# Design of a CPW-Fed Band-Notched UWB Antenna  Using a Feeder-Embedded Slotline Resonator

Authors: Amin M. Abbosh
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

A complete design method for a compact uniplanar ultra-wideband antenna with subband rejection capability is presented. A slotline resonator is incorporated in the coplanar waveguide feeder of the antenna to act as a bandstop filter, hence enabling the rejection of any undesired band within the passband of the antenna. Two samples of the proposed antenna were designed and manufactured. One of the developed antennas does not contain a resonator, whereas the other contains a slotline resonator. The designed antennas feature a compact size of 27 mm× 27 mm. Results of the simulation and measurement show that the designed antennas have a bandwidth from 3 GHz to more than 11 GHz. The results also reveal that the use of the resonator in the feeder of the antenna efficiently rejects any undesired subband, such as the 4.9–5.9 GHz band assigned for IEEE802.11a and HIPERLAN/2. The gain of the antennas with the resonator is about 2.2 dBi at the passband, while it is less than −8 dBi at the rejected subband.


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## Body

## 1. Introduction

Ultra-wideband (UWB) is a short range communications technology. It has gained a lot of popularity among research-ers and the wireless industry after the FCC permitted the marketing and operation of UWB within the range 3.1 GHz to  10.6 GHz [1]. The global interest in the UWB technology is increasing very fast due to the capability of this license exempt
wide bandwidth system to yield low cost, low energy, short range, and extremely high capacity wireless communication links.In addition to the requirement of the 3.1–10.6 GHz bandwidth, the UWB antennas should also have the capability to reject any interference with existing wireless networking technologies such as the subband 4.9–5.9 GHz for
IEEE 802.11a in the USA, and HIPERLAN/2 in Europe. Therefore, UWB
antennas with notched characteristics at certain bands are desired. The ability to provide this function in the antenna can significantly relax the requirements imposed upon the filtering electronics within
the wireless communication system.One of the first attempts to design a
band-notched ultra-wideband antenna was presented in [2]. The design of a UWB antenna with a good impedance matching over the desired band was achieved using a genetic algorithm optimization code. A manual trial-and-error approach was then utilized to modify the design towards a band-notched antenna. In a later
stage, the authors of [2] extend their optimization technique to improve the
radiation pattern of their band-notched design [3].A systematic
method for designing frequency notched ultra-wideband antennas was presented in
[4]. By deliberately introducing a narrow band resonant structure, an antenna
may be made capable of rejecting particular frequencies. This technique is
useful for creating UWB antennas with narrow frequency notches, or for creating
multiband antennas.Reviewing the literature shows that there
are many different methods used to achieve the band-notched function. The
conventional methods are cutting a slot of different shapes on the radiating patch [5–7], inserting a slit
on the patch [8], embedding a quarter-wavelength tuning stub within a large
slot on the patch [9], putting parasitic elements near the radiator as filters
to reject the limited band [10], or introducing a parasitic open-circuit
element, rather than modifying the structure of the antenna’s tuning
stub [11].This paper describes a different
design method for a UWB antenna with a subband rejection capability. Instead of
modifying the radiator as with the proposed methods in the literature, a slotline
resonator is incorporated in the coplanar waveguide feeder of the antenna to
filter out any undesired subband with negligible effect on the passband of the
antenna. The method presented in this paper is used to design a
uniplanar UWB antenna which rejects the 4.9–5.9 GHz band, which is assigned for the IEEE802.11a and HIPERLAN/2. The
results of simulations and measurements for two manufactured antennas (one
without a resonator and the other with a slotline resonator) are presented in
order to prove the validity of the proposed method.


