Abstract-A W-band dual-polarized Cassegrain antenna for cloud radar is proposed. The aperture diameter of the main reflector of the antenna is 50 cm. By using a modified Magic-T structure in the feed horn, the antenna is dual-polarized with high port isolation. The measured results show that the port isolation is 44.7 dB. The gains are 47.3 dB and 49.5 dB for the two ports at 94 GHz, respectively, and the efficiency of the antenna is better than 87%

Bing Yu

Fayu Wan

Fei Yang

Han Xia

Junxiang Ge

Xiaoye Chen

Yong Zhou

Zhenhua Chen

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Progress In Electromagnetics Research Letters, Vol. 61, 99–103, 2016
A Novel W-Band Dual-Polarized Cassegrain Antenna
for Cloud Radar
Bing Yu*, Han Xia, Xiaoye Chen, Fei Yang,
Fayu Wan, Zhenhua Chen, Yong Zhou, and Junxiang Ge
Abstract—A W-band dual-polarized Cassegrain antenna for cloud radar is proposed. The aperture
diameter of the main reflector of the antenna is 50 cm. By using a modified Magic-T structure in the
feed horn, the antenna is dual-polarized with high port isolation. The measured results show that the
port isolation is 44.7 dB. The gains are 47.3 dB and 49.5 dB for the two ports at 94 GHz, respectively,
and the efficiency of the antenna is better than 87%.
1. INTRODUCTION
Dual-polarized Cassegrain antennas at W-band have found extensive applications in millimeter cloud
radar [1–3]. Millimeter cloud radar with dual-polarization capability is highly desirable for the current
estimation of microphysical properties of cloud [4]. As we know, the polarization of the linear polarized
incident wave will be rotated by the non-spherical cloud particle such as water-drop or ice, and in this
case a dual-polarized Cassegrain antenna is deployed to receive the vertical and horizontal polarization
components in the backscattering wave respectively. The ratio between the two polarization components,
which is named linear depolarized ratio (Ldr), is an important parameter for the cloud radar to perform
cloud particle inversion [5, 6]. High precision measurement for Ldr is necessary since the rotation of the
polarization is usually very tiny, and thus high port isolation (> 40 dB) for the antenna at the same
frequency is required [7]. However, the co-frequency interference will be a serious challenge to fulfill
such high port isolation in W-band. Although a polarizer can be used to separate the two orthogonal
waves by transmitting one and reflecting another one [8], the loss caused by the polarizer will be a
serious issue in W-band. In [3], a 2 × 2 array square waveguide feeding structure is used in a W-band
Cassegrain antenna, and in [9], an ortho-mode transducer with multi-feed is deployed in an X-band
Cassegrain antenna, but their complex structure will increase the antenna cost and make it hard to
assembly in W-band. In this paper, a novel dual-polarized Cassegrain antenna with a simple structure
and high port isolation (> 44 dB) is proposed by using a modified Magic-T structure in the antenna
feed. We will first state the design of the antenna feed and then validate it through the measure results
of the proposed antenna.
2. ANTENNA DESIGN
For Cassegrain antenna, the port isolation feature is mainly determined by the antenna feed, which
we will give a detailed description in the following section. As well known, the conventional Magic-T
as shown in Fig. 1(a) is often used in microwave system with 4 ports (indexed by P1, P2, P3, P4),
and for an ideal lossless Magic-T, if port P1 is used as input port, port P2 will be isolated, and vice
Received 18 May 2016, Accepted 19 July 2016, Scheduled 22 July 2016
* Corresponding author: Bing Yu (yubing@nuist.edu.cn).
The authors are with the Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University
of Information Science & Technology, China.
100 Yu et al.
(a) (b) (c)
Figure 1. The evolution of the antenna feed structure with (a) conventional Magic-T, (b) modified
Magic-T and (c) antenna feed.
Figure 2. Simulated port isolation of the antenna feed.
(a) (b)
feed horn
sub-reflector
main reflector
F
y
x
p 2c
D
1 2
D
Figure 3. Cassegrain antenna in which (a) cross section and (b) 3D model.
versa. In this paper, the port isolation feature of Magic-T is deployed into the antenna feed design.
Firstly, the conventional Magic-T is modified by rotating the right part of the conventional Magic-T 90◦
counterclockwise as shown in Fig. 1(b). Secondly, for antenna to be used in cloud radar, the aperture
size of P3 should be enlarged to receive different linear polarization waves, and P4 is unused. The final
Progress In Electromagnetics Research Letters, Vol. 61, 2016 101
