A Bandwidth Improved Circular Polarized Slot ANT Using a Slot Composed of Multiple Circular Sectors

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 59, NO. 8, AUGUST 2011 3065

VI. CONCLUSION

A novel dual circularly-polarized monofilar spiral slot antenna was

modeled, fabricated and tested. The proposed antenna can achieve

RHCP and LHCP for the low and high frequency bands, respectively.

The antenna realizes an 18% impedance bandwidth for both bands

and AR bandwidths of 4.5% and 3.5% with respect to the centre

frequencies of 1616 MHz and 2655 MHz, respectively.

REFERENCES

[1] H. W. Kwa, X. M. Qing, and Z. N. Chen, “Broadband single-fedsingle-patch circularly polarized antenna for UHF RFID applications,”in Proc. IEEE Antennas and Propagation Society Int. Symp., July2008, pp. 1–4.

[2] X. L. Bao and M. J. Ammann, “Compact annular-ring embedded cir-cular patch antenna with a cross-slot ground plane for circular polar-ization,” Electron. Lett., vol. 42, no. 4, pp. 192–193, 2006.

[3] F. Jou, J. W. Wu, and C. J. Wang, “Novel broadband monopole an-tennas with dual-band circular polarization,” IEEE Trans. Antennas

Propag., vol. 57, no. 4, pp. 1027–1034, 2009.[4] C. H. Chenand E. K. N. Yung, “Dual-band dual-sense circularly-polar-

ized CPW-fed slot antenna with two spiral slots loaded,” IEEE Trans.

Antennas Propag., vol. 57, no. 6, pp. 1829–1833, 2009.[5] X. L. Bao and M. J. Ammann, “Dual-frequency Dual-sense circularly-

polarized Slot antenna fed by microstrip line,” IEEE Trans. Antennas

Propag., vol. 56, no. 3, pp. 645–649, 2008.[6] W. L. Curtis, “Spiral antennas,” IRE Trans. Antennas Propag., vol. 8,

pp. 298–306, May 1960.[7] C. J. Wang and D. F. Hsu, “A frequency-reduction scheme for spiral

slot antenna,” IEEE Antennas Wireless Propag. Lett., vol. 1, pp.161–164, 2002.

[8] R. T. Gloutak and N. G. Alexopoulos, “Two-arm eccentric spiral an-tenna,” IEEE Trans. Antennas Propag., vol. 45, no. 4, pp. 723–730,1997.

[9] D. J. Muller and K. Sarabandi, “Design and analysis of a 3-arm spiralantenna,” IEEE Trans. Antennas Propag., vol. 55, no. 2, pp. 258–266,2007.

[10] N. A. Stutzke and D. S. Filipovic, “Four-arm 2nd- mode slot spiral an-

tenna with simple single-port feed,” IEEE Antennas Wireless Propag. Lett., vol. 4, pp. 213–216, 2005.

[11] J. M. Laheurte, “Dual-frequency circularly polarized antennas basedon stacked monofilar square spirals,” IEEE Trans. Antennas Propag.,vol. 51, no. 3, pp. 488–492, 2003.

[12] C. W. Jung, B. A. Cetiner, and F. De. Flaviis, “A single-arm circularspiral antenna with inner/outer feed circuitry for changing polarizationand beam characteristics,” in Proc. 2003 IEEE Antennas and Propaga-

tion Society Int. Symp., pp. 474–477.[13] H. Nakano, J. Eto, Y. Okabe, and J. Yamauchi, “Tilted- and axial-

beam formation by a single-arm rectangular spiral antenna with com-pact dielectric substrate and conducting plane,” IEEE Trans. Antennas

Propag., vol. 50, no. 1, pp. 17–23, 2002.

A Bandwidth Improved Circular Polarized Slot Antenna

Using a Slot Composed of Multiple Circular Sectors

Sai Ho Yeung, Kim Fung Man, and Wing Shing Chan

Abstract—A circular polarized (CP) slot antenna is designed with a slot

composed of multiple circular sectors (MCS). The design has advantagesof having a wide 3 dB axial ratio (AR) bandwidth of 57.4%, achieving agood AR smaller than 2 dB in most areas of the frequency range. The de-sign of the antenna follows a multi-objective optimization procedure thatapplies computational power rather than human tuning. A comparison of the optimized antenna design with other wideband CP antennas in the lit-erature shows that it has advantages of having a smaller physical size in thecross-sectional area than the antennas with multiple feeding structures anda wider operating bandwidth than all the compared antennas.

