I Wireless Corner

Christos Christodoulou Department of Eledrical and Computer Engineering

University of New Mexico

Albuquerque, NM 87131-1356 USA Tel: +1 (5 05) 277 6580 Fax: +1 (505) 277 1439

E-mail: christos@ece.unm.edu

Eva Rajo-lgl8sias Departamento de Teona de la Senal y Comunicaciones University Carlos III of Madrid, Oespacho 4.3810 Avenida de la Universidad, 30,28911 Leganas, Madrid, Spain Tel: +34 916248774 Fax: +34 916248749 E-mail: eva@tsc.uc3m.es

Wideband Circularly Polarized E-Shaped Patch Antenna for Wireless Applications

Ahmed Khidre, Kai Fang Lee, Fan Yang, and Ate' Eisherbeni Center of Applied Electromagnetic Systems Research (CAESR), Electrical Engineering Department

The University of Mississippi University, MS 38655 USA

E-mail: ahmed.khidre@ieee.org. leek@olemiss.edu, fyang@olemiss.edu, atef@olemiss.edu

Abstract

A new technique to achieve a circularly polarized probe-fed single-layer microstrip-patch antenna with a wideband axial ratio is proposed. The antenna is a modified form of the conventional E-shaped patch, used to broaden the impedance bandwidth of a basic patch antenna. 8y letting the two parallel slots of the E patch be unequal, asymmetry is introduced. This leads to two orthogonal currents on the patch and, hence, circularly polarized fields are excited. The proposed technique exhibits the advantage of the simplicity of the E-shaped patch design, which requires only the slot lengths, widths, and position parameters to be determined. Investigations of the effect of various dimensions of the antenna have been carried out via parametric analysis. Based on these investigations, a design procedure for a circularly polarized E-shaped patch was developed. A prototype has been designed, following the suggested procedure for the IEEE 802.11 bIg WLAN band. The performance of the fabricated antenna was measured and compared with simulation results. Various examples with different substrate thicknesses and material types are presented and compared with the recently proposed circularly polarized U-slot patch antennas.

Keywords: Circular polarization; axial ratio; E-shaped patch; microstrip antennas; wideband; wireless communication; local area networks

1. Introduction

C ircularly polarized (CP) microstrip antennas are attractive for wireless applications because they combine advantages of

both microstrip antennas and circular-polarization characteristics. Microstrip antennas have a planar profile, low cost, and mechani­cal robustness, while circular polarization can reduce the transmis­sion loss caused by the misalignment between antennas of station­ary and mobile terminals. Circular polarization can thus provide better mobility, weather penetration, and system-performance

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

enhancement than linear polarization. Circularly polarized micro­strip antennas have been widely used in different wireless applica­tions, such as GPS and RFID systems.

Many studies and research efforts have been reported in the literature directed toward obtaining circularly polarized microstrip antennas. These antennas can be classified according to the number of feeds and the number of layers used. In general, single-feed sin­gle-layer structures yield the smallest axial-ratio CAR) bandwidth, defined at the 3 dB level. However, these structures are attractive because of their simplicity. They do not need external circuitry or

219

larger space compared to configurations consisting of multi-feed, multi-layer, or sequential-feed configurations [ I]. Techniques used to obtain circular polarization in single-feed, single-layer micro­strip-antenna configurations include trimming opposite comers, cutting a narrow slot at 45° to a square patch, or adding tabs to an elliptical patch [2]. However, the axial-ratio bandwidth obtainable using these techniques is usually less than I %, which limits their practical applications.

For the case of linear polarization, a number of impedance­broadening techniques have been extensively investigated and developed in the past. In particular, the V-slot [3, 4], L-probe [5], and E-shaped patches [6] have gained a lot of popularity. This is because compared to other methods, they do not require a complex matching network or stacked configuration. More recently, the V­slot and L-probe techniques have been applied to single-layer sin­gle-feed circularly polarized antennas [7-10]. These techniques enable the use of thick substrates, and yield axial-ratio bandwidths as large as 13%.

In this paper, the well-known technique of an E-shaped patch used for a broad impedance bandwidth [6] is modified to generate circularly polarized radiation. This leads to a new alternative method of acquiring circular polarization, using a single-layer probe-fed patch antenna with a relatively wideband axial ratio, without the necessity of it being square or comer-trimmed. A sys­tematic design procedure is also demonstrated by various design examples. Finally, a comparison with the V-slot technique recently published in [7, 8] for circularly polarized single-layer microstrip antennas is given.

