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1194 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 3, MARCH 2005

Planar Miniature Tapered-Slot-Fed Annular SlotAntennas for Ultrawide-Band Radios

Tzyh-Ghuang Ma, Student Member, IEEE,and Shyh-Kang Jeng, Senior Member, IEEE

AbstractA novel planar tapered-slot-fed annular slot antennais proposedin this paper. Theantenna utilizes a uniquetapered-slotfeeding structure and simultaneously possesses ultrawide band-width, almost uniform radiation patterns, and low profile. It is,hence, adequate forultrawide-band (UWB) applications. By meansof a normalized antenna transfer function, both frequency domainand time domain characteristics of the antenna are carefully in-vestigated. Two measures, the uniformity related to the radiationpatterns and the fidelity associated with the transient behaviors,are used to quantitatively describe the performance of an antennaover such an ultrawidebandwidth. Effects of varying theantennasgeometric parameters on the performance are then investigated.

Finally, the influence of minimizing the antenna dimension is dis-cussed at the end of the paper.

Index TermsAntenna radiation patterns, antenna transientanalysis, slot antennas, ultrawide-band (UWB) antennas.

I. INTRODUCTION

ULTRAWIDE-BAND (UWB) technology has become the

most promising solution for future short-range high-speed

indoor data communication applications. Differing from con-

ventional narrowband communication systems, the UWB

radio directly transmits and receives trains of extremely short

baseband pulses and requires bandwidth of several GHz. It

features high-speed data rates, excellent immunity to multipathinterference, low power consumption and reduced hardware

complexity [1][4]. In 2002, the Federal Communication

Commission (FCC) in United States officially released the

regulations for UWB technology [5]. In [5], the spectrum from

3.1 to 10.6 GHz is allocated for unlicensed UWB measurement,

medical, and communication applications with EIRP less than

dBm/MHz. Additionally, the FCC redefines an UWB

signal as one whose fractional or occupied bandwidth is greater

than 20% or 500 MHz, respectively, on a dB level. Two

distinct schemes, the single-band operation and the multiband

operation, are then proposed. The single-band operation fol-

lows the conventional way to utilize the whole frequency bandand transmits subnanosecond pulses, whereas the multiband

operation divides the allocated spectrum into subchannels to

transmit much broader pulses and eases the challenges in hard-

ware implementations [3]. Though both schemes undoubtedly

Manuscript received March 17, 2004; revised July 17, 2004. This work wassupported by the National Science Council, Republic of China, under GrantsNSC 92-2213-E-002-067 and NSC 93-2213-E-002-091.

The authors are with the Graduate Institute of Communication En-gineering, National Taiwan University, Taipei, Taiwan, R.O.C. (e-mail:

[email protected]; [email protected]).Digital Object Identifier 10.1109/TAP.2004.842648

demonstrate the fascinating future of UWB radios, only the

single-band operation, which is much more sensitive to the

choice of the antenna, will be considered in this paper.

In addition to pulse shape optimizations [6][8], the antenna

implemented in an UWB system plays a more unique role than it

does in other systems. In such a system, the antenna behaves like

a bandpass filter and reshapes the spectraof the pulses. It, hence,

should be designed with care to avoid undesired distortions.

Generally speaking, it is quite challenging to design a suitable

antenna to fulfill all the critical requirements of UWB radios,

including ultrawide bandwidth, omnidirectional patterns, con-stant gain and group delay over the entire band, high-radiation

efficiency, and low profile. Discussions of antennas for UWB

radios are found in [9][17]. Planar monopoles exhibit excel-

lent bandwidth as well as pattern performance [10], [11]. They

have been investigated thoroughly and have become the most

popular designs in UWB systems. Nevertheless, these antennas

suffer from their nonplanar structures. The diamond dipole and

the planar elliptical dipole are both variations of the bow-tie an-

tenna [12], [13]. They are reported to have high electric near

fields and easily cause unwanted coupling to nearby objects

[14]. The magnetic antennas, including large current radiators,

monoloop antennas, and magnetic slot antennas, have been pro-

posed for UWB radios as well [14]. The tapered slot antennas,belonging to traveling wave antennas with endfire patterns, have