Index Terms—Circuit optimization, circular polarization, slot antennas,wideband.

I. INTRODUCTION

Circular polarized (CP) antennas [1]–[5] radiate electromagnetic

waves with circular polarization. They are useful in satellite commu-nication and global positioning systems (GPS) because the linearly

polarized wave can be rotated as it passes through the ionosphere, but

the circular polarized wave will not be affected [6], [7]. Moreover,

communication systems with CP antennas also provide better flex-

ibility in the orientation angle between the transmitter and receiver

[8]. The use of CP antennas can also reduce multipath reflections and

other interferences [8].

Various CP antennas have been developed. To operate CP antennas,

two orthogonal modes with equal amplitude but in phase quadrature

should be excited. An elliptical dielectric resonator antenna (DRA) ex-

cited by a probe has the advantage of small size and energy efficiency

and achieves 5.2% AR bandwidth [9]. A stair-shaped DRA with aper-

ture feed possesses the same advantages and achieves 10.2% axial ratio

(AR) bandwidth [10]. The patch antenna with a U-slot, truncated cor-

ners, and L-shaped probe (L-probe) feed has the advantage of small

size with measurement result of 16.6% AR bandwidth [11].

To increase AR bandwidth further, other antenna configurations can

be used, but the antenna would be larger. An antenna array with four CP

antenna elements with sequential feeding and rotation of the elements

can be designed to enhance AR bandwidth [12], [13]. A four sequential

feed elliptical CP DRA subarray with a hybrid ring-feeding network

can provide a wide AR bandwidth of 26.1% [13]. However, the antenna

size increases since the subarray consists of four radiating elements and

a feeding network.

Multiple feeding structures can also provide wideband characteris-

tics for CP antenna design [8], [14]. A quadruple strip feed cylindrical

DRA canprovide a wide CP bandwidth of 25.9% [8]. However, the sizeof the antenna increases with the inclusion of the broadband power di-

vider with quadrature phase distribution. A circular patch antenna fed

by a switch line balun with printed L-probes can provide broadband

operation with 41% AR bandwidth [14].

Manuscript received June 14, 2010; revised November 21, 2010; acceptedJanuary 15, 2011. Date of publication June 09, 2011; date of current versionAugust 03, 2011. This work was supported by the Hong Kong RGC GeneralResearch Fund (GRF) under Project CityU 119308(9041373).

The authors are withthe CityUniversity of HongKong,Kowloon, HongKong(e-mail: shyeung@ ee.cityu.edu.hk).

Color versions of one or more of the figures in this communication are avail-able online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2011.2158953

0018-926X/$26.00 © 2011 IEEE



3066 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 59, NO. 8, AUGUST 2011

Fig. 1. A simple wide-slot antenna.

The designs of wide-slot antennas with L-probe feeding can excite

two orthogonal field components for CP antenna operations [15]–[17].

Wide-slot antennas with L-probe feeding also have the wideband

characteristic of wide-slot antennas [18], [19]. A recent design of a

L-probe feeding wide-slot antenna with an L-shaped slot [17] can

provide 46.5% bandwidth. In this communication, the bandwidth is

further extended by replacing the L-shaped slot with a slot composedof multiple circular sectors (MCS) [20]–[22]. The MCS slot consists of

a number of radii parameters of the sectors to maximize the operating

bandwidth of the antenna, hence improving the bandwidth to 57.4%.

MCS structures have been previously used to shape the phase shifters

[20] and the ultra-wideband antenna [21].

Although designs for an MCS slot CP antenna have already been

presented [22], the experimental result yielded a mere 46.0% AR band-

width. This result is not the best performance that can be achieved using

the MCS slot structure. Thus, this communication will further improve

the result to yield a measured AR bandwidth of 57.4%.

The communication is organized as follows. In Section II, the

antenna configuration of the MCS slot CP Antenna is introduced. In

Section III, the optimization of the antenna is discussed. In Section IV,the performance of the optimized antenna is presented. In Section V,

the current distribution of the antenna is investigated. In Section VI,

the designed antenna is compared with other wideband CP antenna

designs in the literature. Finally, conclusions are given in Section VII.

