Abstract. Two different antipodal Vivaldi antennas design with better performances are proposed. The one is a small antipodal Vivaldi antenna for a 4.1-20 GHz frequency range, and its dimension is only 34 × 50 mm 2 . Another proposed high gain antenna can achieve the maximum gain of 15.2 dBi within its operating band. Measured results of the manufactured antipodal Vivaldi antennas are in good agreement with the simulated ones. Besides the |S11|, far field radiation pattern, current distribution, phase, gain, and group delay which are important parameters of impulse distortion in ultra-wide band antenna are well analyzed. All results show that the proposed antipodal Vivaldi antennas have better performances and they are well suited for ultra-wide band communications

Chen Xu

Guo Zhu Jia

Xiao Qing Yang

Ying Ya Zhai

CiteSeerX

International Journal of Applied Electromagnetics and Mechanics 46 (2014) 527–536 527
DOI 10.3233/JAE-141937
IOS Press
Design of antipodal Vivaldi antenna with
better performances for ultra wideband
applications
Xiao Qing Yanga, Ying Ya Zhaia, Chen Xub,∗ and Guo Zhu Jiac
aCollege of Electronics and Information Engineering, Sichuan University, Chengdu, Sichuan, China
bInstitute of Nuclear Science and Technology, Key Laboratory of Radiation Physics and Technology,
Ministry of Education, Sichuan University, Chengdu, Sichuan, China
cSichuan Normal University, Chengdu, Sichuan, China
Abstract. Two different antipodal Vivaldi antennas design with better performances are proposed. The one is a small antipodal
Vivaldi antenna for a 4.1–20 GHz frequency range, and its dimension is only 34 × 50 mm2. Another proposed high gain antenna
can achieve the maximum gain of 15.2 dBi within its operating band. Measured results of the manufactured antipodal Vivaldi
antennas are in good agreement with the simulated ones. Besides the |S11|, far field radiation pattern, current distribution, phase,
gain, and group delay which are important parameters of impulse distortion in ultra-wide band antenna are well analyzed. All
results show that the proposed antipodal Vivaldi antennas have better performances and they are well suited for ultra-wide band
communications.
Keywords: Ultra-wide band, small antenna, high gain, Vivaldi antenna
1. Introduction
During the last few years, the ultra-wide band (UWB) technology development is getting faster and
faster. The antenna is one of the important components in UWB systems. Many kinds of UWB antennas
have been proposed in recent years such as monopole, fractal, printed circuit board antennas, and so
on [1–6].
Many antennas are studied in order to improve broadband, such as the exponentially tapered slot
antenna can provide a broad band (up to a ratio 4.5:1) within an array featuring a scan angle up to
45◦ [7]. But asymmetric feed structures tend to produce high cross-polarization. Since in 1979, Gibson
proposed a tapered slot antenna, also known as a Vivaldi antenna [8]. Various forms of Vivaldi antennas
have been developed so far. Furthermore, antipodal Vivaldi antennas have been proposed as a solution
for the above problem. It is found that in the design of a printed UWB antenna, the radiator and the
ground plane shape as well as the feeding structure can be optimized to achieve a good broad impedance
bandwidth [9–11].
∗Corresponding author: Chen Xu, Institute of Nuclear Science and Technology, Key Laboratory of Radiation Physics and
Technology, Ministry of Education, Sichuan University, Chengdu 610065, Sichuan, China. E-mail: xuchen9269@163.com.
1383-5416/14/$27.50 c© 2014 – IOS Press and the authors. All rights reserved
528 X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances
Fig. 1. The model of antenna 1.
(a) (b)
Fig. 2. The model of antenna 2, (a) The total model of antenna
2, (b) The amplifying U shape slot in the antenna 2.
The study of antipodal Vivaldi antennas has attracted the attention of many researchers, and the differ-
ent characteristics of them are studied. In [12], adding an elliptical strip on the substrate can decrease the
lower frequency band while the variation of |S11| remains smaller than −10 dB in the rest of the band.
In [13], an ultra broadband direction finding antenna array covering the whole ultra high frequency range
from 0.3 to 3 GHz is introduced. In [14], a small antipodal Vivaldi antenna is designed for two substrates,
FR4 and Rogers RO3006. The peak gain of antenna achieves 5 dBi.
With the development of communication technology, the characteristics of some conventional anten-
nas cannot meet some communication development requirements, such as broadband, small size, high
gain, and so on. As an UWB antenna, small size is advantageous for conformal system, and high gain
can enhance the degree of energy radiation in the case of ensuring broadband. On the one hand, in some
communication fields such as PDAs, laptop, MP3, 3G wireless phone, videophone, and so on, small
size is to pay close attention to the case of keeping other parameters available because the volume of
those systems is limited. On the other hand, in those fields such as military communication, satellite