also proved to be capable of transmitting baseband pulses with

low dispersions [15]. Though the wide-band characteristics of

the tapered slot antennas appear to be very attractive, their end-

fire patterns definitely draw some constraints on the applica-

tions. Recently, a planar antenna has been proposed to improve

the radiation characteristics of the tapered slot antennas [16],

[17]. This design can be viewed as an antenna consisting of

an ultrawide-band (UWB) tapered slot feeding structure and

dipole-type radiating elements. The radiation patterns of this an-

tenna are more uniform than those of a conventional tapered slot

antenna over the operating band. A detailed study has been car-ried out in [17].

Though the antenna in [16] and [17] exhibits desirable UWB

characteristics, its dimension is still large for most UWB ap-

plications. To further minimize the required antenna dimension

but maintain the UWB properties, in this paper a novel tapered-

slot-fed annular slot antenna is proposed. The size of the newly

proposed antenna is merely half of that of [17]. The geometry

and design guideline of this antenna are introduced in Section II.

With the help of a dimensionless normalized antenna transfer

function, the frequency domain as well as the time domain an-

tenna characteristics are carefully investigated in Sections III

and IV, respectively. Two measures, the uniformity associated

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MA AND JENG: PLANAR MINIATURE TAPERED-SLOT-FED ANNULAR SLOT ANTENNAS 1195

Fig. 1. Geometry of the antenna. (a) Top layer. (b) Bottom layer. (c)Cross-sectional view.

with the radiation patterns and the fidelity related to the tran-

sient properties, are adopted to quantitatively describe the per-formances of UWB antennas in both frequency and time do-

mains. In Section V, the effects of varying the geometric param-

eters on the antenna performance are discussed. To achieve ad-

ditionalflexibility in circuit integration, a miniaturized version

of the proposed antenna is investigated at the end of this paper.

II. GEOMETRY ANDDESIGNGUIDELINES

The geometry of the proposed antenna with its parameters

is depicted in Fig. 1. The antenna lies in the plane and the

normal direction is parallel to the axis. The radiating annular

slot and its tapered-slot feeding structure are on the top layer of

the substrate whereas the microstrip line and its open stub areprinted on the bottom layer of it. The substrate is with height

and dielectric constant . The energy isfirst transferred from

the microstrip line to the slotline by a wideband transition [18].

The tapered-slot feeding structure serves as an impedance trans-

former and guides the wave propagating from the slotline to the

radiating slot without causing pernicious reflection. The radi-

ating slot is then curved to distribute part of the energy to the

reverse side of the feeding aperture. It therefore forms a ta-pered-slot-fed annular slot antenna.

The geometry of this antenna can be mainly determined by

four parameters: , and . The tapered pro-

file of the feeding structure is described by an equation of an

ellipse with the semimajor and semiminor axes equaling

and , respectively. The inner boundary of the annular slot

is described by this elliptical profile and a semicircle centered

at . The radius of this semicircle isfixed to half of the semi-

major axis of the elliptical profile, i.e., . The outer

boundary of the annular slot is depicted with an arc that inter-

sects the midline of the antenna at . This arc is cocentered

at with radius . The width of the an-

nular slot is constant along the concentric arcs and increasesslightly in the vicinity of the feeding aperture. Finally, the pa-

rameter determines both the required matellization around

the radiating slot and the overall antenna dimension.

In designing the antenna, the lowest operating frequency is

first evaluated by

(1)

where is the

approximated longest current path along the inner boundary of

the annular slot, is the speed of light andis the effective dielectric constant. The dimension of the wide-

band microstrip line-to-slotline transition is then designed to

maximize the antenna impedance bandwidth. The highest oper-

ating frequency of the antenna is related to that of the wide-

band transition. After determining the operating bandwidth, the

matellization around the antenna is adjusted to minimize the re-

quired dimension while preserving the impedance bandwidth.

