# Printed UWB Antenna with Coupled Slotted Element for Notch-Frequency Function

Authors: X. L. Bao; M. J. Ammann
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

A novel printed monopole antenna employing a slotted-plate, which is electromagnetically coupled to the microstrip-fed planar element, is proposed to provide notch-frequency function. This technique enables stopband characteristics with improved control, compared to placing the slot in the microstrip-fed element. A detailed investigation of the rejectband properties has been made for the UWB antenna. Measured data for the optimized case show the 10 dB return loss bandwidth to be 9.8 GHz (from 2.80 GHz to 12.60 GHz) with a notchband frequency from 5.15 GHz to 5.825 GHz. Propagation measurements indicate that the electromagnetically coupled slot provides a greater reduction in stopband gain for the three principal planes, compared to placing the slot in the fed element. This is desirable to mitigate interference from WLAN systems. A full parametric study of the antenna is presented.


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## Body

## 1. Introduction

Ultrawideband (UWB) antennas have commanded increased
interest in the last 4 years, due to the rapid development of wideband wireless
communication systems. The planar monopole antenna is one of the candidate antennas proposed, due to low-cost, broad bandwidth, and attractive profile.
These antennas have been reported with many various shaped planar elements,
such as rectangular, disc, elliptical and triangular geometries [1–6]. Most UWB systems currently operate as Mode 1
devices, with an allocated spectrum from 3.1 GHz to 4.8 GHz. However, as demand
increases, and the limitations in silicon technology are surpassed, the higher
bands will become more populated. For
these UWB systems, it may be necessary to provide rejection of interference
from 802.11a WLAN
or Unlicensed-National Information Infrastructure (U-NII), in the band from
5.15 GHz to 5.825 GHz. Thus, the provision of a narrow
rejectband in the UWB allocated spectrum is desirable.
Printed antennas with notch function have recently been reported [7–9], where various shaped narrow slots are
embedded into the planar radiator to obtain the notch-frequency function. In [7–9], either U or H-shaped slots are
embedded in the radiator element to obtain notchband frequencies. A different
approach is to use parasitic elements which are coupled to the planar radiating
element to provide the notch function [10].
An alternative method to provide the notch function is using a novel coplanar
waveguide resonant cell in the CPW feedline as a narrow stopband filter [11]. In
this technique, control of the notch bandwidth is more difficult due to the
small dimensions involved. The proposed
antenna differs from [7–9] because the H-slot is coupled to the planar radiator
and is easier to tune. In comparison
with [10], the proposed antenna provides better stopband rejection. Employing the filter into the antenna
geometry can provide some stopband suppression with the advantages of low cost
and zero passband insertion loss. This
can be used alone or to complement additional filter circuitry where
required.In this
paper, the notching function is provided by adding an extra slotted plate which
is electromagnetically coupled to the microstrip-fed planar element, yielding
greater stopband rejection.It is
printed on the rear side of the substrate and provides the notch-reject
function. The parameters of the slot and the coupled-plate are analysed in terms of the notch-frequency and bandwidth.
A compact optimized UWB antenna with notch-frequency function was then fabricated
and examined both experimentally and numerically. The measured results show
that the proposed UWB antenna can achieve good impedance bandwidth
characteristics, with improved control of bandnotch function.


