Design of Wide Beam Hexagonal Shaped Circularly

Polarized Patch Antenna for WLAN Application

M. Saravanan ✉ and M.J.S. Rangachar

( )

Electronics and Communication Engineering, Hindustan Institute of Technology and Science,

Chennai, India
msarawins@gmail.com, mjsranga@gmail.com

Abstract. The design of hexagonal shaped patch antenna with a narrow slot for
wireless local area network (WLAN) is reported here. The slot oriented at 450, in
order to achieve left-hand circular polarization (LHCP). The ground is defected
by cross-shaped slots oriented at 00 and 450 in order to improve the axial ratio
beam width of the patch antenna. The antenna achieves reflection coefficient
bandwidth (S11 ≤ 10 dB) of 6.25% (4.9–5.2 GHz) with a center frequency of
5 GHz and gives a gain of about 4.81 dB. The antenna designed is appropriate
for Wireless Local Area Network (WLAN) application. A parametric analysis of
ground plane slot dimension and orientation for achieving improved axial ratio
beam width is carried.

Keywords: Axial ratio · Defected ground structure · Edge feed · Reflection

coefficient · Impedance matching technique

1 Introduction

Microstrip antennas are widely used in recent days because of its planar structure and
light weight. These devices are easily incorporated with wireless devices which are very
small in size and often are mobile. Hence the antenna alignment needs to be matched with
the source antenna node. Hence there is a need for incorporating circularly polarized patch
antenna in order to avoid polarization mismatches. In [5], proximity coupled patch antenna
is shown. Here wide axial ratio bandwidth is achieved by creating V-slot in the radiating
patch. Various techniques of circularly polarized patch design and the different kinds of
modes generated are discussed in [6]. In order to suppress higher order harmonics, a
defected ground structures (DGS) is utilized in the ground plane as discussed in [2–4].
These DGS structures increase effective capacitive and inductive components in the
antenna model [7]. In [8], a patch antenna operating at tri-band for WLAN/WiMAX
application is discussed. The antenna includes a circular ring, a strip in the shape of Y, and
a DGS structure in ground plane for reducing impedance mismatches. A planar slot
antenna is presented in [9]. It uses a c shaped slot and inverted L shaped slot to resonate
at multiple modes. A miniature antenna operating at triple-band for WiMAX and WLAN
applications is presented in [10] where an inverted L-shaped radiating patch in addition
to stub element in the ground plane. Multi band antennas are developed in [11].

© Springer International Publishing AG 2018

A. Abraham et al. (eds.), Proceedings of the Eighth International Conference on Soft
Computing and Pattern Recognition (SoCPaR 2016), Advances in Intelligent
Systems and Computing 614, DOI 10.1007/978-3-319-60618-7_15
 Design of Wide Beam 143

The model uses stacked architecture. Here circular polarization is achieved by means of
four cross shaped slots and truncated corners. In [12–14], miniaturization of patch antenna
with improved beam width is presented. In [13], gain of the antenna is improved by means
double L shaped feed structure placed at offset position below the radiating element.
Last few years, a lot of research has been carried on the design of circular polarization
for good gain and wide axial ratio beam width. However, there is need of miniature
antenna for portable wireless devices having a wide axial beam width so as to connect
with source antenna for a wider line of angle sight for better connectivity and reliable
services irrespective of antenna orientation alignment. Therefore, a novel approach to
design a low profile circularly polarized patch antenna, providing having good radiation
characteristics at 5 GHz operating frequency for WLAN application is presented. The
model consists of a diagonal slot in the radiating element for generating circularly
polarized wave (LHCP) and a DGS structure in order to improve the axial ratio beam
width without degrading the gain performances. The slot dimensions are tuned to have
a better impedance mismatch between load antenna with input feed network. Various
parametric analyzes have done to clearly depict the effect of DGS over operating
frequency and the Axial Ratio beam width and the results are tabulated.

2 Geometry of Proposed Antenna Model

2.1 Hexagonal Patch Antenna (Without DGS Structure)

Figure 1 shows the schematic of hexagonal shaped patch antenna without Defected
Ground Structure (DGS). The antenna is fabricated over flame retardant fiberglass epoxy
(FR4) substrate having a permittivity εr = 4.4 and dielectric loss tangent ζ = 0.02. The
resonant frequency of a circular patch antenna for the dominant TM110 is given by [1]

1.8411 ∗ c
fr = √ (1)
2𝜋 ∗ rc ∗ 𝜀r

1.8411 ∗ c
rc = √ (2)
2𝜋 ∗ fr ∗ 𝜀r

Where, rc = Radius of the circle.