Following this design guideline, two versions of the proposed

antenna, the reference antenna and the miniature antenna, were

designed on a Rogers RT/Duroid 5880 substrate with

mm and . Their geometric parameters are summarized

in Table I. The dimensions of the reference and miniature an-tennas are 46.5 by 66.3 mm and 35.6 by 40.3 mm , respec-

tively, and are only 50.5% and 23.5% that of the antenna in [17].

The measured return losses of the antennas are then depicted in

Fig. 2. The operating bandwidth of the reference antenna with

dB (or ) covers almost the whole

allocated UWB spectrum from 3.1 to 10.6 GHz. The miniature

antenna, a miniaturized version of the proposed antenna, oper-

ates only from 4.8 to 10.2 GHz. Though the miniature antenna

fails to cover the whole UWB spectrum, as will be discussed

in Section V, the corresponding performance does not degrade

significantly. Besides the reference and miniature antennas, six

additional antennas were fabricated on the same substrate to

verify the empirical formula (1), and their corresponding param-eters are also summarized in Table I. According to the table, the

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TABLE IGEOMETRICPARAMETERS OF THEANTENNAS AND THE CORRESPONDINGPREDICTED ANDMEASUREDOPERATINGBANDS

Fig. 2. Measured return losses of the reference and the miniature antennas.

error between the measured and predicted is within 6.5%, and

therefore validates the effectiveness of (1). Discussion of the ra-

diation characteristics of these additional designs will be given

in Section V.

III. FREQUENCYDOMAINCHARACTERISTICS

A. Radiation Patterns and Uniformity

Fig. 3(a) and (b) shows the representative measured radia-

tion patterns of the reference antenna at 6.5 GHz in both and

planes, respectively. According to the figures, the plane

( plane) pattern is rather uniform as expected whereas the

plane ( plane) pattern exhibits dual-polarized properties. The

cross-polarization component in plane is generally much

lower than the dominant one . Though the radiation patterns

inevitably vary with frequencies, the dominantfield component

of the antenna manifests as a donut-like shape up to 8.5 GHz.

The absolute gain of the reference antenna is around 46 dBi for

frequencies up to 9 GHz.

To achieve a more comprehensive understanding of the ra-

diation patterns over the entire band, we define the uniformity

to quantitatively describe the performance of the radiation pat-

terns over an ultrawide bandwidth as [see (2) at the bottom of

the page]. In (2), is the measured radiation pat-tern of the antenna under test (AUT) at a specific plane cut and

frequency, and is the direction where maximum

radiation occurs. This maximum value is taken for every single

frequency and is not involved with the absolute gain. That is, the

uniformity is a statistical parameter related to the normalized

measured radiation patterns, and is defined as the probability

that the deviation of the radiation pattern from its maximum

value is less than 6 dB at a specific plane cut and frequency.

With the uniformity, the dependence of the radiation patterns

on frequencies can then be easily observed.

To experimentally evaluate the uniformity of the proposed an-

tenna, in this paper the radiation patterns were measured in an

anechoic chamber from 1 to 18 GHz with a 0.02125 GHz step.

The spatial angle steps with a 0.9 interval. This corresponds to

a two-dimensional (2-D) sampling of 801 frequency points by

400 spatial points. Only the copolarized component in plane

is taken into account in the following discussion since its uni-

form property is rather preferred in UWB radios. Fig. 4 illus-

(2)

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Fig. 3. Measured radiation patterns of the reference antenna at 6.5 GHz in (a) plane. (b) plane.

trates the uniformity of the reference antenna in plane from 2

to12 GHz.From the figure, we observe that the uniformity curve

exhibits dual peaks over the frequency band of interest and re-

mains almost greater than 0.8 for frequencies up to 8 GHz. It

eventually deteriorates due to some higher order modes at the

upper edge of the operating band. Thefluctuations in Fig. 4 can

be attributed to the insufficient sampling in both frequency and

spatial domains.

