Circularly polarized microstrip antenna with reactive load design for wireless local area network application

Journal of Engineering and Development, Vol. 15, No. 3, September (2011) ISSN 1813-7822
55
Circularly Polarized Microstrip Antenna with Reactive Load
Design for Wireless Local Area Network Application
Asst. Lect. Basim Khalaf Jarulla
Electrical Engineering Department, College of Engineering
Al-Mustansiriya University, Baghdad, Iraq
Abstract
This paper presents a design of microstrip antenna for IEEE 802.11b and for IEEE
802.11g using a nearly square patch antenna, excited by a standard miniature adapter
probe feed line. The patch and ground plain are separated by a substrate; the radiating
patch is loaded by a central shorting pin and feeding probe loaded by a series capacitor.
This antenna has wide bandwidth in the frequency band of (WLAN) and with a return loss
≤ −10 dB from 2.4 GHz to 2.48 GHz exhibits circularly polarized far field radiation pattern.
The proposed antennas have been simulated and analyzed using method of moments
(MoM) based software package Microwave Office 2008 v8.0. The results show that the
bandwidth of the antenna increases by using reactive load. The simulated gain of the
antenna is over 6 dB.
‫الخالصة‬
ٍ‫نًُظىير‬ ٍ‫ششَط‬ ٍ‫نهىائ‬ ‫ذصًًُا‬ ‫قذو‬ ‫انثحث‬ ‫هزا‬IEEE802.11b‫و‬IEEE802.11g‫انشكم‬ ‫تاسرخذاو‬
ٌ‫يسرى‬ ٍ‫ي‬ ٌ‫َركى‬ ٍ‫انهىائ‬ .‫انًصغش‬ ٌ‫انًعُاس‬ ‫انرغزَح‬ ‫خط‬ ‫تىاسطح‬ ‫ذغزَره‬ ‫ذرى‬ ٌ‫وانز‬ ٍ‫انكهشويغُاطُس‬ ‫نهثث‬ ‫انًشتغ‬ ‫شثه‬
‫انًادج‬ ٌ‫ذكى‬ ‫تُُهًا‬ ‫يا‬ ‫و‬ ٍ‫األسض‬ ٌ‫يسرى‬ ‫و‬ ‫يشتغ‬ ‫انشثه‬ ‫انششَحح‬‫يغ‬ ‫قصش‬ ‫تذائشج‬ ‫حًم‬ ‫انًشتغ‬ ‫شثح‬ ‫انشكم‬ .‫انعاصنح‬
‫ذشدداخ‬ ٍ‫ذغط‬ ‫ذشدد‬ ‫حضيح‬ ‫ًَرهك‬ ٍ‫انهىائ‬ ‫هزا‬ .‫انًىائًح‬ ‫إلغشاض‬ ٍ‫ذىان‬ ‫تًرسعح‬ ‫حًم‬ ‫انرغزَح‬ ‫خط‬ ‫كزنك‬ ‫و‬ ٍ‫األسض‬ ٌ‫يسرى‬
ٍ‫ي‬ ‫اقم‬ ‫ساخعاخ‬ ‫يفقىداخ‬ ‫ويغ‬ ‫انًحهُح‬ ‫انالسهكُح‬ ‫انشثكح‬-01dB‫د‬ ‫ذشد‬ ٍ‫ي‬2.4 GHz‫ذشدد‬ ً‫إن‬2.48 GHz‫رو‬ ‫و‬
‫دائ‬ ‫تث‬.‫انًذي‬ ‫انثعُذ‬ ‫اإلسسال‬ ‫نشعاع‬ ‫االسرقطاب‬ ٌ‫ش‬
‫انًاَكشوَح‬ ‫انًىخاخ‬ ‫يكرة‬ ‫تاسرخذاو‬ ‫انًقرشذ‬ ٍ‫نههىائ‬ ‫انًُىرج‬ ‫وذحهُم‬ ‫يحاكاج‬ ‫ذًد‬8112.ٍ‫انثاي‬ ‫اندُم‬
ٍ‫انهىائ‬ ‫نًحصهه‬ ‫انًحاكاج‬ ‫َرائح‬ .‫فعال‬ ‫انغُش‬ ‫انحًم‬ ‫تاسرخذاو‬ ‫اصدادخ‬ ٍ‫نههىائ‬ ٌ‫انرشدد‬ ‫االشرغال‬ ‫حضيح‬ ٌ‫ا‬ ‫تُُد‬ ‫انُرائح‬
‫أكثش‬ ٍ‫ه‬ٍ‫ي‬6 dB.