## 2. Design

The configuration of the proposed UWB
antenna with the capability of rejecting frequencies over a certain subband is
illustrated in Figure1. The radiating
structure is formed by the connection of half an ellipse with a rectangular
patch. The antenna is assumed to be fed using a coplanar waveguide (CPW) to
enhance its broadband characteristics. The ground plane, which is located
around the CPW feeder, is in the shape of half an ellipse.(a) Configuration of the proposed antenna, (b) details of the slotline resonator, and (c) the transmission-line model of the resonator.
(a)(b)(c)The design procedure starts by finding dimension of the antenna
feeder to give 50 Ω characteristic impedance
(Zo). This can be achieved using the following
equations [12]: (1)Zo=30πεeK'(k)K(k),k=ss+2wc,εe=εr+12[A+B],A=tanh{1.785log(h/wc)+1.75},B=kwch{0.04−0.7k+0.01(1−0.1εr)(0.25+k)}, where K(k) is the first kind elliptical integral and K′(k)=K(1−k2), s is the central conductor width, whereas wc is the slot width of the CPW, h is the substrate thickness, and εr is the dielectric constant of the
substrate.Depending on the lowest frequency of operation (fl=3.1 GHz), thickness of the substrate and its
dielectric constant, width (w), and length (l) of the antenna structure are calculated as (2)w=l=c2flεr, where c is the speed of light. The antenna is assumed to be in the xy plane, with the dimension w extending along the x-axis. Note that the length and width of the antenna structure,
according to (2), are equal to half of the wavelength inside the substrate
medium.It is worthwhile to mention that (2) gives an accurate
estimation of the required dimension of the antenna for a low value of the
dielectric constant of the substrate, that is, less than 4. If it is required
to find a rough estimation of the required dimension for any value of the
dielectric constant, the following formula can be used: (3)w=l=c2fl(εr+1)/2.The ground plane of the antenna, which is located around
the CPW, is half of an ellipse with major diameter equal to (w). The secondary diameter of the ground plane
can be chosen to be around (0.5w).The radiating structure, which is located at a distanceg from the end of the ground plane, consists of
two parts. The first part, which is connected directly with the feeder, is half
an ellipse with the same dimensions as for the ground plane. The second part is
a rectangular patch which extends from the end of the half ellipse till the end
of the substrate. Parametric analysis on
the best value for g indicates that it should be less than
thickness of the substrate h in order to get the widest bandwidth. In the
design procedure of this paper, g is assumed to be equal to (h/3).Note that the above choice for dimension of the radiator
and the ground plane results in a horizontal elliptical dipole of a length
equal to a half wavelength, and it is elevated from the ground by a distance
which is about a quarter wavelength.The explained design procedure (1)–(3) results in an
antenna which covers the whole UWB range from 3.1 GHz to 10.6 GHz. If it is
required to reject a certain subband within that range, a slotline resonator
can be incorporated in the feeder of the antenna in the manner shown in Figure 1(a).
Configuration of the resonator is shown in
Figure 1(b). Length of the
slotline resonator is assumed to be equal to a quarter of the effective wavelength
calculated at the center of the rejected band.To understand effect of the slotline used in this paper,
a transmission line model of the coplanar waveguide and the slotline resonator
is shown in Figure1(c). The resonator is equivalent to a short-circuited end
series stub. At the center of the rejected band, the stub’s length is equal to
a quarter wavelength which means that the stub appears effectively as an open
circuit at its point of connection with the main transmission line, which is
the coplanar waveguide for the design presented in this paper. This comes from
the fact that the input impedance of a quarter-wavelength stub is equal to Zo2/Zt [13], where Zo is the characteristic impedance of the stub
and Zt is the terminal impedance of the stub, which
is equal to zero in the stub considered in this paper. At the frequencies, which are far away from
the resonance frequency of the stub, the resonator has negligible effect on the
performance and the feeder and hence, the antenna performs as if there is no
resonator.It is to be noted
that in order to use the space available for the stub efficiently and to make
the antenna compact in size, the stub is folded in the manner shown in Figure1(b).


