Design of a Compact Circular Microstrip Patch Antenna for WLAN Applications

International Journal on AdHoc Networking Systems (IJANS) Vol. 11, No. 3, July 2021
DOI: 10.5121/ijans.2021.11301 1
DESIGN OF A COMPACT CIRCULAR MICROSTRIP
PATCH ANTENNA FOR WLAN APPLICATIONS
Chaitali Mukta1
, Mahfujur Rahman2
and Abu Zafor Md. Touhidul Islam1
1
Department of Electrical and Electronic Engineering,
University of Rajshahi, Rajshahi 6205, Bangladesh
2
Department of Information and Communication Engineering,
University of Rajshahi, Rajshahi 6205, Bangladesh
ABSTRACT
This paper presents the design of a compact circular microstrip patch antenna for WLAN applications
which covers the band 5.15 to 5.825 GHz. The antenna is designed using 1.4mm thick FR-4
(lossy)substrate with relative permittivity 4.4 and a microstrip line feed is used. The radius of the
circular patch is chosen as 7.62mm. To reduce the size and enhance the performance of the proposed
antenna, a circular slot is loaded on circular patch and a square slot is etched on the ground plane of
dimension 30mm×30mm. Design of the antenna is carried out using CST Microsoft Studio Sonimulation
Software. The proposed antenna resonates at 5.5 GHz with a wider bandwidth of 702 MHz and it provides
low return loss of -31.58 dB, good gain of 3.23 dB and directivity of 4.28 dBi and high efficiency of around
79% against the resonance frequency. The geometry of the proposed circular antenna with reduced size
and its various performance parameters such as return loss, bandwidth, VSWR, gain, directivity, efficiency
and radiation pattern plots are presented and discussed.
KEYWORDS
Circular patch antenna, WLAN, return loss, bandwidth, slot, FR-4 substrate, CST.
1. INTRODUCTION
Nowadays microstrip patch antenna can be used in variety of applications from military to
commercial because of its distinguished features like small size, low weight, low profile, low
fabrication cost, conformability to different shapes, consumes very less space, easy integration
with printed circuit boards, easy fabrication and efficient directive radiation pattern [1,2,3]. The
WLAN now plays a vital role for short distance wireless communication and users can access
internet in their portable devices through this network.
Microstrip patch antennas have a plannar structure consisting of three layers, central insulating
substrate layer and metallic radiating patch on its one side and a metallic ground on its other side.
The energy radiations from the patch antenna take place due to fringing fields from the edges of
the conducting patch [4]. There are many available shapes of radiating patch of the microstrip
antennas such as rectangular, circular, square, triangular, elliptical, etc. Among them rectangular,
circular and square shaped patch antennas are more commonly used because they are easy to
design and analyze the performance [5]. As the circular patch antenna has one degree of freedom
to control (radius), it is, therefore, simpler to design and its radiation can be easily controlled [6].
Also the size of the circular patch antenna is 16% less than that of the rectangular patch antenna
at the same design frequency [7].