L

Lsi

Ws! 1 W F •

I Ls2

z

I y

Figure 1. The antenna geometry.

220

.. ..

!� I . ----.

.� I .. A. ...

-----------. • I I

-------,� Figure 2. The current flow across the patch: (top) linearly polarized E-patch, (bottom) circularly polarized E-patch.

2. Antenna Configuration and Circularly Polarized Mechanism

2.1 Geometry

The antenna geometry is shown in Figure I. It consists of a modified E-shaped patch antenna with unequal parallel slots. Both slots have the same width (Ws). The patch, with dimensions Wand L, is placed at a height h from the ground plane. The medium between the patch and the ground plane is air. The antenna is fed with a coaxial probe at position F from the middle of the right edge of the patch, as shown in Figure I.

2.2 The Circularly Polarized Design

The conventional E-shaped patch is shown in Figure 2a. Its radiation is linearly polarized and symmetric in the xz plane. To make this antenna circularly polarized, the x-directed current needs to be equal in magnitude to the y-directed current, with a 90° phase

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

, ,

,

, ,

, ,

,

,

,

90" ase

180" phase

, ,

, ,

, ,

Figure 3. The current-vector distribution on the circularly polarized E-shaped patch, and zoomed portions of it for four different phase states.

, ,

, ,

, ,

, ,

,

, ,

,

O· phase

/' -, 270· phase

, ,

, ,

,

ISO· phase

, ,

, ,

,

Figure 4. The current-vector distribution on the linearly polarized E-shaped patch and zoomed portions of it for four different phase states.

difference. This is accomplished by making one of the slots in the E-patch shorter than the other. This introduces an asymmetric cur­rent around the yz plane, which increases the x-directed current such that its magnitude is almost equal to the y-directed current, as shown in Figure 2b. The antenna dimensions are then adjusted such that these two orthogonal currents are in phase quadrature.

It is instructive to present the current-vector distribution on both optimized linearly polarized and circularly polarized E-

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

shaped patches as a function of the source-phase variation. Fig­ures 3 and 4 show the current-vector distribution on the circularly polarized and linearly polarized E-shaped patch at 2.45 GHz, respectively. The zoomed parts at the 0°, 90°, 180°, and 270° phase states mimic current animations. From Figure 3, at 0° cur­rent flows in the x direction, while at 90°, current flows in the y

direction. Similarly, at 180° and 270°, both currents are in opposite directions. This implies quadrature phase between the x- and y­directed currents. It is also clear that the tips of the current vectors rotate in a circular path with a phase progression, which depicts circular polarization. It is therefore expected that Land W, together with the two slot lengths, control the resonant length of the x- and y-directed currents. That is why there is no need for the patch to be nearly square or comer-trimmed. Instead, circular polarization is achieved by incorporating unequal parallel slots into the patch, and adjusting their length, width, and position so that the phase differ­ence between the orthogonal currents is 90°.

On the other hand, from Figure 4, at 0°, current flows in the y direction, while at 90° and 270°, current is almost zero (due to the existence of cross polarization). At 180° it then flows in the oppo­site direction to the flow at 0°. The tip of the current vector there­fore alternates between the "+" and "-" y directions during one cycle. This implies linear polarization. The next section shows how to adjust the antenna's dimensions to obtain these circularly polar­ized characteristics, and demonstrates the physical effect of various parameters.

3. Parametric Analysis and Design Procedure

3. 1 Parametric Study

Basically, W and L control the resonant lengths of the x and y orthogonal currents excited across the edges of the patch. The incorporated parallel slots change the electrical length of both cur­rents, and hence they have significant effects on the resonant lengths. We start with the conventional linearly polarized E-shaped patch to cover the 2.45 GHz WLAN band. The dimensions of such an antenna are tabulated in Table I. Details of the E-shaped patch antenna design were discussed in [6].

When the slot LsI gets shorter, it is expected that the axial ratio will decrease. Figures 5 and 6 show the effect of LsI on the

return loss represented by SII' and on the axial ratio, respectively,

based on simulation results obtained using Ansoft's HFSS [ I I]. As expected, the shorter is LsI, the higher is the resonant frequency. Making Ls I shorter also drastically improves the axial ratio. It

drops from 40 dB at LsI = 40 mm (the case of linear polarization)

to around 7 dB at LsI = 20 mm (elliptical polarization). A further decrease in the length of LsI won't improve the axial ratio; rather, it again begins to increase. These are consistent results, because LsI changes the electrical path length of the x-directed current. At a certain length of LsI, the phase difference between the orthogo­nal currents therefore approaches 90°. Beyond this critical value, the phase difference between the orthogonal currents retreats from 90°. It was found that the optimum value of LsI for the current

antenna configuration was 17 mm, which gives the minimum axial-ratio level.