antenna feed structure is shown in Fig. 1(c). From this figure, it can be seen that a rectangular-circular
waveguide transformer is used to realize the resistance match for P1 to P3, and a metal screw is deployed
to realize the resistance matching of P2. P1 and P2 are both ended with WR-10 waveguide. Due to
the requirement of cloud radar, an elliptical aperture is adopted in P3 to obtain a similar contour of
the radiation patterns for E- and H-planes. The whole antenna feed has a symmetric structure in
the vertical direction which is benefitial for antenna fabrication and assembly. As shown in Fig. 2,
the simulated field distributions for source P1 on and source P2 on is performed by the high frequency
structure simulator (HFSS) respectively. From this figure, it can be seen that a high isolation between
the two ports can be realized by using the modified Magic-T structure, and the isolation is better than
−50 dB from 90 GHz to 100 GHz. The Cassegrain antenna diagram is shown in Fig. 3(a) designed by
a standard procedure [10], where D1 is the aperture diameter of the main reflector, D2 the aperture
diameter of the sub-reflector, F the focal length of the main reflector, p the length between the vertex of
the main reflector and the phase center of the antenna feed, and c the focal length of the sub-reflector.
The main reflector is formed by a rotational paraboloid which is written as
y2 + z2 = 4Fx (1)
The set of F/D1 is equal to 0.33, and the sub-reflector is formed by a rotational hyperboloid which is
written as
(x − c − p)2
a2
− y
2
b2
= 1 (2)
where a = 15 cm, b = 20 cm. In order to minimize the blockage of the sub reflector, the ratio of D2/D1
is set < 0.07 with
D2 =
√
kwλF (3)
where kw is the beam width constant of the feed and λ the wavelength. For optimizing antenna gain.
A −10 dB edge taper is used, which corresponds to a feed pattern with a cosine to the power, q equal
to 68.6 [11].
3. RESULTS AND DISCUSSION
The S-parameter of the feed horn as shown in Fig. 4(a) is measured by the Agilent PNA-X network
analyzer N5244A and millimeter wave VNA extender (75–110 GHz). The measured and simulated
results of the S-parameter are shown in Fig. 4(b). From this figure, it is seen that the measured isolation
(S12) is better than 44.7 dB at 94 GHz and also shows a good agreement between the simulation and
measurement results. The impendence bandwidth of P2 (S22) is narrow compared with P1 since the
impendence matching of P2 is achieved by using a metal screw which can be equivalent to a series
(a) (b)
Figure 4. Antenna feed (a) photograph of the fabricated feed and (b) measured S-parameter.
102 Yu et al.
Figure 5. Radiation pattern of P1. Figure 6. Radiation pattern of P2.
Table 1. Measure results at 94 GHz.
Ports Gain [dB] Side-lobe level [dB] 2θ0.5[◦] XPD [dB] Efficiency
P1 49.5 −17.3 0.434◦ 31 91.9%
P2 47.3 −14.4 0.38◦ 37 87.9%
capacitance and inductance circuit. The proposed Cassegrain antenna is measured by using a compact
range and the Co-pol and X-pol far-field radiation patterns of the proposed Cassegrain antenna, and
simulation results are shown in Fig. 5 and Fig. 6 for P1 and P2, respectively. From the two figures, it
can be seen that the gain of P1 is bigger than P2 mainly for the resistance mismatch of P2, and the
loss caused by the metal screw also contributes to the gain decline of P2. The measured results of gain,
side-lobe level, half power beam width (2θ0.5), cross polarization discrimination (XPD) and efficiency
for the two ports are shown in Table 1.
4. CONCLUSION
In this paper, a novel dual-polarization Cassegrain antenna for cloud radar is proposed. By introducing
a modified Magic-T structure in the antenna feed, the port isolation is better than 44.7 dB at 94 GHz,
and the efficiency of the antenna is better than 87%. Thanks to the modified Magic-T structure used
in antenna feed, the antenna is easy to be fabricated and assembled.
ACKNOWLEDGMENT
This work is supported in part by the Priority Academic Program Development of Jiangsu Higher
Education Institutions, State Postdoctoral Scientific Research Foundation, the National Special
Research Fund for Nonprofit Section (GYHY201206038), Jiangsu Innovation & Entrepreneurship Group
Talents Plan and High Gain Dual-mode Satellite Communication System, Motion Platform Development
and Industrialization Project.
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A Novel W-Band Dual-Polarized Cassegrain Antenna for Cloud Radar

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A Novel W-Band Dual-Polarized Cassegrain Antenna for Cloud Radar

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