2
There are several methods to feed a microstrip patch antennas. The frequently used ones are
microstrip line, coaxial probe and aperture coupling [2]. In the proposed circular patch antenna
design, a microstrip line feed is used which consists of a conducting strip of a very small width
compared to that of the patch. It is a contacting feed method where electromagnetic field
coupling is used to transfer power between microstrip line and radiating patch.
Although microstrip patch antenna has been widely used in various wireless communication
systems, there is a key requirement to have an efficient antenna that would be capable of
transmitting signals with a low return loss, high data rate, high directivity and bandwidth.
However, patch antennas suffer from low gain, small bandwidth, and comparatively large size at
low frequencies [8]. In past years extensive research has been carried out to overcome these
shortcomings of the patch antenna. A number of useful techniques have been proposed by the
researcher to increase gain and enhance bandwidth [9,10].
Some applications of the microstrip antenna in communication systems require smaller antenna
size in order to meet the miniaturization requirements and hence size reduction of the patch
antenna has attracted much attention. Circular microstrip patch antenna is very useful in this
respect because of its small size and ease of fabrication. Different techniques have been
suggested for achieving size reduction [11-16] like implementing slots in patch, implementing
defected ground structure, use of high permittivity substrates, use of shorting pins, and
introduction of parasitic patches.
The aim of this work is to design a compact circular microstrip patch antenna that shows good
performance with respect to state-of-the-art designs. The proposed antenna use the method of
implementing slots on the patch and ground layers for achieving size reduction and microstrip
line feed to operate in the WLAN applications which cover 5.15 to 5.825 GHz band. The antenna
is design and simulated in CST Studio Suite 2015 software.
The rest of the paper is organized into several sections. Section II presents a review of the related
literature, while Section III formally describes the antenna design methodology. Simulation
results are presented and analyzed in Section IV. Finally, Section IV ends the paper with some
concluding remarks.
2. LITERATURE REVIEW
Authors in [20] proposed a circular microstrip antenna with a circular slot is etched on the
circular patch to provide wideband operation. Design parameters were FR-4 epoxy substrate (εr =
4.4) with ground plane dimension 25mm×25mm, radius of the circular patch 6.96mm and
substrate thickness 1mm.They design and simulate the antenna to operate in 5.8GHz in Finite
Element Method (FEM) based High Frequency Structural Simulation (HFSSTM
) tool. They found
a low return loss of -29dB with a wider bandwidth of 660MHz.
A coplanar waveguide (CPW) fed circular patch monopole antenna for WLAN/WiMAX
applications is presented in [21]. To reduce the size of conventional antenna and widen its
bandwidth the patch is loaded with slots. Their simulated result has a -10 dB impedance
bandwidth of 2.46 GHz in the band of 4.97-7.43 GHz.
An overview of circular microstrip antenna design is presented in [22] where it was concluded
that circular microstrip antenna can be designed for different frequency band and different
feeding methods like coaxial, strip line, aperture coupling or proximity coupling are available for
different substrate materials to excite the antenna.

3
In [23], a comprehensive review of the recent research work done by various authors on the
design and optimization of the slotted circular microstrip patch antenna operating in C band is
presented. The slotted antennas are used in C Band applications like satellite communication,
WLAN, WiMAX, Wi-Fi etc. They concluded that the introduction of different slots on the
antenna have resulted in improvement of various performance parameters of the antenna like
gain, bandwidth, return loss etc.
A composite dual band circular microstrip patch with space opening for WLAN applications is
presented in [24]. The antenna configuration is simulated and analyzed with HFSS. A narrow
band high gain circular patch antenna has been designed for GSM1800 band and presented in
[25]. They found the return loss is -21 dB at 1.8 GHz for VSWR≤ 2. The gain of antenna was
increased by inverted suspended circular patch with same dimension of radiating patch from 7.5
dB to 8.5 dB and also it was increased from 8.2 dB to 8.5 dB by increasing the gap between top
and bottom patch from 1 mm to 4 mm.
Authors in [26] presented the design of a circular patch antenna operating at 5GHz for C band
applications. The antenna is designed using FEM based EM simulator software – Ansoft’s HFSS
v13. In [27], author reported that the bandwidth of microstrip patch antenna increases with the
help of slots. By using different shape of slot the gain of microstrip patch antenna can also be
improved and antenna straucture can be made more compact.
3. ANTENNA DESIGN
The proposed circular microstrip patch antenna is designed using CST Studio Suite 2015
software. Once the design is complete, the antenna can be simulated in the software to estimate
its real world performance. For antenna design, it is assumed that the dielectric constant of the
substrate (𝜀𝑟), the resonant frequency (𝑓
𝑟 in GHz), and the height of the substrate h (in mm) are
known. Then a set of simplified equations of cavity model is used for calculating design
parameters of circular microstrip patch antenna as follows.
Radius of the circular patch is given by [28]:
𝑎 =
𝐹
[1 +
2ℎ
𝜋𝐹𝜀𝑟
(ln⁡
{
𝜋𝐹
2ℎ
} + 1.7726)]0.5
where,
𝐹 =
8.791 × 109
𝑓
𝑟√𝜀𝑟
𝜀𝑟= Dielectric constant of substrate
Length and width of the substrate can be calculated by
Substrate length, 𝐿𝑠𝑢𝑏 = 2 × 2𝑎
Substrate width, 𝑊𝑠𝑢𝑏 = 2 × 2𝑎