56
1. Introduction:-
Microstrip antenna is a type of antennas which can be used for transmitting and
receiving signals. Microstrip or printed antennas are low profile, small size, light weight and
widely used in wireless and mobile communications, as well as radar applications. Microstrip
antennas can be divided into two basic types by structure, namely microstrip patch antenna
and microstrip slot antenna. The microstrip patch antennas can be fed by microstrip line and
coaxial probe [1]
.
Nowadays, wireless local area network (WLAN) standardized by IEEE 802.11 is
becoming common in communication system [1]
.
Microstrip patch antennas have received considerable attention for wireless
communication applications [2]
. This proposed microstrip antenna designed to utilized in 2.4-
2.4835 GHz frequency range (IEEE 802.11 b/g), fed by 50Ω standard miniature adapter
(SMA) at a designed frequency of 2.4425 GHz.
2. The proposed antenna:-
The antenna configuration is nearly square microstrip patch antenna shown in Fig.l. In
this case the circular polarization (CP) is obtained because the two modes of resonance
(corresponding to the adjacent sides of the rectangle), are spatially orthogonal. The antenna is
excited at a frequency in between the resonant frequencies of these two modes in order to
obtain the phase quadrature relationship between the voltages (and therefore magnetic
currents) of two modes. Corner or diagonal feeding is required to allow both the modes to be
excited with a single feed [3]
.
Fig.1.The nearly square patch antenna configuration.
The ratio of the two orthogonal dimensions W/L should be generally in the range of
(1.01–1.10) depending upon the substrate parameters. When the patch is fed along the
diagonal, then the two resonance modes corresponding to lengths L and W are spatially
orthogonal.

57
The CP is obtained at a frequency, which lies between the resonance frequencies of
these two modes, where the two orthogonal modes have equal magnitude and are in phase
quadrature [4]
. The feeding probe, i.e. inner conductor of SMA is connected to the patch
through a small opening in the ground plane. The outer conductor of SMA is directly
connected to the ground plane. A cylindrical-capacitor with larger diameter than that of the
probe is inserted between the probe and patch for the impedance matching optimization [5]
.
By using a central pin to short the upper patch to the ground plane, one may improve
the purity of the resonant mode. An addition of a shorting pin acts as an extra parameter to
control the mode excitation. For a given antenna dimension, Since the introduction of the pin
increases the resonance size of the patch, perhaps the most important property of the pin is to
control the antenna gain by increasing the patch size. This may be a useful parameter to use in
the design of higher-gain patch antennas [6]
. This kind of design is the simplest form to
generated circular polarization and is very suitable for the WLAN RHCP or LHCP microstrip
antenna design.
3. The proposed antenna design:-
The calculation of the square Microstrip antenna length is based on transmission-line
model [7]
. The width W of the radiating edge, which is not critical, chosen first. The length L
is slightly less than a half wavelength in the dielectric. The precise value of the dimension L
of the square patch has been calculated using expression [7, 8]
.
…………………………….. (1)
Where the effective dielectric constant, and ∆L is the fringe factor [9]
.
For the frequency of 2.4425 GHz and using (FR-4) with a relative dielectric constant of
4.2 and loss tangent of 0.017, with substrate height of 1.575 mm, this yields nearly square
patch antenna length (Fig.1) of L=28.5 mm and W=29.5 mm. The ratio of the two orthogonal
dimensions W/L is 1.035, lies in the range of generating two spatially orthogonal resonance
modes [4]
.
The feed-point of the antenna, it is defined in terms of input impedance Zin of the
antenna and the characteristic impedance Zo of the feed line. The feed point position should
be placed at the location where the input impedance of the antenna matches the characteristic
impedance of the feed [4]
. The patch can be fed by a coax line from underneath. The
impedance varies from zero in the center to the edge resistance approximately as [10]
.
…………..(2)
Where Ri is the input resistance, Re the input resistance at the edge, and x the distance
from the patch center. By shifting the feed-point along the diagonal to x =-0.4875 cm and y =
-0.475 cm, from the center of the patch; a perfect match with a 50Ω feed line is obtained, with
cylindrical-capacitor [5]
.