The goal after this step is (a) to improve the axial-ratio level so it is above the 3 dB level across the desired band of operation, and (b) to align the impedance and axial-ratio bandwidths together. The probe position is expected to play a critical role for impedance

221

Table 1. The dimensions of the linear polarized E-shaped patch in mm.

w L 76 4S

Ws lsI Ls2 7 40 40

p F 14 10

h 10

O ��==�:-�:-:-��I : ..... , . " -5 "·, .... ... ;.... ... . . . . . . . .. . . . ... . " ' ;1> : ' '" ,

- 1 0 \' .' " -::--->"" i co � '. "!, ' ''''

� -15 . . \. L . -'' . , . -.'<,/ (J)

\: : .- f 1-" .>,,, .... 1 . . �.-, ... ,�. :'. . . .... , ... 1 .--'---,--'-:---:--::-'1"--20

-25

.................. , ......... ' ..... '\.� ...... . ... , ... - Ls1= 10 . '\ j ---Ls1=20 ,'\ .I

. . . . ..

. . . . . . . . : . . . · · · · · · · ; \ r " ; .. -'-" Ls 1 =30 .

. \,/ Ls1= 40 -30!--�--"-i,=---=-i,-�---,,i=--=�=====:i,-�

2 2 .1 2.2 2.3 2.4 2 .5 2.6 2.7 2.8 2.9 3 Frequency(GHz)

Figure 5. Sll at different values of lsI while the other parame­

ters in Table t were fixed.

40 ���r-��--� ... �--�======� i - Ls1=10 ...... ,,/.: ..

- -' ---Ls1=20 30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... .. Ls1=30

fg 25 .. ..... .. - - 'Ls1=40 o ............... . � 20· n::: , � 15', � ' ...

10 ....... �, _________________

�"

5· ................ ...... .......... .

°2�������� �������� 2 . 1 2.2 2 .3 2.4 2.5 2 .6 2.7 2 . 8 2 . 9 3 Frequency( GHz)

Figure 6. The axial ratio at different values of LsI while the other parameters in Table I were fixed.

-5

'" ... :-� .... , . .... , .. ... c ........ . .. .

, :. \" :''i iiJ-10 .. ·· .. ·, , · /\

� , :\ (i) -15 .

-F=10 -20 . ---F=16

.... · .... F=26

\ . .... Lc.

\ ' " I \..'

• . .. , .. , .. � ... , II 01 . .. '·" ,,·,,"',,···,·· .. .. •••

-25����������������� 2 2.1 2 .2 2.3 2.4 2 .5 2.6 2.7 3

Frequency( GHz)

Figure 7. SJ I at different values of F and Lsi = 17 mm while the

other parameters in Table 1 were fixed.

222

Figure 8. The axial ratio at different values of F 1 and LsI = 17 mm while the other parameters in Table 1 were fixed.

-5

iiJ-10 ......... , . . � (i) -15 .

-20 ---Ls2=35 .......... ·Ls2=44

-25 L-�--�������--������ 2 2.1 2.2 2.3 2.4 2.5 2.6 2. 7 2.8 2 .9 3

Frequency(GHz)

Figure 9. S) J at different values of Ls2 while LsI = 17 mm,

F = 16 mm, and the other parameters in Table 1 were fixed.

: : .... -. �:::;"("";r·�r�.-... " /:

.

.: ... -{ ... . : , , ,

. . ,, '( i . . L.

I ' I '

.. -

/-., ..

. , .

I "1 I

.,.1 .. . .. . , .. , I 'J ..... :'t .

°2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

Frequency(GHz)

Figure 10. The axial ratio at different values of Ls2 while LsI", 17 mm, F '" 16 mm, and the other parameters in Table I were fixed.

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

matching as well as for improving the axial-ratio level. Because its position changes the impedance seen by the source, it can improve the matching to 50 Q. The effect of probe position on S,' and

axial ratio are shown in Figures 7 and 8, respectively. As expected, at some position good matching an improved axial-ratio level could be obtained and aligned together within the band of interest.

