The Effect of Element Width on the Impedance Bandwidth and Mode for the
Rectangular Monopole Antenna
J. A. Evans and M. J. Ammann
School of Electronic & Communications Engineering
Dublin Institute of Technology, Kevin St., Dublin 8, IRELAND
Abstract
The rectangular planar monopole is examined. The paper investigates the effect of varying the width of the planar element
for a fixed height. Parameters such as impedance bandwidth, current distribution, mode excitation and radiation pattern are
examined. It is shown that the width has significant effects on the impedance bandwidth, mode excitation and radiation
pattern.
Background
Monopole antennas with planar radiating elements were first outlined by Meinke and Gundlach in 1968 [1], who
described it as a variant of the cylindrical and conical monopole. It was described in more detail by Dubost and
Zisler in 1976 [2] who observed the impedance and radiation characteristics of this antenna. In the last 5 years,
many geometries for the planar element have been reported, with most work being carried out on the circular and
square shapes [3-5]. The planar monopole has become a popular candidate for modern communications systems
due to its wide impedance bandwidth. The square element has a fractional impedance bandwidth of about 70%
whereas the circular disk element has been reported to have an impedance bandwidth ratio of greater than 10:1
[Honda et al, 6]. Recent modifications to the geometry have been shown to introduce additional modes [Lee et
al, 7] and to give good control of upper edge frequencies [Ammann et al, 8].
Introduction
The height of the planar monopole antenna is the determining factor for the lower edge of the impedance
bandwidth, whereas the upper edge frequency has been shown to be dependent on feedgap height. This paper
investigates the effect of varying the width of the planar element for a fixed height. Parameters such as
impedance bandwidth, current density, mode and radiation pattern are investigated.
Antenna geometry
The antenna comprises a rectangular planar radiator made of brass. This element is mounted on an SMA
feedprobe, which is fed through the 100 mm square groundplane. The feedprobe has a radius of 0.6 mm, and the
feedgap, g=2 mm. The length, l, of the planar element is fixed at 40 mm, while the width, w was varied from 20
mm to 80 mm. The antenna geometry and coordinate system are shown in Figure 1.
z
l
θ
w
y
g
x
φ
Figure 1. The rectangular planar monopole and coordinate system.
Modelling
These elements were modelled using both transmission-line modelling (TLM) and method-of moment (MoM)
methods. The MoM methods employed a finite-gap voltage-feed and a wire-grid for the antenna with piecewise
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sinusoidal basis functions. For the TLM method, a 0.375 mm cell size was used. Both methods are in good
agreement with experimental data. This can be seen in Figure 2, which illustrates the return loss for a
rectangular monopole of height 40 mm and width 60 mm, over the range 1 to 6 GHz. The feedgap was 2 mm.
MoM
Measure
TLM
0
-5
S11 (dB)
-10
-15
-20
l=40, w=60, g=2 (mm)
-25
-30
0
1
2
3
4
5
6
7
8
Frequency (MHz)
Figure 2. Modelling of a rectangular monopole using MoM and TLM show good agreement with measured data.
Effect on impedance bandwidth and mode
Return loss plots are shown in Figure 3 for w=20 mm to w=80 mm in 20 mm steps. Intermediate widths are
omitted for clarity. The impedance bandwidth for this work is defined as separation between the two 10 dB
return loss frequencies, for the fundamental mode. The presence of several modes can be seen, in particular for
w=20 mm. These modes overlap for some values of width (w= 30); thus a large impedance bandwidth. As w
increases, the modes move in frequency and some diminish, separate or disappear. The first mode is seen to
diminish at w=60mm, giving a reduced bandwidth. A higher mode now exists at 5.7 GHz due to the increased
width. The frequency of this mode reduces to 4.7 GHz for w=80 mm and the first mode vanishes. A plot of
impedance bandwidth for the first mode is shown in Figure 4 and the optimum width is shown to be w=30 mm.
