PIERS ONLINE, VOL. 4, NO. 3, 2008
326
Study of a Circular Disc Monopole Ultrawide-band Miniature
Antenna
Lingling Zhong, Bo Sun, Jinghui Qiu, and Ning Zhang
Department of Electronics and Communication Engineering
Harbin Institute of Technology, Harbin 150001, China
Abstract— Study of a novel planar circular disc monopole ultrawide-band antenna fed by
coaxial line is presented in this paper. The radiator, ground plane and the feeder equipment of
the antenna are placed on the same plate. The special structure reduces the spatial volume, and
it is used to realize the miniaturization of the antenna. The basic theory and design method
are analyzed, and detailed exploration is conducted to determine the antenna’s properties. The
current distributions, return loss, radiation patterns and gain of the antenna are discussed. In
addition, the time domain performance of the proposed antenna is also evaluated in simulations.
The research results show that this kind of planar antenna can radiate and receive short pulse
signals without distortion. The 10 dB return loss bandwidth is from 2.5 GHz to 55 GHz. And
it can realize near omnidirectional pattern in the H-plane. It is a real planar structure and
can really reduce the spatial volume. On this basis, the structure of the planar circular disc
monopole antenna is improved. The radiator moves around the upper edge of the ground plane.
The new structure reduces the antenna height to a certain degree and it is used to achieve further
miniaturization. Analyses are conducted to determine the novel antennas’ properties. The result
of study indicates that the improved antenna can realize good bandwidth performance as the
planar circular disc monopole antenna, and it has low-cost, simple structural characteristics.
The novel miniature antenna and the improved type are suitable for the wireless communications,
satellite communications and mobile communications systems with good prospects.
1. INTRODUCTION
With the development of RF (Radio Frequency) technology and UWB equipment, there has been
considerable research effort put into ultrawide-band antenna. In recent years, monopole antennas
are the focus of UWB antenna. Several broadband monopole configurations, such as circular,
square, elliptical, pentagonal and hexagonal, have been proposed so far [1–4]. However, they are
not planar structures as the ground planes are perpendicular to radiators.
In this paper, a novel circular disc monopole antenna fed by coaxial line is proposed. It is a real
planar structure. The radiator, ground plane and the feeder equipment of the antenna are placed
on the same plate. This reduces the spatial volume to a great degree. And then the structure of
the planar circular disc monopole antenna is improved. Its structure reduces the antenna height
and further miniaturization is realized.
2. BASIC THEORY OF THE PLANAR CIRCULAR DISC MONOPOLE ANTENNA
The proposed planar monopole is illustrated in Figure 1. Copper can be chosen as the material,
and its thickness d ≈ 1 mm. The sheet is cut as a disk with radius r. Ground plane is composed of
two pieces of rectangle metal (length of side is m and n), and they are placed on the same plane
with the circular radiator. The antenna is fed by a coaxial line. Core of the coaxial is exposed out
as the feed point, and the outer metal is connected with ground planes. The lower end of the disk
is adopted as the feed point. The height of the feed gap is h.
Theoretically, circular disc monopole with radius r could be equivalent as cylindrical monopole
with height l and radius a. Its first resonant frequency can be determined by equating the area of
the circular disk to that of the cylindrical monopole, and equating the height of the disk 2r to the
height of the cylindrical monopole l, i.e., πr2 = 2πal, 2r = l [5]. The length of monopole for real
input impedance is given by
l = 0.24λF
(1)
where F = (l/a)/(1 + l/a).
From the above equations, the first resonant frequency of the circular disc monopole is given by
f=
3.2
r
(2)
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327
where l and r are in centimeters.
y
y
r
z
x
d
circular
monopole
h
n
m
coaxial line
ground
plane
Figure 1: Geometry of the planar circular disc monopole.
3. SIMULATED RESULTS AND ANALYSIS OF THE PLANAR CIRCULAR DISC
MONOPOLE ANTENNA
A dimension of the proposed circular disc monopole antenna is chosen, i.e., r = 10.7 mm, m = n =
16.4 mm, h = 0.6 mm, and d = 1 mm. And the corresponding first resonant frequency is 3 GHz.
The simulation of the antenna is performed using the CST Microwave Studio package which utilizes
the finite integration technique for electromagnetic computation [6].
3.1. Current Distributions
Current distribution at 3 GHz is showed in Figure 2. The current is mainly distributed along the
edge of the disc. As circular disc monopole could be equivalent as infinite dipole antennas, and all
of them have the same feed point, the disc dimension mainly defines the first resonant frequency [7].
Besides the disc, current is distributed on the upper edge of the ground plane. The portion of the
ground plane close to the disc acts as the radiating structure. Figure 3 shows the return loss for
different dimensions of the transverse length of the ground plane m (return loss at 3 GHz is showed
in the block). With increase of m, the first resonant frequency decreases.
Aˋm
10
9.69
8.44
7.19
5.94
4.69
3.44
2.19
0.938
0
Figure 2: Simulated current distributions at 3 GHz.