According to the above discussion, it is expected that Ls2 can be as effective as Ls 1, because it changes the x-directed current's electrical path length, and hence affects both S,' and the axial­

ratio level in a similar fashion. Figure 9 shows the effect of Ls2 on S,', which demonstrated that as Ls2 increases, the frequency at

which S,' is a minimum decreases, and the -10 dB band moves

toward a lower frequency range. Figure 10 shows that the effect of Ls2 on axial ratio is similar to that of lsI, but it is observed that Ls2 is less sensitive, which could be useful for fine tuning the S,' and axial ratio.

3.2 Design Procedure

Based on the above parametric study and discussion, the fol­lowing systematic design procedure is proposed:

Step 1. Start with a linearly polarized E-shaped patch at the band of interest, as given in [6]. It is not necessary to optimize the design for maximum-obtainable imped­ance bandwidth, because this will be distorted later by changing one of the slot's dimensions.

Step 2. For left-hand circular polarization, begin with mak­ing the length of lsI shorter (while Ls2 is fixed) until a minimum axial-ratio level is obtained. The opposite procedure should be followed if right-hand circular polarization is required.

Step 3. Change the probe position along the axis of symme­try in the yz plane to improve the axial-ratio level, as well as aligning the S,' band with the axial-ratio

band.

Step 4. As a last step, Ls2 could be changed for a fine enhancement of the axial-ratio level, as well as alignment with the S,' band.

Figure II shows a flowchart of the suggested design and tun­ing procedure. The previous procedure could be repeated if the goal is not satisfied after the first time. However, in the following loop, Lsi could be changed up and down, rather than only down, as in the first loop. The same procedure is valid for the other parameters.

4. Design Examples and Comparisons with Circularly Polarized U-Slot Patches

In this section, a circularly polarized E-shaped patch is designed using the above-suggested procedure on two different substrate materials with different thicknesses. The antenna per­formance is compared with the two recently published circularly polarized V-slot patches in [9, 10].

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

•

LP E- shap e d Patch

10 arameters r 61

Shortening Ls 1 ILs2 for

LHCP/RHCP

Changing pro be position

Change Ls2/Lsl for fine

tuning

Yes

Figure 11. A flowchart for the design and tuning procedure.

4.1 Design 1

The geometry of the antenna was the same as in Figure 1, while the geometrical parameters of the circularly polarized V-slot antennas are shown in Figures 12 and 13. An air substrate with a 10 mm ( 0.08Ao) thickness was used. The tuned dimensions of the

designs to work in the IEEE 802.llb/g band (2.4-2.5 GHz) are tabulated in Tables 2 to 4. Figure 14 shows the S'l of the three

antennas. From the results, the impedance bandwidth defined by the -10 dB level of S,' was 10.1 % for the E-patch (2.35-2.6 GHz).

For the unequal-arms V-slot, it was 9.25% (2.37-2.6 GHz), and for the truncated-comer V-slot, it was 9.64% (2.27-2.5 GHz). The axial ratio as a function of frequency for the three antennas is shown in Figure 15. It was clear that all axial-ratio bandwidths were aligned with the corresponding S,' bandwidth. The axial­

ratio bandwidth of this E-patch antenna was 6.5% (2.38-

223

\V.I I" �----------------------------� �

s

w p

r

, H

Figure 13. The geometry of the circularly polarized truncated­corners U-slot antenna.

Figure 12. The ge0ltletry of the circularly polarized unequal­arms U-slot antenna.

Cr 1

2.2

224

Ol� __ ��������� � .. . .... ,, : : -5········,· . . . . . \ ......... .

'\ : -10 '. . . , . \+ ...

co \ � -15 . . -.. . . . . . \ Table 2. The dimensions of the circularly polarized

E-shaped patch in mm.

..... \ (J) \ I

-20 ..... .. , ......... ,.. �.\.1. .i ..... .

.. �. . ... r . . . .. X . .. . � .

r-�--�-------. H W L 10 77 47.5 6.7 63 33.5

Ws LsI Ls2 P 7 19 44.5 14 4 27 6 20

F 17 10

-25 ....... ; .... J . . . . .. !.... .. -

--

-

- ��::�ed corner U-slot o Unequal U-slot

-30 .

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 F(GHz

Figure 14. SII of Design 1 for the E-shaped unequal-arms U­

slot and truncated-corners U-slot patch antennas.

