The Design and Analysis on Broadband Circularly-Polarized
Slot Antenna
Chenguang Li, Hanwei Wu, Liping Han, Liyun Yan and Wenmei Zhang
College of Physics and Electronics Engineering, Shanxi University, Taiyuan 030006, China
Email: zhangwm@sxu.edu.cn
Abstract — A design of a broadband circularly-polarized
(CP) slot antenna is proposed in this paper. The CP radiation
of the slot antenna is excited by the proximity coupling of a
microstrip line. To obtain a broad CP bandwidth, three CP
modes contained in the CP band are excited. The results
show that the proposed antenna can provide a CP operating
bandwidth of 56% (3.17 GHz - 5.65 GHz) within an
impedance bandwidth of 64% (2.96 GHz - 5.75 GHz).
Index Terms — Broadband, circular polarization, slot
antenna.
I. INTRODUCTION
Recently, the circularly polarized (CP) antenna has been
widely used in satellite communications and pulse radars,
in which wider impedance and axial ratio (AR) bandwidth
are required. However, antenna designs with a CP
bandwidth of more than 30% are relatively few in the open
articles, and hence are still a challenge for antenna
engineers. Some antennas with relatively narrow CP
bandwidth (>10%) have been proposed [1-2], but feed
network is complexed. Recently, some wide slot antennas
are proposed. In ref. [3] and [4], a circularly-polarized
(CP) slot antenna loaded with a parasitic patch and L-fed
was proposed and CP bandwidth can achieved 33% and
45% respectively. In [5], a low-profile, low-cost, L-shaped
monopole slot antenna with a C-shaped feed was
introduced, and it has a CP bandwidth of 23%. In [6], a
partially covered circular slot has been presented, the
resulting CP bandwidth increase to 40%.
In this paper, a new method to obtain broadband
circularly-polarized (CP) bandwidth is proposed. In order
to get a wider CP bandwidth, we design more than one CP
mode in the CP band [7]. Details of the simulated results
are shown and discussed.
II. ANTENNA STRUCTURE
Fig. 1 shows the geometry of the proposed antenna. It
comprises three different circular slots with radius of r
1
, r
2
and r
3
fabricated on a rogers substrate with a thickness of
1.57 mm and a permittivity of 3.3. They are fed by the
proximity coupling of a microstrip feed line that is printed
on the other side of the rogers substrate. The whole
microstrip feed line consists of a 50Ω microstrip line, an
impedance transformer with the dimensions of l
1
× c
1
, and
a taper segment with the dimensions of l
3
. Every circular
slot can generate one CP mode. When the three modes
generated by three different circular slots combine
together, a wider-band CP antenna can be obtained.
III. BROADBAND CP DESIGN
The numerical analysis for different dimensions is made.
The results indicate that the second circular slots have
larger influence on the axial ratio bandwidth and the l
1
and
l
3
mainly affect the impedance match. The simulated
results are shown in Figs. 2 - 6.
A. Coupling Three CP Modes by w
2
Fig. 2 shows the axial ratio against frequency when w
2
is decreased from 6 to 4 mm and the other dimensions are
fixed. It is clearly seen with the decreasing of w
2
three
resonant frequencies approach each other. Meanwhile, the
axial-ratio variation between the three resonant
frequencies is gradually stable. When w
2
is 4 mm, the
axial-ratio values are less than 3 dB from 2.95 GHz to 5.5
GHz.
w
4
=3mm
w
2
=3mm
5.4mm
w
3
=2.8mmm
c
2
=2mm
l
3
=11mm
32.8mm
29mm
60mm
r
2
=11mm
c
1
=0.5mm