## 3. Results

The proposed UWB antenna was designed using GML1032
substrate with a dielectric constant equal to 3.2, tangent losstanδ=0.004, and thickness of 1.52 mm. Values
of the design parameters for the antenna and the incorporated resonator were first calculated using the proposed design procedure and then optimized using the software Ansoft HFSSv10. The optimized values are: w=l=27 mm, s=2.9 mm, wc=0.25 mm, g=0.5 mm, lr=11 mm, ws=0.3 mm, and sr=0.1 mm.Two samples of the proposed antenna were manufactured,
one without and the other with the slotline resonator. Characteristics of the
developed antennas were tested via simulations using the software HFSSv10 and
via measurements using a vector network analyser in an anechoic chamber. Figure2 shows variation of the SWR with frequency for the developed antennas. The simulated and measured characteristics of the
antenna without a resonator reveal UWB behavior with bandwidth from 3 GHz to
more than 11 GHz assuming SWR = 2 (or 10 dB return loss) as a reference. It is also clear from Figure 2 that, for the
antennas with a resonator, the undesired subband was tuned out, whereas the
wideband behavior of the antenna was maintained. The simulated and measured
results are in good agreement.Figure 2
Variation of the SWR with frequency.It is worthwhile to mention that the slot width of the resonatorsr can be used to adjust width of the rejected
subband. A parametric analysis using the software HFSS shows that increasing sr increases width of the rejected band and its level
of rejection, that is, a higher VSWR at the rejected band with a larger slot
width. Moreover, the parametric analysis shows that width of the part of the feeder
bounded by the resonator, that is, ws in Figure 1(b), has a significant effect on the level of rejection. Optimization techniques can be utilized to achieve a
certain compromise between the required width of the rejected band and the level of rejection.From the UWB applications point of
view, the antenna is usually required to have an omnidirectional radiation.
Concerning the designed antenna, this requirement is fulfilled over the whole
bandwidth as shown in Figure3 for the antenna without a resonator. The
radiation patterns of the antenna with a resonator are similar to the results
shown in Figure 3 and therefore, they are not shown here.The measured radiation pattern of the antenna at (a) 3 GHz and (b) 6 GHz.
(a)(b)Figure4 shows variation of the
measured gain of the developed antennas across the ultra-wideband. It is clear
from the results in Figure 4 that the antennas have a low gain, which agrees
with the expected behavior of the omnidirectional antennas. The gain of
the antenna without a resonator
increases with frequency from around 0 dBi at 3.1 GHz to
3 dBi at the range 9–11 GHz. Concerning gain of the developed antenna with the slotline resonator, the measured results, presented in Figure 4, show that the general behavior of the
gain is similar to that of the antenna without a resonator except for the
rejected subband. Across the band 4.9 GHz to 5.9 GHz, the gain can be as low as −8 dBi, when a resonator is used, compared
with 2.2 dBi for the antenna without
a resonator. This proves the high capability of the proposed resonator
to reject the undesired subband.Figure 4
Variation
of the measured gain with frequency for the manufactured antennas.The last investigation concerns
variation of the radiation efficiency for the designed antennas with and
without the resonator. Results of the calculations using the software HFSS indicated that the proposed antennas feature a good efficiency, being greater than 92% across the desired band.


## 4. Conclusion

This paper has presented a complete design method for a compact ultra-wideband antenna with band-notched characteristics. The subband rejection is accomplished by incorporating a slotline resonator in the coplanar waveguide feeder of the antenna. To validate the proposed method, two samples of the proposed antenna were designed and manufactured: one does not contain a resonator, whereas the other contains a slotline resonator. Results of the simulation and measurement have shown that the designed antennas have an ultra-wideband performance with bandwidth from 3 GHz to more than 11 GHz. The results have also shown effectiveness of the resonator in rejecting any undesired band, such as the 4.9-5.9 GHz band, where the gain of the antenna with the resonator was reduced by more than 10 dB at the rejected band.



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*Source: 564317-2008-04-16.xml*