II. PRINTED WIDE-SLOT ANTENNA CONFIGURATION

Printed wide-slot antennas are useful in satellite and communica-

tion applications [18], [19]. The antennas are printed on double-sided

substrates. A simple configuration of wide-slot antenna to generate a

linear polarized wave is shown in Fig. 1. The lower side of the substrate

consists of the microstrip feeding line, while the upper side of the sub-

strate is the ground plane with a wide radiating slot. The simple wide

slot antenna can be analyzed using the equivalence principle, in whichthe aperture is closed and then replaced by a pair of magnetic surface

currents below and above the ground plane [18]. The advantages of

wide-slot antennas include wide operating bandwidth, good isolation,

and negligible radiation from feed network [18].

For the wide-slot antenna design in [15]–[17], the microstrip feeding

become L-shaped probe (L-probe) as shown in Fig. 2 for CP wave

excitation. The E

component is produced by the vertical part of the

L-probe, while theE

component is produced by the horizontal part of

the L-probe [15]. Sincethe electricallength of the horizontal part of the

L-probe is designed to be 90 , the current phase on the horizontal com-

ponent lags behind that of the vertical components by 90 [16]. There-

fore, there is a 90 out-of-phase orientation betweenE

andE

of the

polarization, which eventually generates the CP radiation. Among the

CP wide-slot antennas, different shapes of slots give different returnloss, AR, bandwidth, and radiation pattern characteristics. The use of

Fig. 2. MCS slot CP antenna.

an L-shaped slot can achieve 46.5% 3 dB AR bandwidth with a trun-

cated corner [17] and 40% bandwidth without a truncated corner [15].

The length of the L-shaped wide-slot is chosen as half wavelength at

around the lowest operating frequency [15]. If a circular slot is used

instead, the AR bandwidth improves to 58% [16]. In this antenna, the

measured0 1 0 d B S

1 1

bandwidth is from 2.4 to 4.3GHz, while the AR

bandwidth is 2.3 to 4.2 GHz. Therefore, the overlapped bandwidth is

from 2.3 to 4.2 GHz (54.5%). Although the bandwidth is very wide, the

main beam direction is tilted away from the broadside in both xy- and

yz-planes, probably due to the unsymmetrical slot shape of the modi-

fied circular structure [16]. Moreover, itsS

1 1

in some frequency points

is larger than 0 1 0 d B in the measurement result of the operating band-

width [16]. Since different shapes of the wide-slot result in differentreturn loss, AR, bandwidth, and radiation pattern characteristics, fur-

ther optimization of the shape is necessary to enhance the antenna’s

performance.

To improve the return loss, bandwidth, and AR of the CP wide-slot

antenna, and to optimize the radiation pattern so that the main beam

direction is in the broadside, the slot proposed in this communication

is composed of many optimized circular sectors. The configuration of

the MCS slot CP antenna is shown in Fig. 2. The MCS slot CP Antenna

is fabricated using a double-sided printed circuit board with a substrate

thickness of 1.5 mm and a dielectric constant of 2.65. The top layer of

the substrate consists of an L-shaped feeding path with a shorting pin

located at the end of the path. The bottom layer of the substrate consists

of a slot composed of 18 circular sectors, eachhaving a different radius,

and occupying an angle of 20 . These 18 circular sectors form a com-

plete circular shape of 360 , and are shown in Table I. Aside from the

radii parameters of the MCS, other dimensional parameters, as shown

in Fig. 2, include l , w

h

, w

x

, w

l

, l

h

, l

x

, and s

x

. To increase the broad-

side gain of the antenna, a copper reflector is placed h mm below the

substrate with a size of ( l + 2 0 ) 2 ( l + 2 0 ) m m .

Since the MCS structure consists of many radii parameters, it pro-

vides a large number of degrees of freedom in the design of the slot

[20]. Hence, the MCS structure provides the capacity to enhance the

return loss, AR, bandwidth, and radiation characteristics.

III. ANTENNA OPTIMIZATION

There are 26 dimensional parameters for the MCS slot CP antenna

design, and thus determining all the parameters through humantrial-and-error tuning is extremely difficult. Instead, the parameters




TABLE IPARAMETERS OF THE MCS SLOT

are determined using computational power through multi-objective

optimization algorithm. Genetic algorithm [23]–[27], particle swarm

optimization [28], and simulated annealing [29] can all be used for

optimization because these methods have been successfully applied

to optimize similar problematic structures such as a folded patch feed

antenna with multiple optimization objectives [25]. Genetic algorithm

has also been applied to CP dielectric resonator antenna optimization

[26] and ultra-wideband antenna optimization in [27].

