Wideband CP magnetoelectric dipole
antenna with microstrip line
aperture-coupled excitation
Xuewu Cui, Feng Yang✉ and Min Gao
A novel wideband circularly polarised (CP) magnetoelectric dipole
antenna, which is excited by microstrip line aperture-coupled
feeding structure, is proposed and investigated. The wide impedance
bandwidth and broadband CP characteristic are achieved based on
the simple feeding structure. An antenna prototype is fabricated and
exemplified to validate the proposed concept. Measured results show
that the proposed antenna achieves an impedance bandwidth of 65%
for voltage standing wave ratio <2 from 2.97 to 5.85 GHz, the broadside maximum gain is up to 10.2 dBi. Moreover, the 3 dB axial ratio
bandwidth is about 75%, ranging from 2.7 to 5.8 GHz. Stable and symmetric unidirectional radiation patterns with low backward radiations
are obtained over the operating band.
thickness of 1 mm, εr = 2.65 and tan δ = 0.0007. Its length is about
ground (GD) = 120 mm. The ground plane with a crossed-type feed slot
located at the centre portion of the four patches is on the top surface of
the substrate. The feed network composed of three Wilkinson power
dividers, two wideband 90° phase shifters and a wideband 180° phase
shifter is printed on the bottom surface, as shown in Fig. 1c. Owing to
the similar principle of the wideband 90° and 180° phase shifters, the
structure of the wideband 90° phase shifter connected with Wilkinson
power divider is shown in Fig. 2. Path 1 is the reference line with characteristic impedance of 50 Ω. Path 2 has two shunted branches. One side is
open and the other is shorted. The dimensions are tabulated in Table 1.
Wilkinson
power divider
SPw1
Dw
Ds
port1 Lw Dl
Introduction: In 2006, Luk et al. designed a novel wideband antenna
with great electrical characteristics including broad bandwidth, stable
gain, low cross-polarisation levels, symmetrical radiation patterns and
high front-to-back ratios [1–4]. This antenna is named as magnetoelectric (ME) dipole antenna well known in the antenna community,
which is composed of an electric dipole and a magnetic dipole.
During the past decade or so, various kinds of modified ME dipole
antennas have been presented and include the circularly polarised
(CP) designs [5–8]. In [5], formed by two bow tie patches and trapezoidal shaped dipoles, a wideband CP antenna is reported, whose
3 dB axial ratio (AR) bandwidth can achieve 33%. In [6], a wideband
CP antenna is reported, which is realised by a CP crossed dipole and
an ME dipole. Its 3 dB AR bandwidth is about 27.7% [6]. In [7], a
CP ME dipole antenna is proposed, whose 3 dB AR bandwidth is
up to 71.5%. However, a specially designed cavity can limit its wide
application. A CP ME dipole antenna using the substrate integrated
waveguide technique is proposed, but its 3 dB AR bandwidth is only
about 25.9% [8].
In this Letter, based on the aperture coupled, a wideband CP ME
dipole antenna fed by a slot located on the ground of microstrip line
is proposed. The wider CP bandwidth is attributed to the aperturecoupled excitation and sequential rotation elements. The measured
impedance bandwidth is 65% (2.97–5.85 GHz) for voltage standing
wave ratio (VSWR) <2 and the 3 dB AR bandwidth is up to 75%
(2.7–5.8 GHz). The broadside maximum gain is up to 10.2 dBi. In
addition, symmetrical and stable radiation patterns with low crosspolarisation level and high front-to-back ratio are also obtained.
wideband
90° phase shifter
SPw 2
SPl 2
port2
SPl1
path 2
SPl 3
port3
R=100 Ω
path 1
Fig. 2 Structure of Wilkinson power divider and wideband 90° phase shifter
Table 1: Optimised dimensions
Parameter
Value (mm)
Pw
20
Ps
6.75
Sw
1.3
Sl
54
H
16
Dw
1.5
Dl
14
Parameter
Value (mm)
SPl1
22.8
SPl2
7
SPl3
33.2
SPw1
0.41
SPw2
2.02
Ds
0.3
Lw
2.75
Operating principle: The simulated current distribution and electric
field over the apertures are exhibited in Fig. 3 to understand the
operating principle. At t = 0 and T/2 (T is a period of time at 4 GHz),
the electric dipole in the x-direction is excited from the current distribution on the patches along the x-direction. Moreover, the electric
field on the portions of the apertures along the y-direction is dominant,
which indicates that the equivalent magnetic dipole in the y-direction is
excited simultaneously. Similarly, at t = T/4 and 3T/4, the electric dipole
is excited strongly in the y-direction, and the magnetic dipole is excited
strongly in the x-direction.
