Compact Band-Rejected Ultrawideband Slot

Compact Band-Rejected Ultrawideband Slot Antennas Inserting With =2and =4Resonators

Zhi-An Zheng,Qing-Xin Chu ,Member,IEEE ,and Zhi-Hong Tu

Abstract—Two compact ultrawideband (UWB)slot antennas with band-rejected characteristics are proposed for UWB com-munication applications.The entire size of both antennas is as small as 15

mm 22

mm 08mm ,which is smaller than most of the proposed UWB antennas.The radiator is a stepped slot excited by a 50 feed line.By etching a half-wavelength and a quarter-wavelength slot from the ground plane of UWB slot antennas,respectively,two UWB antennas with band-notched characteristics can be obtained.The potential interference be-tween UWB and WLAN/WiMAX can be reduced.Omnidirectional radiation patterns and constant gain can be observed in the pass bands.Time-domain characteristics of both proposed antennas are analyzed under different conditions.It demonstrates that the time-domain characteristics of the antennas are good enough for UWB applications.The results show that proposed antennas are very suitable for various portable UWB systems.

Index Terms—Band-rejected UWB antenna,compact size,slot antenna,time-domain characteristics.

I.I NTRODUCTION

S

INCE the Federal Communications Commission (FCC)’s allocation of the frequency band 3.1–10.6GHz for com-mercial use [1],ultrawideband (UWB)system has attracted sig-ni?cant research power in the recent years.As one of main issue of UWB systems,UWB antenna has received increased attention [2].Most of the proposed antennas have UWB perfor-mances of the impedance matching,radiation stability and con-stant https://www.360docs.net/doc/e6331421.html,ually,the size of such printed antenna with broad impedance bandwidth is about 40

mm 50mm to 25

mm 25mm [3]–[5].As the requirement of reducing the size of the UWB communication system,especially those portable systems,the miniaturization of UWB antennas becomes a signi?cant topic.Recently,some UWB antennas with more compact size have been proposed [6]–[9].The size of the compact UWB antenna is about 13

mm 30mm in [6],[7].The antenna with smaller size was proposed,but the 10dB impedance bandwidth is only from 3.1to 5GHz [8].On the other hand,there are some other wireless communication systems in the UWB band,such as wireless local-area network (WLAN)IEEE 802.11a oper-ating in the 5.15–5.825GHz band.Besides WLAN,in some Eu-rope and Asia countries,world interoperability for microwave access (WiMAX)service from 3.3to 3.8GHz also operates in

Manuscript received March 19,2010;revised June 24,2010;accepted June 28,2010.Date of publication December 03,2010;date of current version Feb-ruary 02,2011.This work was supported in part by the Science Fund of China (U0635004)and in part by the Science Fund of Guangdong (No.60571056).The authors are with the School of Electronic and Information Engineering,South China University of Technology,Guangzhou 510641,China (e-mail:zhian.zheng@https://www.360docs.net/doc/e6331421.html,;qxchu@https://www.360docs.net/doc/e6331421.html,).Digital Object Identi?er 10.1109/TAP.2010.2096399

the UWB band.As a result,the potential interference between UWB and the two wireless communication systems should be minimized.However,using ?lters will increase the complexity of the UWB systems.Therefore,various UWB antennas with one or dual even triple notched bands have been proposed in recent years [10]–[13].The notched bands are mainly imple-mented by etching slots on the radiator or adding some stubs around the radiator or feed line.The lengths of those etched slots or additional stubs are about quarter-wavelength or half-wavelength corresponding to the designed notched frequencies [12]–[16].Moreover,the notched band can be achieved by con-trolling the resonant frequency of the antenna [17].

It is well known that the ?rst resonant frequency mainly de-pends on the size of the antenna [18].With smaller dimen-sions,the ?rst resonant frequency of the UWB antenna becomes higher.Therefore,to the same type antennas,if the start fre-quency of the antenna band is bigger than 3.1GHz,the size of the antenna can be designed to be smaller than those whose band starts from 3.1GHz.To miniaturize the antenna size and the potential interferences between UWB system and the two nar-rowband systems,the impedance bandwidth of UWB antenna is suggested to start from 3.8GHz,but not 3.1GHz as normal.The band of 3.1–3.3GHz can be negligible compared to the whole UWB band.Therefore,the dimension of the antenna can be reduced.As the UWB antennas are employed to transmit/re-ceive time domain transient pulse signal,the key is whether the time-domain characteristics will be affected or not.Some work has been done by authors in the study of time-domain character-istics of UWB antennas [19],[20].Based on the previous work,the time-domain characteristics of proposed antennas are ana-lyzed.