0
1.8
-5
10dB Bandwidth (GHz)
1.6
S11 (dB)
-10
-15
-20
20mm
40mm
60mm
80mm
-25
-30
-35
-40
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Frequency (GHz)
Figure 3. Measured return loss for w=20 mm to w=80 mm..
6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
20
25
30
35
40
45
50
55
60
Antenna width, w
Figure 4. Measured Impedance Bandwidth (10dB return
loss) for w=20 mm to 60 mm for the first resonance.
Surface current density
TLM analyses of the surface current densities on the planar elements and on the groundplane for the different
values of width w, show the existence of different modes. The surface current density for w=60 mm is shown in
Figure 5 for frequencies of 2.08 and 5.71 GHz. Darker shades indicate areas of high current density.
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Figure 5.Surface current density on the planar element for w=60 mm.
Radiation patterns
Radiation patterns have been shown to be stable with change of frequency over the bandwidth for the square
element, (typically 70% impedance bandwidth). TLM simulated patterns for the w=20 mm w= 60 mm are
shown in Figure 6. Figure 6a represents the patterns for both w= 20 mm (1.68 GHz) and for w=60 mm (2.08
GHz), which are typical monopole patterns. Figs 6b and 6c are for w=20 mm at 5.95 GHz and w=60 mm at 5.71
GHz, respectively. Previous work has reported a maximum gain of about 4.8 dBi at the first resonance and this
increases by about 1 dB at the higher frequencies [Chen et al, 9]. The H-plane is Eθ (φ , θ = 90) and the E-plane
is Eθ (θ , φ = 0) .
20mm 5.95GHz
20mm 1.68GHz & 60 mm 2 GHz
90
120
60
150
30
180
0
330
240
300
270
Figure 6. (a)
120
60
180
0
210
300
270
(b)
H-Plane
E-Plane
60
30
180
0
-40 -30 -20 -10 0
0
330
240
120
150
30
-60 -40 -20
90
H-Plane
E-Plane
150
-40 -30 -20 -10 0
210
60mm 5.71GHz
90
H-plane
E-plane
210
330
240
300
270
(c)
Conclusions
The rectangular planar monopole impedance bandwidth and mode is dependent on element width as well as
feedgap height. The geometry for maximum bandwidth is rectangular with an aspect ratio of 3:4.
References
1. Meinke, H and Gundlach, F.W, 1968, Taschenbuch der Hochfrequenztechnik, Springer-Verlag, Berlin, New
York, 1968, pp. 531-535.
2. Dubost, G. and Zisler,, Antennas a Large Bande, Masson, Paris, New York, 1976, pp.128-129.
3. J. A. Evans and M. J. Ammann, “Planar Trapezoidal And Pentagonal Monopoles with Impedance
Bandwidths in Excess of 10:1,” IEEE International Symposium on Antennas and Propagat., 1999, (3), 15581561.
4. Agrawall, N. P, G. Kumar, and K. P. Ray, , “Wide-Band Planar Monopole Antenna,” IEEE Trans.
Antennas & Propagat., 1998, AP-46, (2) 294-295
5. Ammann, M. J., “Square Planar Monopole Antenna, ”IEE NCAP, 1999, 37-40
6. Honda, S, Ito, M, H. Seki, and Y. Jinbo, “A Disk Monopole Antenna With 1:8 Impedance Bandwidth And
Omnidirectional Radiation Pattern,” ISAP“92, 1992, Sapporo, Japan, 1145-1148
7. Lee, E, Hall, P.S. and Gardner, P., ‘Compact wideband planar monopole antenna’ Electron Lett. 1999, (35),
2157-2158.
8. Ammann, M. J and Chen, Z. N, A Wideband Shorted Planar Monopole with Bevel, IEEE Trans. Antennas &
Propagat.. 2003, AP-51 (4), 901-903.
9. Chen, Z N, Ammann M J, Chia, M Y M and See S P, Annular Circular Planar Monopole Antennas
IEE Proceedings, Microw. Antennas & Prop, 2002, 149, (4), 200-203.
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