4
For WLAN band (5.15-5.825) GHz, antenna design parameters are calculated as follows. Here
FR4 (lossy) substrate is used with dielectric constant 4.4 and the of the substrate height is chosen
as 1.4 mm. Then
(1) Resonance frequency 𝑓
𝑟=
5.15+5.825
2
= 5.4875⁡𝐺𝐻𝑧 ≈ 5.5𝐺𝐻𝑧
(2) 𝐹 =
8.791×109
𝑓𝑟√𝜀𝑟
= 7.619 mm
Radius of the circular patch =
𝐹
[1+
2ℎ
𝜋𝐹𝜀𝑟
(ln⁡
{
𝜋𝐹
2ℎ
}+1.7726)]0.5
= 6.4 mm
(3) Substrate length, 𝐿𝑠𝑢𝑏 = 2 × 2𝑎 = 25.6 mm
Substrate width, 𝑊𝑠𝑢𝑏 = 2 × 2𝑎 = 25.6 mm
Once we get calculated values of the antenna parameters, trial and error method is used to
improve the performance of the proposed antenna along with its size reduction. The geometry of
the proposed circular patch antenna is shown in Fig. 1. The radius of the circular patch is taken as
7.62mm and the antenna is printed on a FR4 (lossy) substrate having relative permittivity of 4.4
and substrate thickness of 1.4 mm. The dimension of ground plane, which is printed in the bottom
side of the substrate, is chosen to be 30mm×30mm. A circular slot is cut from the circular patch
and a square slot is cut from the ground plane to enhance the performance of the proposed
antenna. The radius of the circular slot is used as 4.5 mm and the dimension of the square slot
is19mm×19mm. Amicrostrip feed line with dimensions 6mm×2mm is used with the circular
patch to achieve the impedance matching 50𝛺 between the patch antenna and the transmission
line. The detailed dimensions of the proposed antenna are shown in table 1.
Table 1. Design Parameters and corresponding values
Parameter Length(mm) Width(mm) Height(mm) Material
Ground 30 30 0.035 Copper (annealed)
Substrate 30 30 1.4 FR-4 (lossy)
Microstrip line feed 6 2 0.035 Copper (annealed)
Patch Radius = 7.62 mm Copper (annealed)
Figure 1. Geometry of proposed circular patch antenna

5
4. SIMULATION RESULTS AND DISCUSSION
The proposed circular microstrip patch antenna is designed and simulated using CST simulation
software. An optimization between size reduction and performance enhancement is maintained in
this work until satisfactory results were obtained. The performance of the proposed antenna is
investigated in terms of the following parameters: return loss, bandwidth, VSWR, gain,
directivity, total efficiency, and radiation pattern.
4.1. Return Loss
Return loss or S parameter indicates how much electromagnetic power from the microstrip patch
antenna is returning back (reflected power) and therefore it is known as reflection coefficient. It
determines the quality with respect to impedance match between the sending end (source) and the
receiving end (measured load). To obtain a successful radiation mode, return loss must be below -
10 dB. Figure 2 shows the S parameter versus frequency plot of the proposed antenna. It is seen
that the resonance frequency of the proposed antenna is 5.5 GHz. The curve shows return loss of
the antenna is -31.58 dB at 5.5 GHz which indicates the impedance match is better and the energy
loss is lesser.
The bandwidth of the designed antenna can be determined from Fig. 2 as the frequency range
bounded by a return loss of less than -10dB. The estimated -10 dB impedance bandwidth of the
antenna is 0.7019 GHz  702 MHz (from 5.129 GHz to 5.8309 GHz) which implies that the
proposed antenna satisfying the WLAN bands of (5.15-5.825) GHz.
Figure 2. S-Parameter Vs. frequency curve of the proposed antenna
4.2. Voltage Standing Wave Ratio (VSWR)
The impedance mismatch between antenna and transmission line is measured by VSWR. The
VSWR value of less than 2 is acceptable for a good design antenna. Fig. 3 shows VSWR Vs.
frequency plot of the proposed antenna and it is seen that at the resonance frequency of 5.5 GHz
the value of VSWR is 1.054 which much less than 2. The lower VSWR indicating better
impedance match of the proposed antenna to the transmission line, and the higher the power
supplied to the antenna.