In this paper,two band-notched UWB slot antennas with

the size

of

are proposed.They are very small in dimension and simple in structure.With such a compact dimension,the 10dB impedance bandwidth of the antenna covers the band of 3.8to more than 11GHz.A Z-shaped slot and a split rectangle ring slot were etched nearby the slot radiator in the ground plane,respectively.Both of them can achieve a notched band in frequency range from 5.15–5.825GHz,and the notched band can be adjusted easily by varying the width and length of the slot.Good agreements are observed between the simulation and measurement.More-over,the H-plane radiation patterns are nearly omnidirectional as the frequency increases.By analyzing the correlation factor and pulse wide stretch ratio,the time-domain characteristics of proposed antennas are demonstrated to be good enough.The results show that the proposed antennas are very suitable for the use of portable UWB systems.

0018-926X/$26.00?2010IEEE

Fig.1.Geometry of the UWB slot antenna(antenna

1).

Fig.2.Measured and simulated re?ection coef?cients of antenna1.

II.A NTENNAS D ESIGN

Generally,the radiators with gradient,tapered or cambered

shapes can implement wide-band function[12],[21].Similarly,

the slot antennas with those shapes can achieve wide-band char-

acteristics as well[5],[22].Fig.1is the geometry of a UWB

antenna printed on a substrate with

permittivity and

thickness of0.8mm.A stepped slot in the ground plane is ex-

cited by a

50microstrip line.The stepped radiator can result

in a transition from one resonant mode to another.By using An-

soft’s High Frequency Structure Simulation(HFSS)software to

optimize the parameters,good impedance match over a broad

frequency range can be obtained.As shown in Fig.2,the mea-

sured and simulated bandwidth of the antenna is from3.8to

more than11GHz.

To achieve band-notched characteristic,slots have been

etched on UWB antenna patch as discussed in[12],[13].Each

slot acts as a resonator,and the resonant frequency mainly

depends on the length of the slot.The length of the slot is

about a half or a quarter of the wavelength corresponding to the

resonant frequency

[14].

Fig.3.Geometry of band-notched UWB slot antenna with half-wavelength slot

(antenna2).

The required notched band is5.15–5.825GHz,so the de-

signed resonant frequency is set to be about5.5GHz.The wave-

length can be calculated

by

(1)

where is the speed of the

light,is the effective dielectric

constant,

and is the designed resonant frequency.Two slots

were etched from the previous UWB slot antenna to implement

the notched band,respectively.Both of the two slots can achieve

the required notched band at5.5GHz.But the lengths and loca-

tions of the slots are different.If the slot is placed in the radiator,

the length is about a half of the wavelength.While one end of the

slot is open,the length is only about a quarter of the wavelength.

The geometries of the antennas with band-rejected function are

shown in Figs.3,4.The half-wavelength slot is a split rectangle

ring with the two ends surround the radiating slot to enhance the

coupling between them,and the radiating slot is in the middle of

the opening.The length of the split rectangle

ring can

be deduced

by

(2)

The quarter-wavelength slot is a Z-shaped slot,and one end of

the slot is open.The length of the Z-shaped

slot can be

deduced

by

(3)

The optimized parameters obtained in Ansoft HFSS are as

follows:

,

,

,

,

,

,

,

,

,

,

,

,

,

,,

Fig.4.Geometry of band-notched UWB slot antenna with quarter-wavelength slot (antenna

3).

Fig.5.Photograph of fabricated antennas (front view and back views of an-tenna 1,antenna 2,antenna

3).

,

,

,

,

.

III.R ESULTS AND D ISCUSSION

For simplicity,the antennas in Fig.1,Fig.3and Fig.4are named antenna 1,antenna 2,and antenna 3,respectively.The three antennas were fabricated and measured using Agilent N5230A vector network analyzer (VNA).The photographs of the three fabricated antennas are shown in Fig.5.The designs of antenna 2and antenna 3are based on antenna 1.A.Frequency-Domain Characteristics of Antenna 2

The measured and simulated re?ection coef?cients of antenna 2are plotted in Fig.6.The simulated bandwidth is 3.8–5.15and 5.9–11GHz,and the measured result is 3.7–5.2and 6.5–11GHz.The measurement and simulations match well.Because of the machining accuracy,the centre frequency of the notched band is shifted a little,and the measured bandwidth is a little bigger than simulation.The simulated result of antenna 1is added for comparison.Fig.7depicts the simulated re?ection

coef?cient of the antenna with

different

and .The centre frequency of the notched band can be changed by adjusting the total length of the split rectangle ring slot.Increasing the length of the slot,the centre frequency of the notched band will accord-ingly decrease.