By equating area of circle with area of hexagon,
√ √
′
S = rc ∗ ( (3 3)∕𝜋)−1 (3)

′
√
S = rc ∗ ( 2.898∕𝜋)−1 (4)

The addition of slot increases the electrical length of the slot. Hence the antenna
resonates at lower band nearer to operating frequency. In order to compensate this effect,
the size of the antenna has to trimmed by a factor Δl as given in Eq. (5) of so the antenna
resonates at its operating frequency.
144 M. Saravanan and M.J.S. Rangachar

′
S = S − Δl
′/ (5)
Δl = S 2𝜀r

Where,

c = Velocity of light in free space (3 * 108 m/s)

S = Length of side of Hexagon
εr = Dielectric substrate permittivity (Fr4 = 4.4)

Fig. 1. Hexagonal patch antenna (without DGS structure)

The radiating patch has a face length of Patch_Face and Radius Patch_R and is tuned
to operate at WLAN operating frequency. A diagonal slot of length Slot_L and width
Slot_W is made in the radiating patch element and its dimensions are adjusted to achieve
circular polarization. The introduction of slot adds the electrical length of the radiating
element. However, the surface current is not disturbed by the slot.
In order to reduce the secondary radiation from the slot, it is necessary to keep the
slot narrower. The model is excited at the radiating edge by means of coaxial cable
having 50 Ω impedance. At resonant frequency, the impedance at the radiating edge is
given by

1
Zin = (6)
2Ge

S
Ge = 0.004 (7)
𝜆0

A quarter wave transformer is used for impedance matching between antenna impe‐
dance ZL and the input impedance Zin. The matching impedance Z0 for the quarter wave
transformer is obtained from the equation given below.
 Design of Wide Beam 145

√
Z0 = Zin ∗ ZL (8)

Based on the above equations, the overall dimensions of the hexagonal patch with
diagonal slot antenna are given in Table 1 given below.

Table 1. Optimized value for antenna dimensions

Parameters Specifications
Operating frequency 5 GHz
Sub_X * Sub_Y * Sub_Z 30 mm * 30 mm * 1.6 mm
Patch_Face 8.171 mm
Patch_R 6.968 mm
Slot_L 7.55 mm
Slot_W 0.25 mm
Qw_Length 8 mm
Qw_Width 0.5 mm
Feed_Length 4 mm
Feed_Width 1.5 mm

2.2 Hexagonal Patch Antenna (With DGS Structure)

Figures 2 and 3 shows the schematic of proposed hexagonal patch antenna model with
DGS structure in the ground plane. Here the conducting metal of particular shape is
etched off from the ground in order to disturb the overall surface currents in the radiating
elements and to suppress the higher order harmonics. The slot is oriented in 00 and 450
to analyze the effect of DGS structure over the radiation characteristics of the antenna
model as shown in Figs. 2 and 3. The width and length of DGS_slots are 0.25 mm and
3 mm respectively.

Fig. 2. Antenna with 00 inclined DGS Fig. 3. Antenna with 450 inclined DGS
146 M. Saravanan and M.J.S. Rangachar

3 Design and Parametric Analysis of Proposed Antenna

The performance of the proposed antenna is analyzed by varying the ground plane slot
orientation at 00 & 450 and also without DGS structure. The result obtained are discussed
below. Figure 4 shows that at operating frequency, the patch having DGS with slot
oriented at 450 gives good return loss of about |S11| = −37.89 dB providing a bandwidth
of 6.26% (4.89 GHz–5.20 GHz) which makes it suitable for WLAN applications. The
antenna without DGS structure shifts the operating frequency by 200 MHz and are
resonating at 5.19 GHz.

HFSSDesign1
Curve Info
0.00 Tb(Hexagon)
Return Loss : dB
Tb(Hexagon_DGS(0deg))
Return Loss : dB
Tb(Hexagon_DGS(45deg))
-10.00 Return Loss : dB
Return Loss (dB)

Name Tb(Freq) Y
m1 5.0510 -37.8906
-20.00

-30.00

m1

-40.00
3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00
Tb(Freq) [GHz]

Fig. 4. Return loss (dB)

Figure 5 shows the voltage standing wave ratio for a given model. It is observed
from the graph that the VSWR is less than 2 for both the DGS structures at operating
frequency range.