Table 3. The dimensions of the circularly polarized U-slot patch 151 in mm.

Cr H Lp Wp LuI Lur Lub Wu Ws Lf I 10 43.7 43.7 27.3 19.8 10.3 16.9 2.3 12.5

2.2 6.7 32.7 32.7 20 13.5 8.9 11.4 1.5 12

Table 4. The dimensions of the circularly polarized U-slot patch [6] in mm.

Cr H L W a d Ua Ud Ux Uy 1 10 48.2 48.2 12.2 10 2.1 19.8 23 19

2.2 6.7 36 36 7.7 6.5 1.2 19.6 15.5 11.5

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

2.54 GHz), which was pretty wide for a single-layer single-feed microstrip-patch antenna.

The 4% (2.39-2.49 GHz) axial-ratio bandwidth obtained from the unequal-arms V-slot was comparable to 4.5% (2.39-2.2.5 GHz) for the truncated-corner V-slot.

4.2 Design 2

To show that the proposed design also worked for a material substrate, we used a substrate with 8r = 2.2 and a thickness of

6.7 mm (O.08A.g). The optimized dimensions are tabulated in

Tables 2 to 4. The results are shown in Figures 16 and 17. The modified E-shaped patch provided a 10.6% (2.39-2.66 GHz) impedance bandwidth and a 3.6% (2.41-2.5 GHz) axial-ratio bandwidth, which was pretty wide for such permittivity. On the other hand, the unequal-arms V-slot gave a 6.9% (2.38-2.55 GHz) impedance bandwidth and a 2.8% (2.4 15-2.465 GHz) axial-ratio bandwidth, while the truncated-corners V-slot gave a 13.1% (2.4 I-2.75 GHz) impedance bandwidth and a 3.3% (2.41-2.49 GHz) axial-ratio bandwidth.

5. Experimental Work

Design 1 was chosen for fabrication and measurements. A photo of the prototype is shown in Figure 18. The metallic patch

4 . . . ..... � .... . iii' � 3 ······;·· · ·· ··:"·" ···'·

o : : :p It! It: (ij 2 ...... : ......... � .... .

� .� . . . . • • • . . . �. . • . • . ' 1 ' "

.. ': ......... � .......... ;.

. : ......... �..... . .

- E-patch --- Truncated corner U-slot

o Unequal U-slot O

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 F(GHz)

Figure 15. The axial ratio of Design 1 for the E-shaped unequal-arms V-slot and truncated-corners V-slot patch antennas.

O __ "�TT���==��I ,.. - E-patch

-5 . . . ... . . � .. .

---truncated corner U-slot o Unequal U slot '-........

iii'-10 �

...... -[ . . •. . . . . . ; . . . . . .

�/;'r t�/'. ·: · · · · · · ·· �·· . . . .

. /': 'r"

(j) -15 ····i· · · · ·····�···· ·· . . ; ,········r • . • Ii'. � • . • . . . • . . � . • • • • . . • . . �.

-20 ·· · ·· ···;··· ·······l···· ···· ·j···· ····· .. · · · · ·

_25L--L����L-�������� 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

F(GHz)

Figure 16. SII of Design 2 for the E-shaped, unequal-arms V­slot and truncated-corners V-slot antennas.

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

4

- E-patch ---Truncated corner U-slot

o Unequal U-slot

O2 2.1 2.2 2.3 2.7 2.8 2.9 3

Figure 17. The axial ratio of Design 2 for the E-shaped unequal-arms V-slot and truncated-corners V-slot antennas.

Figure 18a. A plane view of the prototype of the circularly polarizedE-shaped patch antenna.

Figure 18b. A side view of the prototype of the circularly polarized E-sbaped patch antenna.

Table 5. The dimensions of the circularly polarized E-sbaped patch prototype in mm.

w L 76 45

Ws LsI Ls2 7 40 40

p F h

14 10 10

225

-5

co-lO

� �

in -15 ....

-20

s-parameters

.. . : •• • • • '-i'" • • •• . �

-25 L-��������������� 2 2 . 1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2. 9 3

f(GHz)

Figure 19. A comparison of the simulated and measured SII value for the circularly polarized E- shaped patch antenna.

co "0 '0 � 0::: (ii �

10

9

8

7 ..... i .... . . .. ! 6 ..... � .. . .,.