The measured E-plane (XZ)and H-plane (YZ)radiation pat-terns at 4.2,6.8and 9.6GHz are normalized and

illustrated

Fig.6.Measured and simulated re?ection coef?cients of antenna 2and antenna

1.

Fig.7.Simulated re?ection coef?cients of antenna 2with different L and L .

in Fig.8.Each radiation pattern is normalized to itself.The H-plane radiation patterns are observed to be nearly omnidirec-tional over the whole UWB frequency range but slightly dete-riorated at high frequencies.The antenna was measured in the outdoor test system due to the limit of condition.As a result,the radiation patterns in some orientations are not good enough.The measured and simulated peak gains of the proposed antenna are plotted in Fig.9.The antenna gain is nearly constant with sharp decrease occurs in the designed notched band.Moreover,the ef?ciency of the antenna has been measured in 4–6GHz [23],[24],and the measured and simulated ef?ciencies are superim-posed in the bottom right corner of Fig.9.It can be observed that both the measurement and simulation decrease sharply in the frequency of about 5.5GH,and the simulated results are bigger than the measurements.It is because of the dielectric loss of the substrate and the conductor loss,and the effect of the SMA con-nector and the soldering will in?uence the measurement as well.Furthermore,the variation trends of the measurement and sim-ulation are the same.

B.Frequency-Domain Characteristics of Antenna 3

The measured and simulated re?ection coef?cients of the antenna 3are presented in Fig.10.The simulated bandwidth is 3.76–5.12and 5.84–11GHz,and the measured result is

Fig.8.Normalized measured radiation patterns of antenna2.(a)E-plane(X-Z

plane);(b)H-plane(Y-Z

plane).

Fig.9.Measured and simulated peak gains of antenna2.(measured and simu-

lated ef?ciencies in the bottom right corner).

3.87–5.18and5.97–11GHz.The measurement and simu-

lations match well.Due to the machining accuracy,the?rst

resonant frequency is shifted a little.The dielectric constant

and dissipation factor are not stable with increasing frequency,

so there are some differences between simulation and measure-

ment in high frequency.The simulation of antenna1is added

for comparison as well.The simulated re?ection coef?cient

of

Fig.10.Measured and simulated re?ection coef?cients of antenna3and an-

tenna

1.

Fig.11.Simulated re?ection coef?cients of antenna3with different L.

antenna3with

different is shown in Fig.11.The centre

frequency of the notched band can be varied easily by adjusting

the total length of the Z-shaped slot.With a shorter slot,the

centre frequency of the notched band will correspondingly

increase.

The measured E-plane(XZ)and H-plane(YZ)radiation

patterns at4.1,6.8and10GHz are normalized and plotted in

Fig.12.It is noticed that the omnidirectivity of the H-plane

radiation patterns is generally good but will be deteriorated

as frequency increases.The measured and simulated peak

gains are shown in Fig.13.Sharp gain decrease occurs in the

designed rejected-band as expected.Besides,the measured

and simulated ef?ciencies of antenna3among4–6GHz are

superimposed in the bottom right corner of Fig.13.The results

are similar to that of antenna2.

IV.T IME-D OMAIN C HARACTERISTICS OF P ROPOSED

A NTENNAS

The UWB antennas are employed to transmit/receive time-

domain transient pulse signals,so the time-domain characteris-

tics of the UWB antennas are very important.The signal propa-

gation characteristics of the UWB antennas have attached much

attention[25],[26].With excellent time-domain characteristics,

little distortion will be introduced by the UWB antenna when

Fig.12.Normalized measured radiation patterns of antenna 3.(a)E-plane (X-Z plane);(b)H-plane (Y-Z

plane).

Fig.13.Measured and simulated peak gains of antenna 3.(measured and sim-ulated ef?ciencies in the bottom right corner).

transmitting or receiving pulse signals.Authors have done some

work on it [19],[20].

In this paper,the time-domain characteristics analyses were carried out with CST microwave studio software.

To satisfy the FCC spectral mask for indoor systems,the an-tenna is assumed to be excited by the UWB signal suggested

in

Fig.14.(a)Antenna input signal,(b)Fourier transform of the shrunken input signal.

[27],[28].This UWB signal is the 5th-derivative of the Gaussian pulse and is given by

(4)

(4)

Here,is a constant that can be chosen to comply with peak power spectral that FCC will permit

and has to be 51ps to ensure that the shape of the spectrum complies with the FCC spectral mask [26].The input signal and the Fourier transform of the shrunken input signal are shown in Fig.14.

In [20],correlation

factor

and pulse width stretch ratio (SR)have been proposed to evaluate the time-domain charac-teristics of the UWB antennas.

Correlation factor of

signals

and is de?ned

by (5)

where is a delay which is varied to make numerator in (5)a maximum.