## 2. Numerical Results for the Printed Monopole Antenna with Slotted-Plate Coupled to Radiator-Plate

The geometry and coordinate system for the proposed printed
monopole antenna fed by a 50 ohm microstrip line is shown in Figure1. It is printed on a Taconic RF35 substrate with a
thickness of 1.52 mm, relative
permittivity of 3.5, and a loss tangent of 0.0018. The width of microstrip feed
line is fixed at 3.5 mm to
achieve a 50 ohm impedance. A rectangular plate with a length L1 and width W1 is connected to the 50 ohm microstrip line. The substrate is of
length L and width Wg. The ground plane has dimensions Lg and Wg. A section of groundplane of width W2 and length L2 is
removed in order to improve the matching and also slightly reduces the lower-edge
frequency [12]. The radiator plate is separated from the groundplane by a
distance Gw1. The simulation was carried out using the Finite-Integration Time-Domain
technique (CST MWS). The optimized printed monopole antenna dimensions are as follows: L = 28 mm, Wg = 30 mm, L1 = 16.5 mm, W1 = 12 mm, L2 = 4 mm, W2 = 3 mm, Gw1 = 1.5 mm, Lg = 10 mm.Figure 1
Geometry of the printed rectangular
monopole antenna showing coupled element and coordinate system: (a) rear side showing groundplane and coupled element with H-slot, (b) front side showing microstrip-fed planar monopole.To realise the proposed UWB antenna with the narrow rejectnotch band, a
rectangular element is placed on the rear of the substrate and couples to the microstrip-fed
plate. The H-shaped slot is removed from this rear plate as shown in Figure1(a). The centre-frequency and width of rejected-frequency band are mainly determined
by the size of H-shaped slot, its location in the coupled-plate, and the distance Gw2 between the coupled-plate and groundplane
edge. The effects of various parameters of the H-shaped slot are discussed
below. In this case, the rear plate size is selected at Ls = 12 mm and Ws = 12 mm, and an appropriate
H-slot is removed from this plate to achieve the required function.


## 3. Tuning of Notch Frequency

A parametric study was made, and a set of design rules to
tune the notch centre-frequency and control the notch bandwidth is presented.


### 3.1. The Effect of the Separation between Coupled-Plate and GroundplaneGw2

The separation distanceGw2
between the coupled-plate and the groundplane was varied, and results show
heavy dependence of the notch bandwidth on this parameter as shown in Figure 2. The dimensions of the H-shaped slot were Ls1 = 4 mm, Ws1 = 1.4 mm, Ls2 = 3 mm, Ws2 = 0.6 mm, and Lsc = 3 mm. It
can be seen that as the separation Gw2
increases, the rejected-frequency band becomes much narrower. However, the
improvement diminishes after a point and for this antenna requirement. In order
to increase the bandwidth of the notch, the separation distance Gw2 should be reduced.Figure 2
The simulated S11 for different values ofGw2.


### 3.2. The Effect of the Distance of Slot from the Plate Edge

The distanceLsc was
varied from 3 mm to 9 mm for H-slot dimensions of Ls1 = 4 mm, Ws1 = 1.4 mm, Ls2 = 3 mm, Ws2 = 0.6 mm,
and Gw2 = 3 mm. The simulated results show that only a small (0.5%)
shift in notch frequency took place with no other significant changes.


### 3.3. The Effect of Slot DimensionsWs2 and Ls2

The slot dimensionsWs2 and Ls2, were varied, for Ls1 = 4 mm, Ws1 = 1.5 mm, Gw2 = 3 mm, Lsc = 3 mm,
and Ls2 = 3 mm. It was observed that the bandwidth and
notch-frequency show no dependence on Ws2,
but that as Ls2 increases, the
centre-frequency is reduced the reject bandwidth remaining constant.Figure3 illustrates the return loss for variation of the
dimension Ls2 (Ws2 = 0.6 mm).Figure 3
Simulated S11 showing notch dependence onLs2.


### 3.4. The Effect of the Slot Length and Width

The return
loss curves in Figure4 illustrate that the slot length Ls1 has a smaller effect on notch frequency than the parameter Ls2, (Ws1 = 1.5 mm), and
Figure 5 shows that the centre-frequency of the stopband shifts down as the
slot width Ws1 increases. Here, Ls2 = 3 mm, Ws2 = 0.6 mm, Gw2 = 3 mm,
and Lsc = 3 mm. The notch frequency is heavily dependent on this
parameter. The effects of H-slot
parametric variation on the reject centre-frequency and bandwidth are displayed
in Table 1.Table 1
The effects of H-slot parametric
variation on stopband centre-frequency and bandwidth.
ParametersStopband centre-frequencyStopband bandwidthLs1Light dependenceConstantLs2Light dependenceConstantWs1Heavy dependenceConstantWs2No dependenceConstantLscLight dependenceLight dependenceGw2Light dependenceHeavy dependenceFigure 4
Simulated S11 showing notch dependence onLs1.Figure 5
Simulated S11 showing notch dependence onWs1.The distributions of surface current are simulated by using
CST microwave studio software, as shown in Figures6(a), 6(b), and 6(c). It is noted that surface current distributions
on the edges of the rectangular plate and groundplane are very strong. It was
also observed that the surface current distributions at the edges of the H-slot
are very strong at the notchband frequency of 5.55 GHz, but weak at other
frequencies. This is because the H-slot shape is highly resonant at the notch
frequency.(a) The distribution of surface
current at 3 GHz, (b) the distribution of surface current at 5.55 GHz, (c) the distribution of surface current at 8 GHz.
(a)(b)(c)