5 . ,. 4 3

2 .... :

O2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

F(GHz)

Figure 20. A comparison of the simulated and measured axial­ratio values for the circularly polarized E-shaped patch antenna.

o

60 60

90 ., ......... 90

120 120

150 150 180

Figure 21a. The radiation patterns of for left-hand circular polarization (blue) and right-hand circular polarization (green) in the yz plane. The simulated pattern is the solid line, while the measured pattern is the dashed or dotted line.

226

0 30 30

60 60 ..

90 :: ....... � ..... 90

.. .'

120 ". . " 120 .. .... -: . . .. .. '

-

150 . .. .. .. .. ... . 150

180 Figure 2 1 b. The radiation patterns of for left-hand circular polarization (blue) and right-hand circular polarization (green) in the xz plane. The simulated pattern is the solid line, while the measured pattern is the dashed or dotted line.

7.5

co "0 5 c 'iij � 2.5 co :; � G

---Measured - HFSS

-... �

... .� ...

-5���������-L���--�� 2 2.1 2.2 2 . 3 2.4 2.5 2.6 2.7 2.8 2.9 3

F(GHz)

Figure 22. A comparison of the simulated and measured broadside gain as a function of frequency.

was fabricated via milling a thin copper-clad substrate. This enabled etching the E-shaped patch dimensions with higher accu­racy. The substrate's relative dielectric constant was 2.2, and its thickness was 0.787 mm. It was mounted above a ground plane with dimensions of 200 mm x 95 mm by using a via of 0.75 mm radius, which was soldered to the center pin of a 50 Q SMA con­necter, as shown in Figure 18 (bottom view). The tuned dimen­sions are tabulated in Table 5, and a comparison of the simulation and measured results for SII is shown in Figure 19. Good agree­

ment between the simulation and measurement was observed. The measured antenna impedance bandwidth was 9.27% (2.34-2.57 GHz), while 10.27% (2.31-2.57 GHz) was obtained from the simulation. The simulated and measured axial-ratio results are shown in Figure 20. The axial-ratio bandwidth based on simulation was 8.1 % (2.28-2.45 GHz), and the measured axial-ratio band­width was 16% (2.3-2.7 GHz). The bandwidth overlapped by SII and the axial ratio was 2.34-2.57 GHz, which was 9.27%. Com-

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

pari sons of the left-hand circular polarization and right-hand cir­cular polarization patterns in the yz and xz planes at 2.45 GHz are shown in Figure 21. The gain as a function of frequency in the broadside direction is shown in Figure 22. The maximum meas­ured gain obtained was 8.3 dBi with a 15.5% 3 dB bandwidth (2.27-2.65 GHz). The maximum simulated gain was 8.7 dBi with a 19.5% 3 dB bandwidth (2.18-2.65 GHz). The overlapped band­

width among impedance, axial ratio, and gain was 9.27%, which was the effective bandwidth of the antenna. This was considered very wideband for a single-feed single-layer microstrip-patch antenna.

6. Conclusion

A new technique to achieve circularly polarized radiating fields from a single-feed single-layer microstrip antenna using an E-shaped patch has been proposed. The technique is simple, and achieves a wideband axial ratio compared to other techniques applied to comparably sized patches. The circularly polarized E­shaped patch has been designed, fabricated, and measured for the 802.11 big WLAN band. An effective bandwidth of 9% was

obtained (2.34-2.57 GHz). Wireless devices that require circular­polarization characteristics are considered one of the potential applications for this antenna.

The proposed design is also attractive for the implementation of a reconfigurable antenna with switchable left-hand circularly polarized/right-hand circularly polarized operation. This future implementation would be useful for diversity in establishing radio links. A .current investigation is exploring this concept with the E­shaped patch antenna of equal parallel slots. Two R F switches (PIN diodes) were inserted in appropriate locations across each

slot. If one of the switches is ON and the other is OFF, the two slot lengths become effectively unequal, and circular polarization is obtained. If the states of the two switches are reversed, circular polarization with the opposite orientation will be obtained at the same frequency. The results of this investigation will be reported in a future article.

7. References

I. R. Garg. P. Bhartia, I. Bahl and A. Ittipiboon, Microstrip Antenna Design Handbook, Norwood, MA, Artech House, 2001.

2. C. A. Balanis, Antenna Theory: Analysis and Design, Second Edition, New York, John Wiley and Sons, 1997.

3. T. Huynh, and K. F. Lee, " Single-Layer Single-Patch Wideband Microstrip Antenna," Electronics Letters, 31, 16, August 1995, pp. 1310-1312.