communication, microwave radio relay communication, and so on, high gain antenna is taken into ac-
count because of high communication quality. In this paper, two antipodal Vivaldi antennas with better
performances are studied by numerical calculation finite-difference time-domain (FDTD) method. One
is the small antipodal Vivaldi antenna whose ground plate is designed by transition curve. Another is
the high gain antenna with U shape structure. Finally, they are fabricated to perform measurements for
validation.
2. Modified Vivaldi antenna model
The models of modified antipodal Vivaldi antenna are shown in Figs 1 and 2. There are three basic
parts in the antipodal Vivaldi antenna: Transmission line, Radiator, Transition between transmission
line and the radiator. The transmission line is a microstrip that is compatible with monolithic microwave
integrated circuits. Generally, the transition between transmission line and the radiator has an exponential
form. Actually, this is the most important part that affects the impedance matching. In this paper, the xy-
plane and yz-plane are referred to as the E and H planes respectively. Two symmetric exponentially
X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances 529
Fig. 3. Top and bottom views of the fabricated antenna 1. Fig. 4. Top and bottom views of the fabricated antenna 2.
tapered patches lie on opposite sides of the substrate. The exponentially taper patch is defined by the
opening rate γ and the two points D1 (x1, y1) and D2 (x2, y2) (the start and the end points of the
exponential taper)
x = c1e
γy + c2 (1)
where
c1 =
x2 − x1
eγy2 − eγy1 (2)
c2 =
x1e
γy2 − x2eγy1
eγy2 − eγy1 (3)
Given the highest frequency (fH), the width W1 of the antipodal Vivaldi antenna should satisfy Eq. (4)
to avoid any grating lobes for the Vivaldi antenna.
W1 <
c
fH
√
εe
(4)
where c is the speed of light in vacuum, εe is the effective dielectric constant.
The optimization method is based on the genetic algorithm (GA). The GA is a robust, stochastic and
global optimization method simulated on the concepts of natural selection and evolution. There are two
sets of four structural parameters needed to be optimized. The fitness function plays an important role
in the GA optimization because it influences the direction of GA optimization. In this paper, the fitness
function should take wide bandwidth and high gain into account, hence it is defined as
Fitness = C1 ∗ Bandwidth + C2 ∗ Gain (5)
where Fitness is the fitness value, Bandwidth refers the |S11| < −10 dB bandwidth, Gain denotes the
gain over a measured frequency band, C1 and C2 are weight factors. Through repeated applications of
selection, crossover and mutation operators, the initial population is transformed into a new population in
an iterative way. New population will obtain better chromosomes and will finally converge to an optimal
population. In the process of simulation, when the fitness function is bigger than the threshold value or
530 X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances
(a) (b)
Fig. 5. The |S11|, (a) antenna 1, (b) antenna 2.
(a) (b)
Fig. 6. Measured radiation pattern of antenna 1 at 5, 15, and 18 GHz: (a) H plane, (b) E plane.
(a) (b)
Fig. 7. Measured radiation pattern of antenna 2 at 3, 6, and 8 GHz: (a) H plane, (b) E plane.
X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances 531
(a) (b)
(c)
Fig. 8. The copolarization and crosspolarization radiation patterns for antenna 1 at (a) 5 GHz, (b) 15 GHz, (c) 18 GHz.
the GA do not find a better population within 400 successive generations, the GA will be terminated and
a solution will be obtained.
The geometry of antenna 1 is shown in Fig. 1, and the U shape slot which can increase gain in antenna
2 is shown in Fig. 2. The two antennas are printed on PTFE (εr = 2.65, tan δ = 0.003) substrate with
the thickness of 1 mm respectively. In Fig. 1, the dimensions are optimized as follows: L1 = 50 mm,
L2 = 8.5 mm, L3 = 3 mm, W1 = 34 mm. It is a small UWB antipodal Vivaldi antenna. In Fig. 2, the
dimensions are optimized as follows: L4 = 85 mm, L5 = 450 mm, L6 = 36 mm, W2 = 300 mm. The
width of U shape in antenna 2 is 0.8 mm. The top and bottom views of the fabricated antennas are shown
in Figs 3 and 4, respectively.
The gain reflects the ability of an antenna to radiate the power of a given waveform. In order to
improve gain, the U shape slot is proposed in current radiation direction. From the point of view of
physical characteristic, it effectively adds the current path.
532 X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances
(a) (b)
(c)
Fig. 9. The copolarization and crosspolarization radiation patterns for antenna 2 at (a) 3 GHz, (b) 6 GHz, (c) 8 GHz.
3. Results and discussion
UWB antipodal Vivaldi antennas are analyzed using FDTD method. By optimizing their parameters,
the better characteristics are obtained in the proposed antennas.
In antenna design, |S11| provides an estimate of an antenna’s match. And it is important that the
|S11| is below −10 dB across the entire UWB spectrum. Measured and simulated |S11| are compared
in Fig. 5. Using an Agilent Vector Network Analyzer, the measured |S11| of the proposed antenna 1 is