6
Figure 3. VSWR Vs. frequency curve of the proposed antenna
4.3. Gain
The parameter gain describes how efficient an antenna can send out or receive power in a
particular direction. In other words, it indicates how much power is transmitted in a given
direction to the radiation intensity that would be produced by an isotropic antenna. Figure 4
shows the 3D plot of far-field gain of the proposed antenna and the estimated peak value of gain
is 3.229dB at the resonance frequency of 5.5 GHz.
Figure 4. 3D plot of far-field gain at 5.5 GHz
4.4. Directivity
The directivity measures how much intensely the antenna radiates power in its preferred direction
and is defined by the ratio of the maximum power density to its average value over a sphere as
observed in the far field. The 3D plot of far-field directivity of the proposed antenna at the
resonance frequency of 5.5 GHz is shown in Fig.5. It is noticed that the peak value of directivity
of the designed antenna is 4.275dBi.

7
Figure 5. 3D plot of far-field directivity at 5.5 GHz
4.5. Total Efficiency
Antenna efficiency is defined as the ratio of the radiated power to the incident power at the
antenna in percentage. Figure 6 shows the total efficiency Vs. frequency curve of the proposed
antenna. It is observed that antenna efficiency is above 67% over the entire operating band (5.13
GHz - 5.83 GHz) with a maximum value of around 79% is achieved at the resonance frequency
of 5.5 GHz.
Figure 6. Total efficiency Vs. frequency curve of the proposed antenna
4.6. Radiation Pattern
The radiation pattern is the distribution of the radiated power from the antenna (in the case of
transmitting antenna), or received by the antenna (in the case of receiving antenna) as a function
of the direction angles from the antenna. In most cases, it is calculated in the far-field region and
explains both the E-field and H-field patterns.
4.6.1. E-field pattern
The polar plot of Far-field E–field pattern of the proposed antenna at 5.5 GHz is shown in Fig.7.
It is seen that the main lobe magnitude is 18 dBV/m and direction of main lobe is 10 degree. Fig.
8 shows the 3D plot of the Far-field E–field pattern of the designed antenna which shows the
maximum radiation intensity Emax is 17.99 dBV/m.

8
Figure 7. Polar plot of Far-field E –field pattern at 5.5 GHz
Figure 8. 3D plot of Far-field E –field pattern at 5.5 GHz
4.6.2. H-field pattern
Figure 9 shows the polar plot of Far-field H–field pattern of the proposed antenna at 5.5 GHz. It
is seen that the main lobe magnitude is -33.5 dBA/m and direction of main lobe is 10 degree. The
3D plot of the Far-field H-field pattern of the designed antenna is shown in Fig. 10 which shows
the maximum radiation intensity Hmax is -33.53dBV/m.

9
Figure 9. Polar plot of Far-field H-field pattern at 5.5 GHz
Figure 10. 3D plot of Far-field H-field pattern at 5.5 GHz
A summary of the above simulation results of the proposed compact designed circular microstrip
patch antenna is presented in Table 2.
Table 2. Summary of simulation results of the proposed antenna
Parameter Value
Resonance frequency 5.5 GHz
Return loss -31.5765
Bandwidth 0.7019 GHz (from 5.129 GHz to 5.8309 GHz).
VSWR 1.054
Total Efficiency about 79%
Gain 3.229dB
Realized Gain 3.226 dB
Directivity 4.275dBi

10
5. CONCLUSION
In this paper, we have proposed a compact circular microstrip patch antenna for WLAN
applications which covers the band 5.15 to 5.825 GHz. To reduce the size of conventional
antenna and to enhance the overall performance of the proposed antenna, a circular slot of radius
4.5 mm is loaded on circular patch and a square slot of dimensions 19×19mm is etched on the
ground plane. The antenna is designed using CST Microsoft Studio Simulation Software. Various
performance parameters like return loss, bandwidth, VSWR, gain, directivity, total efficiency and
radiation pattern are studied. The proposed antenna resonates at the frequency of 5.5 GHz with a
wider bandwidth of 702MHz (from 5.129 GHz to 5.8309 GHz) as expected. The antenna
provides low return loss of -31.58 dB, good gain of 3.23 dB and directivity of 4.28 dBi and high
total efficiency of around 79% at 5.5 GHz. These parametric results indicate that the proposed
compact designed circular microstrip patch antenna attains good performance and is suitable for
WLAN applications. An optimization between size reduction and performance enhancement is
maintained in this work.
In future work, we will focus on fabrication and testing of the proposed antenna and performance
comparison of the simulated and fabricated antenna. The performance of this antenna can be
further improved by cutting modified slots on the patch and ground plane, using different
substrate materials. By integrating this proposed single elemental design antenna into an array on
a single substrate, enhanced gain, directivity and efficiency can be obtained compared to its
singular counterpart.
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