4. K. F. Lee, K. M. Luk, K. F. Tong, Y. L. Yung, and T. Huynh, "Experimental Study of the Rectangular Patch with a U- Shaped Slot," IEEE International Symposium on Antennas and Propaga­

tion Digest, I, July. 1996, pp. 10-13.

5. K. M. Luk, C. L. Mak, Y. L. Chow, and K. F. Lee, "Broadband Microstrip Patch Antenna," Electronics Letters, 34, 15, 1998, pp. 1442-1443.

6. F. Yang, X. X. Zhang, X. Ye and Y. Rahmat- Samii, " Wide Band E- Shaped Patch Antenna for Wireless Communications,"

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

IEEE Transactions on Antennas and Propagation, AP-49, 7, July 2001, pp. 1094-1100.

7. W. K. Lo, J. I. Hu, C. H. Chan, and K. M. Luk, "L- Shaped Probe-Feed Circularly Polarized Microstrip Patch Antenna with a Cross Slot," Microwave and Optical Technology Letters, 25, 4, May 2000, pp. 251-251.

8. F. S. Chang, K. L Wong, and T. W. Chiou, "Low-Cost Broad­band Circularly Polarized Patch Antenna," IEEE Transactions on Antennas and Propagation, AP-51, 10, October 2003, pp. 2006-2009.

9. K. F. Tong and T. P. Wong, "Circularly Polarized U- Slot Antenna," IEEE Transactions on Antennas and Propagation, AP-55,8, August 2007, pp. 2382-2385.

10. S. L. S. Yang, K. F. Lee, A. A. Kishk, and K. M. Luk, "Design and Study of Wideband Single Feed Circularly Polarized Micro­strip Antennas," Progress in Electromagnetics Research, PIER 80, 2008, pp. 45-6 I.

11. High Frequency Structure Simulation (H F S S), Version 11, Ansoft Corp.

12. Y. X. Guo, L. Bian, and X. Q. Shi, "Broadband Circularly Polarized Annular-Ring Microstrip Antenna," IEEE Transactions on Antennas and Propagation, AP-57, 8, August 2008, pp. 2474-2477.

13. C. 1. Wang and C. H. Chen, "CP W- Fed Star- Shaped Slot Antennas with Circular polarization," IEEE Transactions on Antennas and Propagation, AP-57, 8, August 2008, pp. 2483-2486.

IntroduCing the Authors

Ahmed Khidre received his B Sc in Electrical Engineering from Ain Shams University in 2006. From 2006 to 2007, he was

employed at Ericsson Egypt Ltd. From 2007 to 2009, he was appointed to the Electrical Engineering Department of Misr Inter­national University ( MIU) as a teacher assistant. He is currently a graduate research assistant at the Center of Applied Electromag­netic Systems Research (CA ESR), the University of Mississippi, where he is pursuing his postgraduate studies. His research inter­ests include microstrip antennas, reconfigurable printed antennas, beam-scanning antennas, phased antennas/reflectarrays, U WB communication systems, numerical techniques for EM modeling and simulation, optimization techniques for active microwave cir­cuits/antennas, and RF M EMs.

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Kai Fong Lee is Professor of Electrical Engineering at the University of Mississippi, where he served as Dean of Engineering from 2001 to 2009. He received his BSc and MSc degrees from Queen's University, Canada, and his PhD degree from Cornell University, all in Electrical Engineering. In his career, he has held research, faculty, and academic administrative positions in the US and Hong Kong. Administrative positions included the founding Head of the Department of Electronic Engineering at the City Uni­versity of Hong Kong, Chair and Professor of Electrical Engineer­ing at the University of Toledo, and Chair and Lapierre Professor of Electrical Engineering at the University of Missouri. He held research positions at the University of California, San Diego; Uni­versity of California, Los Angeles; National Center for Atmos­pheric Research; National Oceanic and Atmospheric Administra­tion; and NASA. He also taught at the Catholic University of America, the Chinese University of Hong Kong, and the Univer­sity of Akron.

Prof. Lee worked on plasma waves and instabilities from 1966-1981, and has worked on antennas since 1981. He was elected an IEEE Fellow in 1997. He received the 2009 John Kraus Antenna Award of the IEEE Antennas and Propagation Society. He authored Principles of Antenna Theory (John Wiley and Sons, 1984), and was senior editor of Advances in Microstrip and Printed Antennas, (Wiley Interscience 1997). He was the lead author of Microsfrip Patch Antennas with Kwai Man Luk (World Scientific/Imperial College Press, 20 I 0).