In this communication, the MCS slot antenna is optimized based on

a multi-objective genetic algorithm [24]–[27]. “Multi-objective” refers

to the ability of the algorithm to handle multiple optimization objec-

tives. Pareto-dominance concepts are used to distinguish the quality of

the solutions with different objective values [23], instead of combining

the objective values as a weighted sum. Each solution in the population

will be ranked through a non-dominated sorting procedure adopting

the Pareto-dominance principle [24]. For detailed optimization theories

and procedures for multi-objective optimization, interested authors can

refer to [23], [24].

The optimization algorithm will determine all 26 dimensional pa-

rameters of the antenna design based on 3 optimization objectives. The

first optimization objective is to minimize S

1 1 for a desired frequencyrange. This optimization objective can be formulated as follows:

M i n i m i z e F

1

= m a x

f 2 F

f S

1 1

( f ) g (1)

where S

1 1

( f ) represents the antenna return loss at frequency

f : F

s

= f 2 : 2 ; 2 : 3 ; 2 : 4 ; . . . ; 3 : 6 ; 3 : 7 g

is the set of the sampled fre-

quency of the design frequency band. This optimization objective

minimizes the maximum S

1 1

within the selected frequency band.

The second optimization objective is to minimize the axial ratio of

the antenna, which allows the antenna to yield circular polarization.

This can be achieved through the following optimization objective:

M i n i m i z e F

2

= m a x

f 2 F ; 2 f 0 5 ; 0 ; 5 )

f A R ( f ; ; = 0

) g (2)

Fig. 3. Optimized MCS slot CP antenna.

Fig. 4. Relationship between the radii and angle of the MCS slot.

where the notation AR is the axial ratio at frequency f in the angular

direction of ( ; ) , for which and are the elevation angle and the

azimuth angle from the observation point, respectively. The minimiza-

tion of F

2

will reduce the axial ratio withinthe selected frequency band.

The third optimization objective is to minimize the change of gain

along the frequency axis. This can be achieved through the following

optimization objective:

M i n i m i z e F

3

= m a x

f 2 F

f E

R H

( f ) g 0 m i n

f 2 F

f E

R H

( f ) g (3)

where the notation is the right hand polarized electric field gain at fre-

quencyf

. The minimization of F

3

will reduce the interval between

the largest and the smallest maximum antenna gain within the selected

frequency band. During the optimization process, the antenna charac-

teristics are simulated using IE3D.1

All the dimensional parameters of the antennas are optimized

sing genetic algorithm. This results in the antenna configuration

shown in Fig. 3. The optimized dimensional parameters are given

by l = 8 6 : 4 m m , w

h

= 7 : 9 m m , w

x

= 1 : 5 m m , w

l

= 3 : 0 m m ,

l

h

= 1 0 : 8 m m , l

x

= 3 4 : 9 m m , s

x

= 0 : 7 m m , and h = 3 4 : 1 m m .

The radii of the MCS are given in Table I. The relationship between the

radii and the angle of the MCS slot is plotted in Fig. 4 for reference.

IV. ANTENNA PERFORMANCE

The optimized antenna is fabricated for measurement and a photo-

graph of it is shown in Fig. 5. Total size of the circuit including the sub-

strate is9 6 : 4 m m 2 1 0 6 : 4 m m

; and is placed in front of a reflector that

measures1 0 6 : 4 m m 2 1 0 6 : 4 m m

. Results for the return loss is shown1IE3D is a trademark of Zeland Software, Inc.




Fig. 5. Fabricated MCS slot CP antenna.

Fig. 6. Return loss of the MCS slot CP antenna.

Fig. 7. Axial ratio of the MCS slot CP antenna at the direction of ( ; ) =

( 0 ; 0 ) .

in Fig. 6. A 0 1 0 d B fractionalbandwidth of 64.7% (2.06–4.03 GHz) is

obtained in the simulation, compared with 63.4% (2.08–4.01 GHz) ob-

tained in measurement. Both simulation and experimental results agree

well and show a good return loss within the entire bandwidth.