Figure 3: Simulated return loss curves for different
m.
3.2. The Effect of the Height of the Feed Gap Between the Feed Point and the Ground Plane
The simulated return loss curves for different feed height h are presented in Figure 4. Lots of
simulations demonstrate that h would influence the return loss and bandwidth to a great extent. For
circular disc monopole, the ground plane serves as an impedance matching circuit. Consequently,
it tunes the input impedance and hence the 10 dB return loss bandwidth by changing h [8]. With
the increase of h, the first resonant frequency decreases at lower frequencies. When f = 3 GHz, the
return loss is less than −15 dB.
S-Parameter Magnitute in dB
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0
-5
-10
-15
-20
-25
-30
0
Figure 4: Simulated return loss curves for different
h.
328
10
20 30 40
Frequency/GHz
50
60
Figure 5: Simulated return loss of the planar antenna.
3.3. Return Loss
Simulated return loss of the circular disc monopole from 0 GHz to 60 GHz is showed in Figure 5.
The 10 dB return loss bandwidth is from 2.5 GHz to 55 GHz, and 15 dB bandwidth is from 10 GHz
to 28 GHz. As a result, the circular disc monopole antenna has bandwidth ratio of 20 : 1 at least.
3.4. Radiation Patterns and Gain
Radiation patterns and gain are important factors to evaluate the performance of the UWB antenna.
The simulated normalized radiation patterns at 3 GHz, 6 GHz and 10 GHz are plotted in Figure 6.
It is noticed that the simulated E-plane (x-y plane) pattern is like a traditional monopole, and the
H-plane (x-z plane) pattern is near omnidirectional. The radiation patterns are steady within the
bandwidth 3–10 GHz.
90
120
60
f =3 GHz
f =6 GHz
f =10 GHz
180
0
-30 -20 -10 0 10 [dB]
30
180
-20
-10
240
300
270
(a)
0
10 [dB]
z
y
240
0
330
210
330
210
60
150
30
150
f =3 GHz
f =6 GHz
f =10 GHz
90
120
x
300
270
x
(b)
Figure 6: Simulated normalized radiation patterns of the planar antenna. (a) E-plane, (b) H-plane.
Figure 7 illustrates the simulated maximum gains of the proposed antenna. It is shown that the
maximum variation of the gain is 6 dB within the frequency range 3–60 GHz. With the increase of
the frequency, the maximum gain increases too. When f = 50 GHz, the maximum gain is greater
than 5 dB.
3.5. Time Domain Performance
Apart from the consideration of the 10 dB return loss bandwidth and radiation patterns, a good
time domain characteristic is an essential requirement for an UWB antenna. Flat group delay time
indicates all frequencies are delayed for the same time. This is conducive for the maintenance of
the signal waveform. Figure 8 is the simulated group delay curve of the planar antenna. The curve
is nearly constant throughout all the bands. This ensures the planar circular disc monopole can
radiate and receive short pulse signals without distortion.
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7
0 .6
Group Delay Time in ns
6
Gain in dB
5
4
3
2
1
0
0
10
20
30
40
50
60
Frequency/GHz
Figure 7: Maximum gains of the planar antenna.
0 .5
0 .4
0 .3
0 .2
0 .1
0
- 0 .1
0
10
20
30
40
Frequency/GHz
50
60
Figure 8: Group delay curve of the planar antenna.
4. IMPROVED CIRCULAR DISC MONOPOLE MINIATURE ANTENNA
On the basis of the foregoing statements, the structure of the planar circular disc monopole antenna
is improved. The radiator moves around the upper edge of the ground plane φ, as Figure 9 shows.
This reduces the height of the antenna to a certain degree. It is favorable for the miniaturization
of the antenna. The return loss for φ = 10◦ , φ = 30◦ and φ = 45◦ are plotted in Figure 10
respectively. It can be seen from the figure that the improved antenna is able to achieve good
bandwidth characteristics as the planar circular disc monopole antenna.
y
y
z
d
r
x
circular
monopole
h
φ
n
m
coaxial line
ground
plane
Figure 9: Geometry of the improved UWB antenna.
Figure 10: Return loss of the improved UWB antenna.
5. CONCLUSIONS
The novel planar circular disc monopole antenna is investigated detailedly in this paper. The basic
theory and design method are analyzed and research results show that this kind of planar antenna
can radiate and receive short pulse signals without distortion. The 10 dB return loss bandwidth
is from 2.5 GHz to 55 GHz. And it can realize near omnidirectional pattern in the H-plane. It is
a real planar structure and can reduce the spatial volume. On the basis, the disc of the planar
circular monopole moves around the upper edge of the ground plane a certain angle. The result
of study indicates that the improved antenna reduces the height and can realize good bandwidth
performance, and it has low-cost, simple structural characteristics. The novel miniature antenna is
suitable for wireless and satellite communication system.
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