhen the Radiator of the chosen dimensions does not yieldthe bandwidth for UWB operation, the slot can be introducedon the radiator, which increases the perimeter of the radiator,i.e. increased length for the current flow path, which in turnwill shift the resonant frequency of the antenna. The widthof the slot also affects the design. The dimensions of the slotare optimized for desired results.

3.1.3 Feed point variation in Elliptical monopole

Like in circular monopole, the elliptical monopole designalso involves the attachment of a strip of height 0.25mrn atthe base of the radiator as shown in Fig 3. In the specificdesign optimization, the feed point is located at 2.4mmtowards the right with respect to center of the ellipticalmonopole.

3.1.1 Feed point variation in Rectangular monopole

If the antenna is fed at the extreme left edge of the radiator,there will be two resonant frequencies but the bandwidth isnot enough for UWB applications. If the feed point is in thecenter, it exhibits three resonant frequencies. Yet it does notshow UWB performance. The precise location of the feedpoint is determined by varying the position of the feed pointby a step of 0.5 mm and repeating the simulation. Theoptimum feed point for return loss of -I OdB between 3.1-10GHz, is located in between the left edge and the mid pointalong the width ofthe monopole.

3.2.1 Slots on the Rectangular monopole

In the rectangular monopole geometry, both the vertical andhorizontal slots of width 1.5mm are introduced as shown in

Fig I. Trimming the length of the vertical slot improves the

le+lO8e+9

Fig 4: Vivaldi Antenna

2e+9

·12

·14

·16o 4e+9 6e+9

frequency in Hz

Fig 5: SII plot for Rectangular monopole without slots andtuning stubs.

Fig 5 shows the return loss plot of a rectangular planarmonopole antenna without slot and tuning stub. As can beseen from the plot, the bandwidth of the rectangular planarmonopole antenna is not very wide despite many resonantfrequencies with in the UWB spectrum. The primary reasonfor decreased bandwidth can be attributed to intermediate

peaks of the return loss plot with magnitude higher than ­10dB. The presence of the intermediate peaks makes thereturn loss plot a sequence of discrete narrow bands withseparation between them. One of the alternatives to increasethe bandwidth of the rectangular planar monopole antenna isto reduce the magnitude of the intermediate peaks to belower than -10 dB. In order to obtain the continuous

bandwidth for ultra wideband spectrum, the experimentationis carried out further.

·6

·8

·10

o

·2

-4

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o

The feed point is moved along a strip connected at the base(bottom) of the radiator. The optimized feed point position isdetermined by successive comparison of iterations

1e+10 1.2e+10

1e+10 1.2e+104e+9 6e+9 8e+9

frequency in Hz

4e+9 6e+9 8e+9

frequency in Hz

Fig 7: SII plot of Circular Monopole

2e+9

2e+9

Fig 6: SII plot of Rectangular Monopole

·25

·30o

·30

-35

-40o

·25

·20

5115

o

·5

·15

·15

·10

·10

·5

·20

4.2 Circular MonopoleThe design optimization of the circular monopole antennainvolving the determination of precise feed point as well asthe locations and sizes of the tuning stubs helped to realize areturn loss better than -10 dB covering 2.1 - 11.7 GHzfrequency range.

4.1 Rectangular Monopole

The return loss plot of the designed rectangular planarmonopole is shown in Fig 6. The horizantal and vertical slotsas well as the tuning stubs introduced on the radiator havethe advantageous feature to realize very wide bandwidth(2.3 - II GHz) for return loss better than -I OdB.

S115

impedance match at the higher frequencies. Trimming thelength of the horizontal slot reduces the number of resonantfrequencies.

The Vivaldi antenna is designed with slot profile ofexponential taper. Initially, the minimum width of the slot is1.5mm and the maximum width of the slot is 12.8mm. The

location of maximum width of the slot is along the edge ofthe ground plane. The choice of these parameters results inthe three resonant frequencies at 3.4, 5.5, and 8 GHz. Thenminimum width of the slot is reduced to O.5mm, themaximum width is increased and the tapering is alsoadjusted based on the required results. The microstrip feedline is moved up and down to improve the impedancematching. The structure of the feed stub design is alsochanged and optimized for improved performance.

3.3 Tuning stubs for improved performanceSmall stubs are used for tuning the performance of theantenna to realize the return loss of -lOdB in the entire

UWB spectrum. The stubs used are, metallized strip, whichare placed on the slots as shown in Figs 1,2 and 3. Thelocation of the stubs along the slot contour helps to tune theantenna for the desired performance. In the designed UWBantennas, two tuning stubs denoted as TI and T2 are used. Inthe rectangular monopole antenna, T1 is placed on thevertical slot and T2 is placed on the horizontal slot.