obtained at 4.1–20 GHz and that of antenna 2 is obtained at 1.3–12 GHz, as depicted in Figs 5(a) and (b),
respectively. From Fig. 5, it is observed that the measured |S11| reasonably agrees with the simulated
results. Measured results indicate that the antenna 1 can operate across the entire UWB spectrum with
|S11| of lower than −10 dB, and the |S11| of antenna 2 is below −10 dB within the operating frequency
range except for a few points. The frequency shift between measured and simulated |S11| is attributed to
the fabrication inaccuracy.
Figure 6 shows the measured radiation pattern of E and H planes at 5, 15, and 18 GHz for the proposed
X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances 533
(a) (b)
Fig. 10. The peak gain, (a) antenna 1, (b) antenna 2.
(a) (b)
Fig. 11. The current distribution, (a) antenna 1, (b) antenna 2.
antenna 1, and Fig. 7 shows the measured radiation pattern of E and H planes at 3, 6, and 8 GHz for
antenna 2. It can be seen from Figs 6 and 7 that the measured radiation patterns of antennas 1 and 2
are symmetrical. Figure 8 shows the copolarization and crosspolarization radiation patterns for antenna
1 at 5, 15, and 18 GHz. Figure 9 shows the copolarization and crosspolarization radiation patterns for
antenna 2 at 3, 6, and 8 GHz. The measured radiation patterns are similar and stable across the operating
frequency from Figs 6–9. The results clearly show that two proposed antipodal Vivaldi antennas meet
the UWB radiation characteristic.
Figures 10(a) and (b) show the gains of the fabricated antennas 1 and 2 respectively. As can be seen
from Fig. 10, the gain gradually increases with the increase of frequency. Above a certain frequency,
the gain begins to decrease. The decrease can be attributed to the dielectric loss. The maximum gains
in Figs 10(a) and (b) approach 9.6 dBi and 15.2 dBi which are increased by 4.6 dBi and 10 dBi than
ordinary antenna respectively. At 10 GHz, the gains of antennas 1 and 2 are increased by 2.5 dBi and
534 X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances
(a) (b)
Fig. 12. The phase, (a) antenna 1, (b) antenna 2.
(a) (b)
Fig. 13. The group delay, (a) antenna 1, (b) antenna 2.
8.5 dBi than ordinary antenna respectively.
Figures 11 and 12 show the current distribution and phase of the fabricated antennas 1 and 2 respec-
tively. Figure 11 shows current densities on the exponentially tapered patch decrease with the distance
from feed point, and the current densities are higher at the edges of the slot.
Besides the parameters already mentioned above, group delay is also an important parameter in UWB
antenna design, which represents the degree of distortion of pulse signal.
The group delay of an antenna is the frequency dependence of the time delay [15]. It is defined as
follows:
τ(ω) = −dj(ω)
dω
= −dj(f)
2πdf
(6)
where j(f) is the frequency-dependent phase of the radiated signal.
X.Q. Yang et al. / Design of antipodal Vivaldi antenna with better performances 535
The group delay is obtained from the derivative of the phase response. A nondistorted structure is char-
acterized by a constant group delay, i.e., linear phase, in a relevant frequency range. The nonlinearity of
a group delay indicates the resonant character of the device, which implicates the ability of the structure
to store the energy. It produces ringing and oscillation of the antenna impulse response. Figures 13(a)
and (b) show clearly the group delay of the fabricated antennas 1 and 2 respectively.
From Figs 13(a) and (b), it is observed that the variations of group delay on antennas 1 and 2 are only
2.5 ns and 4 ns, respectively. The group delay time is superior to [16] whose group delay time is 58 ns.
This guarantees the stability of transmission signal to some extent.
4. Conclusions
In this paper, two designs for antipodal Vivaldi antenna with better performance are presented and
analyzed. One is the small Antipodal Vivaldi antenna whose ground plate is designed by transition
curve. Another is high gain antenna with U shape slot structure. Finally, they are fabricated to perform
measurements for validation. The different types of UWB characteristics such as |S11|, far field radiation
pattern, gain, current distribution, phase and group delay are well analyzed, especially the group delay as
an important parameter of impulse distortion in UWB antenna. UWB antennas with better performances
are an essential part of UWB radar and communication systems in relevant fields. The proposed antipodal
Vivaldi antennas give good results and they are well suited for UWB communications.
Acknowledgment
This project was supported by 973 programs (No. 2013CB328905), National Natural Science Founda-
tion of China (No. 61372043 and No. 61102044), Sichuan Province Science and Technology Plan (No.
2013GZX0149) and Fund of Chinese Academy of Sciences Laboratory (No. 11H1772).
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Design of antipodal Vivaldi antenna with better performances for ultra wideband applications

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Design of antipodal Vivaldi antenna with better performances for ultra wideband applications

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