B. Antenna Transfer Functions

The antenna transfer functions are another issue of concern,

from which we can judge to what extent the spectra of the pulses

will be modified by the antenna. For ideal UWB applications,

the magnitude of the antenna transfer functions should be as flatas possible in the operating band but drop dramatically outside

Fig. 4. Uniformity of the reference and the miniature antennas in

plane.

the band. The group delay is also required to be constant over theentire band. Various literature has been devoted to evaluating the

antenna transfer functions [19][23]. With the applied voltage

at the transmitting antenna terminal, the output voltage at

the receiving antenna terminal can be expressed by [20]

(3)

where and are the normalized im-

pulse responses (IRs) of the transmitting and receiving antennas,

respectively, and R is the distance between the virtual sources

of the antennas. Converting (3) to frequency domain and using

the fact that , we have [21]

(4)

In (4), and

are defined as

the dimensionless normalized antenna transfer functions of the

transmitting and receiving antennas, respectively, and are mod-

ifications of those in [20] with a normalization factor .

To experimentally evaluate this normalized antenna transfer

function, in this paper the conventional two-antenna gain

measurement arrangement [24] is improved to add the requiredphase information. Referring to Fig. 5, the position and po-

larization of the transmitting antenna were fixed during the

measurement. The AUT or the standard antenna was mounted

on a rotation positioner as the receiving antenna. The virtual

sources of the AUT and standard antenna were first evaluated

[20] and aligned with the rotation center of the positioner.

The transmission scattering parameter in (4) was then

measured by an HP 8722ES network analyzer. Note that the

reference planes were calibrated to the antenna terminals in

advance. The normalized antenna transfer function of the AUT

can now be expressed by

(5)

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Fig. 5. Arrangement for measuring the normalized antenna transfer functions.

where is the measured transmission scatteringparameter of the AUT at a specific angle is

that of the standard antenna in its maximum gain direction, and

is the normalized antenna transfer function of the

standard antenna.

To simplifythe evaluation of , we assume that the

standard antenna is well matched to the measurement system

and has constant group delay over the frequencyband of interest.

With these assumptions, can be derived from [20],

[23]

(6)

where is the ratio of the characteristic impedance of the mea-

surement system to the intrinsic impedance of free space, is

the constant group delay, and and are the ef-

fective height and gain of the standard antenna, respectively. By

substituting (6) into (5), the normalized antenna transfer func-

tion of the AUT can be readily achieved.

In this paper, a Spectrum Technologies International

DRH-118 double-ridged horn antenna was chosen to be

the standard antenna. This antenna has proved to be wellmatched to the measurement system, and with constant group

delay over the frequency band of interest. The group delay

of the standard antenna, which is 630 ps, can be readily de-

termined since the position of the virtual source has been

specified. Referring to the data sheet, the absolute gain can

be obtained as well. Fig. 6(a) then illustrates the magnitude

of the normalized antenna transfer functions of the reference

antenna at in plane. From this

figure, we observe that the transfer functions are ratherflat over

the frequency band of interest except for that at the backward

direction (i.e., 180 in plane). Measured group delays are

depicted in Fig. 6(b) and are nearly constant over the frequency

band of interest. As a consequence, the proposed antenna hasproved to be suitable for UWB radios.

Fig. 6. (a)Magnitudesof thenormalized antennatransferfunctions. (b) Groupdelays of the reference antenna at

in

plane.

The overall response and corresponding total distortion

with a pair of identical antennas are commonly discussed inUWB antenna design [9], [10], [15]. With the normalized

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Fig. 7. Comparisons of the calculated and measured overall response witha pair of reference antennas aligned face-to-face. (a) Magnitudes. (b) Groupdelays.

antenna transfer function, this response can be readily derived

by substituting the corresponding transfer function into (4).

To verify this argument, the overall response with a pair of

reference antennas as the transmitting and receiving antennas

was measured. In the measurement the feeding apertures of the

antennas were aligned face-to-face and the separation distance

R was 6m. In the calculation the normalized antenna transfer

function evaluated at was substituted into (4). Fig. 7(a)and (b) then compares the measured and calculated overall

responses. From the figures, reasonable agreements between

the results can be observed. It validates that the overall response

can be easily determined once the normalized antenna transfer

function has been properly evaluated.

IV. TIMEDOMAINCHARACTERISTICS

In this section, time domain properties of the reference

antenna are investigated using the normalized antenna transferfunctions evaluated in Section III. Referring to the inset of

Fig. 8. (a) Waveform of the incident pulse arriving at thereceivingantenna.(b)Spectrum of the incident pulse normalized to the FCC indoor emission mask.