## 3.1. The Effect of the Separation between Coupled-Plate and GroundplaneGw2

The separation distanceGw2
between the coupled-plate and the groundplane was varied, and results show
heavy dependence of the notch bandwidth on this parameter as shown in Figure 2. The dimensions of the H-shaped slot were Ls1 = 4 mm, Ws1 = 1.4 mm, Ls2 = 3 mm, Ws2 = 0.6 mm, and Lsc = 3 mm. It
can be seen that as the separation Gw2
increases, the rejected-frequency band becomes much narrower. However, the
improvement diminishes after a point and for this antenna requirement. In order
to increase the bandwidth of the notch, the separation distance Gw2 should be reduced.Figure 2
The simulated S11 for different values ofGw2.


## 3.2. The Effect of the Distance of Slot from the Plate Edge

The distanceLsc was
varied from 3 mm to 9 mm for H-slot dimensions of Ls1 = 4 mm, Ws1 = 1.4 mm, Ls2 = 3 mm, Ws2 = 0.6 mm,
and Gw2 = 3 mm. The simulated results show that only a small (0.5%)
shift in notch frequency took place with no other significant changes.


## 3.3. The Effect of Slot DimensionsWs2 and Ls2

The slot dimensionsWs2 and Ls2, were varied, for Ls1 = 4 mm, Ws1 = 1.5 mm, Gw2 = 3 mm, Lsc = 3 mm,
and Ls2 = 3 mm. It was observed that the bandwidth and
notch-frequency show no dependence on Ws2,
but that as Ls2 increases, the
centre-frequency is reduced the reject bandwidth remaining constant.Figure3 illustrates the return loss for variation of the
dimension Ls2 (Ws2 = 0.6 mm).Figure 3
Simulated S11 showing notch dependence onLs2.


## 3.4. The Effect of the Slot Length and Width

The return
loss curves in Figure4 illustrate that the slot length Ls1 has a smaller effect on notch frequency than the parameter Ls2, (Ws1 = 1.5 mm), and
Figure 5 shows that the centre-frequency of the stopband shifts down as the
slot width Ws1 increases. Here, Ls2 = 3 mm, Ws2 = 0.6 mm, Gw2 = 3 mm,
and Lsc = 3 mm. The notch frequency is heavily dependent on this
parameter. The effects of H-slot
parametric variation on the reject centre-frequency and bandwidth are displayed
in Table 1.Table 1
The effects of H-slot parametric
variation on stopband centre-frequency and bandwidth.
ParametersStopband centre-frequencyStopband bandwidthLs1Light dependenceConstantLs2Light dependenceConstantWs1Heavy dependenceConstantWs2No dependenceConstantLscLight dependenceLight dependenceGw2Light dependenceHeavy dependenceFigure 4
Simulated S11 showing notch dependence onLs1.Figure 5
Simulated S11 showing notch dependence onWs1.The distributions of surface current are simulated by using
CST microwave studio software, as shown in Figures6(a), 6(b), and 6(c). It is noted that surface current distributions
on the edges of the rectangular plate and groundplane are very strong. It was
also observed that the surface current distributions at the edges of the H-slot
are very strong at the notchband frequency of 5.55 GHz, but weak at other
frequencies. This is because the H-slot shape is highly resonant at the notch
frequency.(a) The distribution of surface
current at 3 GHz, (b) the distribution of surface current at 5.55 GHz, (c) the distribution of surface current at 8 GHz.
(a)(b)(c)