ground
patch
a
H
feed slot
b
substrate
a
wideband 90° phase shifter
GD
Sw
y
c
Pw
Sl
feed slot
Fig. 3 Current distributions and electric field on apertures over period
Ps
ground
b
d
x
wideband 180° phase shifter
c
Fig. 1 Structure of reported CP ME dipole antenna
a 3D view of CP ME dipole antenna
b Details of radiation patches and slots
c Details of feed network
Antenna configuration: The structure of the proposed CP ME dipole
antenna composed of two parts is given in Fig. 1. One is four identical
horizontal square planar patches, each attached at the top by two
adjacent vertically shorted patches. The other is a square PCB with
a t=0
b t = T/4
c t = T/2
d t = 3T/4
According to the analysis in [4, 8], the simultaneously excited electric
and magnetic dipoles in crossed location can operate as a complementary antenna. Meanwhile, the two sets of combination contributing
to the radiation with orthogonal polarisations are excited with phase
difference of 90° (corresponding to time delay of T/4). Additionally,
due to the similar amplitudes of the combinations, the unidirectional
CP radiation can be expected for this design.
Results: For the verification of the proposed design, a prototype of the
CP ME antenna is fabricated as shown in Fig. 4. It should be noted that
ELECTRONICS LETTERS 12th July 2018 Vol. 54 No. 14 pp. 863–864
in order to maintain the stable gain, a cavity with radius of 53 mm is also
considered in the prototype. The VSWR is measured using the
KEYSIGHT E5071C network analyser, and the radiation patterns AR
and gain are measured by NSI far-field measurement system.
The measured radiation patterns are compared with simulated results
in xoy and xoz planes at 3–5 GHz and presented in Fig. 6. Good agreement is obtained. The measured cross-polarisation level and backward
radiations are all below −20 dB, and the radiation patterns are generally
stable and symmetric.
Conclusion: In the Letter, based on aperture coupled, a novel wideband
CP ME dipole antenna is reported. The measured impedance bandwidth
is 65% (2.97–5.85 GHz) for VSWR<2. Moreover, the measured 3 dB
AR bandwidth is up to 75% (2.7–5.8 GHz). The measured boresight
maximum gain is up to 10.2 dBi. Symmetric and stable radiation
patterns with high front-to-back ratio are also obtained.
Acknowledgments: This work was supported by the National
Natural Science Foundation of China (61201056 and 11176007), the
Sichuan Provincial Science and Technology Support Programme
(2013HH0047), the Fok Ying Tung Education Foundation (141062)
and ‘111’ Project (B07046).
Fig. 4 Pictures of fabricated CP ME dipole antenna
As presented in Fig. 5, the measured VSWR, AR and gain of the proposed CP ME dipole antenna are compared with the simulated results.
The measured impedance bandwidth is up to 65% (2.97–5.85 GHz)
for VSWR<2, and well agreed with the simulated result (66% from
2.85 to 5.65 GHz). The measured boresight maximum gain is up to
10.2 dBi. The measured broadside 3 dB AR bandwidth achieves 75%
(2.6–5.7 GHz), which agreed well with the simulation.
16
VSWR/gain, dBi
12
15
12
9
10
gain
8
6
3
6
4
0
VSWR
2
2.5
3.0
–3
3.5
4.0
4.5
frequency, GHz
5.0
5.5
6.0
Fig. 5 Comparison of VSWR, gain and AR of proposed CP ME dipole antenna
xoz-plane
yoz-plane
0
0
–10
330
0
30
330
60
300
30
60
300
–20
–30
90 270
270
90
–20
–10
240
120
0
210
0
330
240
150
120
210
3.0 GHz
180
0
30
–10
180
0
330
60
300
150
30
60
300
–20
–30
90 270
270
Xuewu Cui, Feng Yang and Min Gao (School of Electronic
Engineering, University of Electronic Science and Technology of
China, Chengdu 611731, People’s Republic of China)
✉ E-mail: yangf@uestc.edu.cn
18
simulated VSWR
simulated gain
simulated AR
axial ratio, dB
measured VSWR
measured gain
measured AR
14
© The Institution of Engineering and Technology 2018
Submitted: 8 April 2018 E-first: 8 June 2018
doi: 10.1049/el.2018.1046
One or more of the Figures in this Letter are available in colour online.
References
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90
–20
–10
240
120
210
0
150
210
30
330
120
4.0 GHz
180
0
0
240
–10
330
60
300
150
180
0
30
60
300
–20
–30
90
90 270
270
–20
–10
0
240
120
210
150
180
240
5.0 GHz
measured co-pol.
simulated co-pol.
120
210
150
180
measured cross-pol.
simulated cross-pol.
Fig. 6 Comparison of radiation patterns
ELECTRONICS LETTERS 12th July 2018 Vol. 54 No. 14 pp. 863–864