Simulated and measured results for the axial ratio are shown in

Fig. 7. In the simulation results, the 3 dB axial ratio fractional band-

width is 61.5% (2.14–4.04 GHz). For an axial ratio smaller than 2 dB

the frequency range covers 2.16–3.90 GHz. In the experimental results,

the 3 dB axial ratio fractional bandwidth is 57.4% (2.16–3.90 GHz).

While for an axial ratio smaller than 2 dB the frequency range covers

2.18–3.83 GHz. Experimental results agree well with the simulation

results, with both results providing a good axial ratio smaller than2 dB over most of the desired frequency range.

Fig. 8. Measured radiation pattern for = 0 plane.

Fig. 9. Measured radiation pattern for = 9 0 plane.

Fig. 10. RHCP gain of the MCS slot CP antenna at the direction of ( ; ) =

( 0 ; 0 ) .

Measurement results of the radiation pattern at = 0

and =

9 0

planes are shown in Figs. 8 and 9, respectively. The antenna is

a right hand circular polarized (RHCP) antenna that provides a large

RHCP gain and a small left hand circular polarized gain at the broadside

direction. Back radiation is small because of the reflector placed at the

rear of the antenna.

Simulation and experiment results of the RHCP antenna gain are

shown in Fig. 10. Both simulation and experimental results agree well

with each other, with a flat gain over the majority of the bandwidth.

The measured peak gain is 8.3 dBi at 2.2 GHz, whereas the gain is 6.7

dBi at 3.0 GHz which is near the centre frequency.

The antenna efficiency and radiation efficiency of the antenna sim-

ulated with IE3D are shown in Fig. 11. The antenna efficiency and the

radiation efficiency of the antenna are larger than 84.8% and 90.1%,

respectively, in the frequency range of 2.14–4.04 GHz (simulated ARbandwidth). This shows that the antenna has high efficiency.




TABLE IICOMPARISON BETWEEN THE MCS SLOT CP ANTENNA AND THE OTHER CP ANTENNAS.

Fig. 11. Simulated antenna efficiency and radiation efficiency.

V. CURRENT DISTRIBUTION ANALYSIS

To understand the physical behavior of how the antenna operates the

magnetic current distribution of the CP antenna at 2.2 GHz is given in

Fig. 12. Since the simulation of magnetic current is based on infinite

ground plane model in IE3D, it may not be very accurate but is enough

to demonstrate the CP operation mechanism of the antenna.

Three time intervals within a cycle of CP radiation are shown in

Fig. 12. In the figure, the sizes of the red arrows correspond to the

magnitude of the magnetic current. The more important information

in the figure is the direction of the magnetic current. In all the three

time intervals, the magnetic current circulates around the center of the

MCS slot, which contributes for the CP radiation.

VI. COMPARISONS

A comparison between ourMCS slot CP antenna andother wideband

CP antennas are presented in Table II. The comparisons are mainly onthe impedance and axial ratio bandwidths, aswellas onthe size in terms

Fig. 12. Magnetic current distribution of the MCS slot CP antenna.

of the cross-sectional area of the antenna. The MCS slot CP antennahas the axial ratio bandwidth of 57.4%, which is wider than that of the




other CP antennas in the comparison [8], [11], [14], [17] (ranging from

10.4% to 46.5%). Moreover, its size in terms of cross-sectional area

including the ground plane and the reflector ( 1 0 6 : 4 m m 2 1 0 6 : 4 m m )

is smaller than those of the antennas in [8], [14] (which are2 0 0 m m 2

2 0 0 m m

, and1 5 0 m m 2 1 5 0 m m

, respectively). A disadvantage of the

MCS slot CP antenna is that its size is slightly larger than that of the

wideband CP antenna with an L-shaped slot [11] ( 8 2 m m 2 8 2 m m ) .

VII. CONCLUSIONS

An MCS slot CP antenna design has been presented. The measured

axial ratio bandwidth is 57.4%, which is larger than the bandwidth

of the CP antenna with an L-shaped slot of 46.5% [17]. The antenna

provides a good axial ratio smaller than 2 dB in the majority of the

bandwidth. The antenna is designed through computational power

using multi-objective optimization algorithm to eliminate human

tuning, which is both difficult and time consuming. The antenna is

compared with other wideband CP antennas in the literature, demon-

strating its advantages of having a wide operating bandwidth and a

small size in terms of the cross-sectional area.

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Documents

A Bandwidth Improved Circular Polarized Slot ANT Using a Slot Composed of Multiple Circular Sectors