Fig. 8(a), the incident wave arriving at the receiving antenna is

assumed to be the fourth derivative of a Gaussian function

(7)

where and ns. The normalized spec-

trum of this pulse is illustrated in Fig. 8(b), and proves

to comply with the required FCC indoor emission mask. Fur-

ther refining the pulse spectrum can be achieved by utilizing

some optimization algorithms [6][8]. The pulse spectrum is

then multiplied by the normalized antenna transfer functions

and an inverse Fourier transform is performed to achieve the

required time domain response. The output waveform at the re-

ceiving antenna terminal can therefore be expressed by

(8)

where represents an ideal bandpass filter from 1 to 18

GHz. Fig. 9(a) and (b) illustrates the received waveforms at thereference antenna terminal as equals 0 , 90 , 180 , and 270

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Fig. 9. Received waveforms at the reference antenna terminal as (a)

. (b)

in

plane.

in plane. The range related effects are well calibrated from

the normalized antenna transfer functions so that only the group

delays of the reference antenna are observed here. According to

thefigures, well-behaved received pulses are demonstrated and

the late time ringing is almost negligible. Again, it manifests the

applicability of the proposed antenna in UWB radios.

In a UWB radio, a template pulse needs to be generated

independently in the receiver for pulse detections. A well-de-

fined parameter namedfidelity is then proposed to evaluate thecapability of pulse detection of an antenna [25][27]

(9)

The fidelity in (9) is in essence the correlation coefficient of

the template and received pulses. It quantitatively describes how

similar the received pulse is to the template pulse. The fidelity

reaches unity as the two pulses are exactly the same in shape,

which means the receiving antenna does not distort the incidentpulse at all. Fig. 10 shows the fidelity of the reference antenna

Fig. 10. Fidelity of the reference and the miniature antennas in plane.

in plane with the template pulse being the same as . Ac-cording to thefigure, thefidelity is mostly greater than 0.8 and

even better than 0.9 for ranging from 0 to 60 and 310

to 360 . It validates that the reference antenna does not dis-

tort the incident pulse significantly. The asymmetry in the curve

can be mostly attributed to the unwanted radiation from the mi-

crostrip radial stub in the transmission line transition. Note that

though the selections of the incident as well as template pulses

inevitably affect the evaluations of thefidelity, the discussion in

this section still applies without loss of generality.

V. DISCUSSION

In this section, various aspects of the proposed antenna willbe discussed. These include the effects of varying the geometric

parameters on the antenna performance and discussion about the

miniaturized version of the proposed antenna.

First the effects of varying the antenna geometric parameters

were investigated. As mentioned in Section II, the geometry of

the proposed antenna is principally determined by four param-

eters , and . Among them, the width of the

annular slot is the most significant parameter that affects

the antenna radiation characteristics. Fig. 11(a) and (b) shows

the variations of the uniformity andfidelity with respect to the

variations of (Designs I, III, and IV), respectively. Refer-

ring to Fig. 11(a), the uniformity degrades as the slot width be-comes wider. This is because the energy tends to radiate from

the feeding aperture rather than the desired annular slot as

increases. The antenna is hence more similar to a conventional

tapered slot antenna. Thefidelity in Fig. 11(b) exhibits a sim-

ilar trend as well. Referring to the figure, it degrades and even

drops to 0.7 as the slot width equals 12 mm. As a consequence,

it is concluded that in designing the antenna the width of the an-

nular slot should be properly adjusted to avoid excess radiation

directly from the tapered-slot feeding aperture.

The semimajor and semiminor axes of the tapered profile de-

termine the lowest operating frequency of the antenna, as dis-

cussed in Section II. Nevertheless, these two parameters are

found to have limited effects on the fidelity. The variations ofthe semimajor axes are from 12 to 18 mm (Designs I, V, and VI)

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Fig. 11. Variations of the (a) uniformity and (b)fidelity with respect to thevariations of the slot width

(Designs I, III, and IV).

and the variations of the semiminor ones are from 15 to 25 mm

(Designs I, VII, and VIII). In addition, the fidelity is proved to

be insensitive to the variations of ( , and mm) as

well. Due to the limited space, these results are not shown here.