58
The feeding point position is 0.323% from the diagonal long. The central shorting pin
added at the center of the patch with x=0 cm, y=0 cm. A shorting pin through the center has
no effect on radiation or impedance, but it allows a low-frequency grounding of the antenna
[10]
. The proposed antenna structure has been modeled using a full-wave numerical Method of
Moment (MoM). EMSightTM
, of the Applied Wave Research (AWR), includes a full-wave
electromagnetic solver that uses a modified spectral domain method of moments to accurately
determine the multi-port scattering parameters for predominately planar structures [11]
. In the
presented design; this software package was applied to simulate the typical characteristic of
the proposed antennas.
4. Simulation results:-
The proposed antenna structure had been modeled at the design frequency of 2.4425
GHz, which are the center frequency of WLAN standardized by IEEE 802.11b/g. It has been
supposed that the antenna element to be located parallel to x-y plane and centered at the origin
(0, 0, 0). The computed input return losses of the antenna patch is shown in Fig.2.
Fig.2 The calculated input returns loss for the modeled patch element.
The computed radiated electric far field intensity right handed and left handed in
broadside direction with respect to frequency is shown in Fig.3. It is clear that, for this feed,
this antenna supports LHCP electric field radiation pattern and if the feed is shifted to the
other diagonal, then RHCP is obtained [4]
.
Fig.3 The computed far electric field intensity left hand and right hand circular
polarization in broadside direction, of the proposed antenna.

59
The required axial ratio AR has been calculated using [12]
:
………………….….(3)
Where ER and EL are the right and left handed circularly polarized radiated electric
fields. The resulting axial ratio as a function of frequency is depicted in Fig.4. It can be seen
that the axial ratio in the broadside direction is below 3 dB throughout a bandwidth of about
80 MHz's.
Fig.4 The computed axial ratios around the IEEE802.11b/g frequency, for the
proposed antenna.
The computed gain around the IEEE802.11b/g frequency range has shown in Fig.5. As
the gain response implies, the proposed antenna possesses an average gain of about more than
(4.5-6) dB throughout the required bandwidth of IEEE802.11b/g antenna.
Fig.5 The computed antenna gain around the IEEE802.11b/g frequency, for the
proposed antenna.
5. Conclusions:-
A nearly square wideband single probe-fed microstrip patch antenna with inserted
central shorting pin and input feed matching capacitor has been investigated. The realized
impedance bandwidths (return loss ≤ −10 dB) and the circular polarization bandwidth (axial
ratio ≤ 3 dB) satisfy the bandwidth requirements for the IEEE802.11b/g operation.

60
6. References:-
1. W. Kueathawikun1, P. Thumwarin1, N. Anantrasirichai1, and T. Wakabayashi2,
“Wide-Band Slot Antenna for IEEE 802.11b/g,” SICE-ICASE International Joint
Conference 2006 Oct. 18-21, 2006 in Bexco, Busan, Korea.
2. Guo, Y. J., A. Paez, R. A. Sadeghzadeh, and S. K. Barton, “A Patch Antenna for
HIPERLAN,” Wireless Personal Communications, Vol. 3, 389–396, 1996.
3. C. Sharma and Kuldip C. Gupta, “Analysis and Optimized Design of Single Feed
Circularly Polarized Microstrip Antenna,” IEEE Transactions on Antennas and
Propagation, Vol. AP-31, No.6, November 1983.
4. G. Kumar and K. P. Ray, “Broadband Microstrip Antennas”, Artech House, Inc.
2003.
5. Y.J. Wang, C.K. Lee, P.S. Tian, and S.W. Lee, “Novel Microstrip- Monopole-
Integrated Ultra-Wideband Antenna For Mobile UWB Devices”, School of EEE,
Nanyang Technological University Nanyang Avenue, Singapore 639798.
6. I. R James and P. S. Hall, “Handbook of microstrip antennas”, Vol. 1. Peter
Peregrinus Ltd., London, 1989.
7. Bahi, and Bhartia, “Microstrip Antennas”, Artech House, Inc. 1980.
8. M. Amamam, “Design of Rectangular Microstrip Patch Antenna for 2.4 GHz Band”,
Applied Microwave and Wireless Nov/Dec 1997.
9. C. A. Balanis, “Antenna Theory, Analysis and Design”, John Wiley & Sons 1997.
10. Thomas A. Milligan, “Modern Antenna Design”, John Wiley & Sons. 2ed. ed. 2005.
11. AWR®
Design Environment, Microwave Office®
(MWO) 2008 V8. 1960 E. Grand
Avenue, Suite 430 El Segundo, CA 90245 USA.
12. Jawad K. Ali, “A New Compact Size Microstrip Patch Antenna with Irregular Slots
for Handheld GPS Application”, Eng. & Technology, Vol.26, No.10, 2008.
Line Plot (Abeer1 18v*118c)
0 200 400 600 800 1000 1200 1400 1600 1800
Deflection (mm)
0
6
12
18
24
30
36
42
48
54
60
66
Load(kN)
S2 Right Column
S2 Left Column

Circularly polarized microstrip antenna with reactive load design for wireless local area network application

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