The simulations are carried out with the open end of the slotat the center; If that slot opening is moved towards right itproduces more number of resonances in the first half of theUWB spectrum and the peaks of the return loss plot of therectangular monopole appearing in the upper half of theUWB spectrum are found to be above -IOdB. If the openend of the slot is moved to the left, all the peaks of the returnloss plot of the rectangular monopole are of magnitude lessthan -lOdB and thereby yielding very wide bandwidth.

3.2.2 Slots in the Circular Monopole

In circular monopole geometry, circular slot of 1.5mm widthis introduced with the slot opening of 3 mm as shown in theFig 2. The antenna radiated well even without slots andcovered the UWB band for -8.5dB of return loss. In order to

reduce the peaks of the return loss plot at 7.5GHz, the slot isintroduced.

3.4 Design Optimisation of Vivaldi Antenna

3.2.3 Slots in the Elliptical Monopole

Similar to the circular monopole geometry, the elliptical slotof width 1.5mm is introduced on the radiator shown in Fig 3.The position and the width of the open end of the slot areoptimized to obtain the return loss better than -lOdBcovering the UWB spectrum.

4. RESULTS AND DISCUSSION

The return loss (SII) plot of the planar monopole antennasand Vivaldi antennas obtained through simulation arediscussed below.

4.3 Elliptical MonopoleThe elliptical monopole geometry is optimized withoutchanging the location of open end of the slot. Similar to the

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circular monopole the feed point is moved along the strip atthe base of the radiator and the optimized feed point isidentified

SU2

0-2-4·6·8-10-12-14-16-18-20

02e+9

Fig 8: SIt plot of Elliptical Monopole

The elliptical slots and the tuning stubs on the slot contour ofthe radiator significantly reduced the amplitude of peaks ofthe return loss response to below -10 dB. The realizedbandwidth of elliptical planar monopole antenna for -10 dBreturn loss is 2.9 - 12 GHz.

4.4 Vivaldi Antenna

The following are the techniques used to improve theimpedance bandwidth of the Vivaldi antenna.

• Changing the position of the microstrip feed• Widening Flare Opening• Increase/ Decrease the linear slot width

• Varying the length of the Feed stub• Changing the shape ofthe Feed stub

Fig 9: S\I plot of Vivaldi Antenna

The Vivaldi antenna of Fig 4 provides the bandwidth of 5.7GHz from 5.3 - 11 GHz, for the return loss < -lOdB, which

can be useful with the UWB devices operating at the second

half of the UWB spectrum. The return loss plot shows threedeep resonances, as shown in the Fig 9.

5. VALIDATION AND COMPARISON

Table 1 summarizes the results of all the planar monopoleantennas proposed in this paper. The monopole antennas arewith slots and tuning stubs on their radiating structures.

RadiatorBandwidth

Peak

ConfigurationDimensions (mm)

(GHz)Gain

(dB)Rectangular20 x 21

2.3 - II+6Monopole

CircularRadius = 11.42.1-11.7+8

Monopole

Elliptical

Major axis=162.9 - 12+4

monopoleMinor axis = 10.4

Table 1: Summary of simulated planar

monopole Results

In [I I], the authors had utilized a ground plane of size 300 x300 mm in their design. In the proposed paper, the size ofthe ground plane is only a small fraction (= I/I oth) of that in[11]. Further the design of the rectangular monopole of[II]does not appear to be intended for UWB.

RadiatorBandwidthConfigurationDimensions

(GHz)(mm)Rectangular

46 x 421-2.0

Monopole CircularRadius = 25

1-10.2Monopole

EllipticalMajor axis = 521-10.7MonopoleMinor axis = 48

Table 2:Results of Wide band Planar Monopole [II]

In fact a bandwidth of only 2 GHz was realized forrectangular monopole of [I I]. In the design of this paper,UWB performance has been realized even though theradiator and ground plane dimensions are significantlysmaller than that of [11]. This substantiates the novelty ofthe slot and tuning stubs proposed in this paper.

6. CONCLUSIONS

The antenna design for ultra wideband spectrum is not a newtopic to the antenna designers, but the geometries,complexities and the technical approach for the desireddesign solutions vary with individual applications. One ofthe primary objectives of this paper is to evolve a designconcept of utilization of planar monopole of compact sizeand relatively simpler structural configurations for UWB

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