Curve Info
75.00 Tb(Hexagon)
VSWR :
Tb(Hexagon_DGS(0deg))
62.50 VSWR :
Tb(Hexagon_DGS(45deg))
VSWR :

50.00 Name Tb(Freq) Y

m1 5.0510 1.0258
VSWR

37.50

25.00

12.50

m1
0.00
3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00
Tb(Freq) [GHz]

Fig. 5. VSWR

For a pure circularly polarized patch antenna, the axial ratio must be equal to 0 dB.
However, it is difficult to obtain the ideal value of 0 dB since the axial ratio tends to
degrade from main beam and hence less than 3 dB is the acceptable value of the axial
 Design of Wide Beam 147

ratio for near circularly polarized patch antenna. Figure 6 shows the axial ratio measured
for patch without DGS and Patch with DGS (00 and 450 oriented slots).
From the Fig. 6 it is observed that for a ground plane with DGS structure, the axial
ratio beam width is improved to 2210.

Curve Info
70.00
Tb(Hexagon)
Axial Ratio : dB
Tb(Hexagon_DGS(0deg))
Axial Ratio : dB
60.00
Tb(Hexagon_DGS(45deg))
Axial Ratio : dB
Name Tb(Theta) Y
50.00
m1 -119.0000 2.8067
m2 102.0000 3.0058
Axial Ratio (dB)

40.00 Name Delta(Tb(Theta))

d( m1,m2) 221.0000

30.00

20.00

10.00
m1 m2

0.00
-200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00
Tb(Theta) [deg]

Fig. 6. Axial ratio beamwidth (AR ≤ 3 dB)

4 Results and Discussions

Based on the discussion in Sect. 3, it is observed that the ground plane with Slots 450
oriented gives wide axial ratio beam width and better impedance matching characteris‐
tics is shown in Fig. 7 and it is observed that at operating frequency, the magnitude of
the impedance is close to input impedance (50 Ω) and also the reactive part becomes
close to zero. This ensures that at resonance capacitive component is having equal
magnitude and out of phase with inductive component and hence cancels each other.
Therefore the antenna is resistive at operating frequency.

Fig. 7. Z Impedance
148 M. Saravanan and M.J.S. Rangachar

Transient current distribution over the surface of the radiating element is shown in
Fig. 8. It shows the electric field vector is rotating left-hand side in the direction of
propagation. Hence it concludes that antenna radiated left-hand circularly polarized
wave.

(a) (b) (c)

(d) (e) (f)

Fig. 8. Transient current distribution at (a) T = 0 (b) T = T/6 (C) T = 2T/6 (d) T = 3T/6
(e) T = 4T/6 (f) T = 5T/6

The radiation Pattern and gain (in dB) of the antenna is given in Figs. 9 and 10. The
radiation pattern obtained is uniform in the azimuthal plane. The maximum gain
obtained by the proposed antenna is about 4.814 dB in the direction of propagation.

Fig. 9. Radiation pattern

 Design of Wide Beam 149

Fig. 10. Gain total (dB)

Table 2 gives performance comparison of proposed antenna models. From the above
table, it is clear that the DGS structure with 450 orientation slots provides better gain
and improved beam width at 5 GHz operating frequency which make suitable for WLAN
applications. Since the antenna is circularly polarized, it gives very reliable path link
with the source node irrespective of antenna alignment.

Table 2. Performance comparison between circularly polarized Hexagonal patch antenna with
and without DGS structure
Parameter Hexagonal without Hexagonal with DGS Hexagonal with DGS
DGS structure (00) structure (450) structure
S11 −34.28 dB −30.78 dB −37.27 dB
VSWR 1.0394 1.0962 1.0258
Z Impedance 47.76 Ω 47.08 Ω 47.93 Ω
Gain 4.75 dB 4.615 dB 4.814 dB
Bandwidth |S11| 6.35% 6.38% 6.25%
≤ 10 dB
Axial ratio beam 1880 2150 2210
width

5 Conclusion

A compact hexagonal shaped patch antenna with DGS structure is presented in this letter.
The diagonal slot in the patch gives a circularly polarized wave and also adds the elec‐
trical length of the radiating element. The designed antenna is operating at 5 GHz (4.9–
5.2 GHz) WLAN application band. The antenna provides a narrow bandwidth of 6.25%
and provides a reflection coefficient |S11| = −37.27 dB and a gain of about 4.814 dB.
The axial ratio of the proposed model is improved (AR (≤3 dB) = 2210) by utilizing
DGS structures in the ground plane. This design is suitable for WLAN application
devices where the line of orientation often changes between transmitter and receiver.
150 M. Saravanan and M.J.S. Rangachar

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