The substrate also has some effect on the antenna performance.

Utilizing a high permittivity substrate can effectively reduce the

antenna size, nevertheless the radiation efficiency decreases cor-respondingly. Regarding the substrate thickness h, the effects

are minor as long as a higher order surface or space wave is not

excited.

The performance of the miniature antenna was then inves-

tigated. As mentioned in Section II, the impedance bandwidth

of this antenna does not cover the whole allocated UWB spec-

trum. As for the radiation characteristics, Figs. 4 and 10 illus-

trate the uniformity and fidelity of the miniature antenna, re-

spectively, along with those of the reference antenna. In Fig. 12,

the peak-to-peak amplitudes [20] of the received pulses of both

the reference and miniature antennas are also depicted. Apart

from the fact that the uniformity, fidelity and peak-to-peak am-

plitudes of the miniature antenna are generally lower than thoseof the reference antenna, they still remain acceptable over the

Fig. 12. Peak-to-peak amplitudes of the received waveforms of the referenceand the miniature antennas in plane.

frequency band of interest. The reason can be explained as fol-lows. Due to the insufficient impedance bandwidth of the minia-

ture antenna, it inevitably introduces more distortions to the in-

cident pulses. However, as long as the operating band of this

antenna covers the spectrum in which the pulse energy concen-

trates, the antenna performance can be mostly preserved. With

this observation, further miniaturizing the antenna dimension

to addflexibility in circuit integration can be readily achieved

even though the antenna itself may not cover the entire allocated

UWB spectrum.

VI. CONCLUSION

In this paper, a novel UWB tapered-slot-fed annular slot an-

tenna has been proposed and demonstrated. With the help of a

dimensionless normalized antenna transfer function, the perfor-

mance of the proposed antenna has been experimentally inves-

tigated. The overall response with a pair of identical antennas

can be also derived with this transfer function. Meanwhile, two

quantitative measures, the uniformity andfidelity, are evaluated

from the measured data and reveal that this antenna not only

exhibits almost uniform radiation patterns but also introduces

limited distortion to baseband signals. It therefore substantiates

the applicability of the proposed antenna in UWB radios. Ef-

fects of varying the geometric parameters on the antenna per-

formance and miniaturizing the antenna dimension with someperformance tradeoff have also been discussed. The future work

will be in examining the effects of pulse shape optimization on

the antenna performance.

ACKNOWLEDGMENT

The authors would like to express their sincere gratitude to the

reviewers, Dr. C.-H. Tseng and Dr. S.-C. Yen for their thoughtful

comments, and to J. Long for reading the revised manuscript.

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Tzyh-Ghuang Ma (S00) was born in Taipei,

Taiwan, R.O.C., in 1973. He received the B.S. andM.S. degrees in electrical engineering from NationalTaiwan University, Taipei, R.O.C., in 1995 and 1997,respectively.

He is currently working toward the Ph.D. degreewith the Graduate Institute of Communication En-gineering, National Taiwan University. From 1997to 1999, he served in the Navy of the Republic ofChina. His research interests include mobile antennadesigns, electromagnetic theory and numerical

techniques, UWB antenna and passive circuit designs.

Shyh-Kang Jeng (M86SM98) received theB.S.E.E. and the Ph.D. degrees from NationalTaiwan University, Taipei, Taiwan, R.O.C., in 1979and 1983, respectively.

In 1981 he joined the faculty of the Departmentof Electrical Engineering, National Taiwan Univer-sity, where he is now a Professor. From 1985 to 1993,he visited the University of Illinois, Urbana-Cham-paign, as a Visiting Research AssociateProfessoranda Visiting Research Professor. In 1999 he visited theCenter for Computer Research in Music and Acous-

tics, Stanford University, CA, for half a year. His research interest includesnumerical electromagnetics, UWB wireless systems, music signal processing,

music information retrieval, intelligent agent applications, and electromagneticscattering analysis.

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