## 4. Comparison of Proposed Antenna Performance with Other Rectangular Planar UWB Antennas

The proposed antenna
with coupled slotted plate (Figure1, antenna A) is now compared to two other
antennas, one with a simple microstrip-fed rectangular monopole (antenna B,
without notch function) and the other with the slots in the microstrip-fed
element (antenna C) as shown in
Figures 7(a) and 7(b), respectively. The basic parameters were kept the same for
all 3 antennas as follows: L = 33 mm, Wg = 30 mm, L1 = 16.5 mm, W1 = 12 mm, L2 = 4 mm, W2 = 3 mm, Gw1 = 1.5 mm, and Lg = 15 mm. The same substrate
was used; Taconic RF35 with relative permittivity of 3.5 and thickness of 1.52 mm.
For the antenna with the H-shape slot in
the microstrip-fed element (Figure 7(b), antenna C), the other optimized
dimensions are Lsc = 2.25 mm, Ls1 = 4 mm, Ws1 = 2.5 mm, Ls = 4 mm, and Ws = 0.3 mm. For the antenna with the coupled slotted plate
(Figure 1, antenna A), the H-shape parameters are selected in Ws = 12 mm, Ls = 12 mm, Gw2 = 3 mm, Lsc = 3 mm, Ls1 = 4 mm, Ws1 = 1 mm, Ls2 = 3 mm,
and Ws2 = 0.6 mm. The substrate had a width of 30 mm and
length of 28 mm in each case. The
simulated results of S11 for three
antennas are shown in Figure 8. It is
seen that both antenna A and antenna C show a notchband between 5.15 GHz and
5.825 GHz in the response.Figure 7
(a) Simple rectangular UWB monopole geometry, (b) rectangular
monopole with H-slot in microstrip-fed element.Figure 8
Comparison of simulated S11 for the three planar UWB antennas.Compared to the
rectangular UWB, printed antenna with the H-shaped slot in the microstrip-fed
element shown as
Figure7(b) (antenna C), this new proposed UWB antenna is much more convenient to tune
the notch frequency and bandwidth by changing the H-slot dimensions,
independent of the other antenna dimensions.
This is in contrast to tuning the H-slot in the microstrip-fed planar
element, which necessitates a change in the dimensions of the planar element,
thus changing the overall S11 response, and requiring an iteration in the
design.Also,
the proposed antenna can provide a deeper drop in notch gain.


## 5. Measurements of the Proposed Antenna

Figure9 presents the simulated and
measured return losses for the proposed antenna, which are in good agreement. The measured 10 dB return loss impedance bandwidth is about 9.8 GHz from 2.80 GHz to 12.60 GHz
with the narrow notch-frequency band from 5.15 GHz to 5.825 GHz.Figure 9
Comparison of the measured and simulated S11 for the proposed
antenna.The S21 was measured for the three antennas in thex, y, and
z directions as illustrated in Figure 10. The illuminating antenna was a
wideband horn antenna. It is observed that the proposed antenna has the
greatest reduction in notchband gain, therefore providing better notch
rejection.The measured S21 for the three antennas in thex, y, and
z
directions.
(a)(b)(c)The CW gain pattern is shown in Figure11, which is
determined from the directional measured transfer function. The plots in Figures 11(a) and 11(b) illustrate
the measured patterns for the XoZ and XoY, planes for the UWB frequency range
(from 3.1 GHz to 11 GHz), respectively. These results show the power
distributions in the 3D space against frequency. The notch can be clearly seen. The
measured group delay for the three UWB antennas is displayed in Figure 12, which show that the group delay is reasonably
constant within about 1nanosecond except for the notchband. It shows that the antenna has low-impulse
distortion and is suitable for UWB applications.The measured copolar gain of the
proposed antenna plotted against frequency for the principla planes: (a)XoZ, (b) YoZ.
(a)(b)Figure 12
The measured
group delay for the three UWB antennas.


## 6. Conclusions

A notch-frequency
band for an UWB antenna is obtained by employing a slotted coupled-plate. The
effects of antenna parameters on the notch bandwidth and centre frequency are presented
in detail. The measured results are in agreement with the simulated results. This
parametric study is valuable in the design of wideband compact antennas with a narrow
notched-band. The novel notchband UWB
antenna provides easier tuning of the notch-frequency function and bandwidth with
good stopband rejection. Group delay and
radiation characteristics are appropriate for this application.



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*Source: 713921-2008-03-17.xml*