Fan Yang received the BS and MS degrees from Tsinghua University in 1997 and 1999, and the PhD degree from University of California, Los Angeles (UCLA) in 2002. From 1994 to 1999, he was a Research Assistant in the State Key Laboratory of Microwave and Digital Communications, Tsinghua University, China. From 1999 to 2002, he was a Graduate Student Researcher in the Antenna Lab, UCLA. From 2002 to 2004, he was a post-doc Research Engineer and Instructor in the Electrical Engineering Department, UCLA. In August 2004, he joined the Electrical Engineering Department at the University of Mississippi as an Assistant Professor, and was promoted to Associate Professor in 2009.

Dr. Yang's research interests include antenna theory, design, and measurements, electromagnetic bandgap (EBG) structures and

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their applications, computational electromagnetics and optimiza­tion techniques, and applied electromagnetic systems, such as radio-frequency identification (RFlD) systems and solar energy concentrating systems. He has published over 100 technical journal articles and conference papers, five book chapters, and two books, Electromagnetic Band Gap Structures in Antenna Engineering and Electromagnetics and Antenna Optimization Using Taguchi's Method.

Dr. Yang is a Senior Member of the IEEE and a Member of Commission B of URSIIUSNC. Dr. Yang serves as the Associate Editor-in-Chief for the Applied Computational Electromagnetics Society (ACES) Journal and as an Associate Editor for the IEEE Transactions on Antennas and Propagation. He is also a frequent reviewer for over twenty scientific journals and book publishers, and has chaired numerous technical sessions in various interna­tional symposiums. Dr. Yang was Secretary of the IEEE AP-S Los Angeles Chapter. He was a Faculty Senator and a Member of the University Assessment Committee at the University of Mississippi. In 2004, Dr. Yang received the Certificate for Exceptional Accom­plishment in Research and Professional Development Award from UCLA. He received a Young Scientist A ward at the 2005 URSI General Assembly and at the 2007 International Symposium on Electromagnetic Theory. He was appointed as a University of Mis­sissippi Faculty Research Fellow in 2005 and 2006. In 2008, Dr. Yang received the Junior Faculty Research Award from the Uni­versity of Mississippi. In 2009, he received the inaugural IEEE Donald G. Dudley Jr. Undergraduate Teaching Award.

Atef Z. Eisherbeni is a Professor of Electrical Engineering and Associate Dean for Research and Graduate Programs, the Director of the School of Engineering CAD Lab, and the Associate Director of the Center for Applied Electromagnetic Systems Research (CAESR) at the University of Mississippi. In 2004, he was appointed as an adjunct Professor in the Department of Elec­trical Engineering and Computer Science of the L. C. Smith Col­lege of Engineering and Computer Science at Syracuse University. In 2009, he was selected as a Finland Distinguished Professor by the Academy of Finland and Tekes. Dr. Elsherbeni has conducted research dealing with scattering and diffraction by dielectric and metal objects; Finite-Difference Time-Domain analysis of passive and active microwave devices including planar transmission lines; field visualization and software development for EM education; interactions of electromagnetic waves with the human body; RFID and sensor development for monitoring soil moisture, airport noise levels, and air quality including haze and humidity; reflector and printed antennas and antenna arrays for radars, UA V, and personal communication systems; antennas for wideband applications; antenna and material properties measurements; and hardware and software acceleration of computational techniques for electromag­netics. Dr. Elsherbeni is co-authored The Finite Difference Time Domain Method for Electromagnetics with MATLAB Simulations (SciTech, 2009), Antenna Design and Visualization Using MATLAB (SciTech, 2006), MATLAB Simulations for Radar Sys-

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

tems Design (CRC Press, 2003), Electromagnetic Scattering Using the Iterative Muitiregion Technique (Morgan & Claypool, 2007),

Electromagnetics and Antenna Optimization using Taguchi's Method (Morgan & Claypool, 2007), and was the main author of the chapters "Handheld Antennas" and "The Finite Difference Time Domain Technique for Microstrip Antennas" in Handbook of Antennas in Wireless Communications (CRC Press, 2001). Dr. Elsherbeni is a Fellow of the IEEE and a Fellow of ACES. He is the Editor-in-Chief of the ACES Journal, and a past Associate Editor of Radio Science. (@

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