# Cross-Slot Antenna with U-Shaped Tuning Stub for Ultra-Wideband Applications

Authors: Dawood Seyed Javan; Mohammad Ali Salari; Omid Hashemi Ghoochani
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

A novel design of an ultra-wideband (UWB) slot antenna is presented. This antenna operates as a transmitter and receiver antenna. Effects of the antenna dimensional parameters are studied through experimental and simulation results. Design procedures are developed and verified for different frequency bands. The experimental and simulation results exhibit good impedance bandwidth, radiation pattern, and relatively constant gain over the entire band of frequency. Antenna gain and directivity at boresight and in their maximum states are close to each other and indicate high radiation efficiency. To use the antenna as a linearly polarized antenna, the radiation pattern in E-plane is better than that in  H-plane.


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## Body

## 1. Introduction

The Federal Communication Commission (FCC) issued a ruling for ultra-wideband (UWB) implementation in data communication [1]. A UWB technology promotes communication systems, particularly in wireless multimedia systems with high data rate. According to FCC, a UWB antenna should
provide a gain and impedance bandwidth from 3.1 to 10.6 GHz. A microstrip slot
antenna may be a good choice as it is low-profile, low-cost, lightweight, easy
integration with monolithic microwave integrated circuits (MMICs). Feed
interactions of wideband slot antennas are analyzed using finite element-optimization
methods and effects of feeding mechanisms on dimensions of slots have been
discussed in [2]. Several methods have been proposed to increase the bandwidth
of microstrip-fed slot or cavity-backed slot antenna,
such as printed radial stub [3]. A printed wide-slot antenna is fed by a microstrip line with a fork-like tuning
stub for bandwidth enhancement [4]. A design of a microstrip-line-fed
printed wide-slot antenna had been studied in [5]. An ultra-wideband
coplanar-waveguide- (CPW-) fed slot antenna was excited by a 50-Ω CPW with a U-shaped
tuning stub [6]. Experimental investigations on a wideband slot antenna element
have been proposed [7] as a building block for
designing single- or multielement wideband or dual-band slot antennas. This
element shows bandwidth values up to 37%, if used in the wideband mode.A circular
slot antenna is fed by a circular open-ended microstrip line to provide UWB impedance
bandwidth [8]. Also an ultra-wideband square-ring slot antenna (SRSA) has been
proposed which is fed by a microstrip line with a U-shaped
tuning stub [9]. However, the SRSA is split inside the U-shaped feed, so it is called split square-ring
slot antenna (SSRSA). A printed rectangular slot antenna with a U-shaped tuning stub is backed with reflector for improvement
in the impedance bandwidth and unidirectional radiation patterns [10].In this paper, we propose a novel structure
that is driven by wide-slot antenna and merged by a cross-slot for improvement
in gain and impedance bandwidth. The measurement and simulation results of the
impedance bandwidth are in good agreement with each other.


## 2. Antenna Structure

Figure1 shows the proposed printed slot antenna. The antenna structure is a split square-ring slot in the ground plane of dielectric
substrate with a cross-slot in the center of the square ring. This structure is
fed by a single microstrip line with a U-shaped
tuning stub. The slot
antenna is fed near an edge by a microstrip line and a fictitious short circuit that
produces more resonant frequencies [7].Figure 1
Configuration of the proposed antenna.The SSRSA can be considered as a
combination of numbers of narrow slot radiators which are connected to each
other, so it can provide a couple of resonances at different frequencies. The
split in one arm actually increases the number of resonances by introducing new
resonant lengths. The cross-slot is located in the center of the square ring. Actually, the combination of the cross slot and
square ring slot can create more resonant
frequencies and this promotes the impedance
matching rather than [9]. This structure is fabricated on a 0.5 mm RO4003B substrate
with a dielectric constant equal to 3.4. A photograph of the proposed antenna is shown in Figure 2. The antenna includes a microstrip feed line with the U-shaped tuning stub. By
splitting the square-ring slot antenna (SRSA) and optimizing the feeding
network, the required impedance bandwidth is achieved over the UWB frequency range (3.1 to 10.6 GHz).The ground
plane size is Lg×Wg=100⁢mm×100⁢mm.Figure 2
Photograph of the antenna: (1) cross-slot; (2) square slot; (3) ground plane; (4) input port is connected to microstrip line.


## 3. Results and Discussions

Effect ofLT variations on the return loss of antenna, which has been evaluated by IE3D
software [11], is shown in Figure 3.Figure 3
Effect ofLT changes on return
loss. LT is the distance between the center of the
cross-slot and the square ring.The longer theLT, the better the impedance  matching, for the coupling between the square ring and the cross-slot.It is
obvious that the upper resonances are created by off-center microstrip feed [7].The
U-shaped tuning stub is employed for wideband performance. The impedance
matching of the proposed antenna is unfavorable when theLT is less than
13 mm. The prototype antenna was simulated by IE3D software and fabricated with LT=13⁢mm and Wt=1⁢mm. The simulation and
measurement results are shown in Figure 4.Figure 4
Measured and simulated return loss of proposed
antenna.The
resonant frequencies of the simulation and measurement results are in good
agreement within the matching frequency band 3 GHz to 12 GHz, which
corresponds to the impedance bandwidth (S11<−10 dB). This
structure is more compact than that in [8]. The antenna provides a VSWR lower than 2 (S11<−10 dB)
from 3 to 12 GHz.The parameter dimensions are obtained
after performing an optimization and identified in Table1. These dimensions
were obtained by performing an optimization for improving the impedance bandwidth
by ADS software [12]. Figure 5 shows the gain of optimized antenna at broadside
(φ=0,θ=0)
from 2 to 12 GHz. The directivity at the direction of maximum radiation is
shown in this figure. The antenna gain and directivity at boresight and in
their maximum states are close to each other and indicate high radiation
efficiency.Table 1
The dimensions of proposed antenna to mm.
ParameterWWsW1W2WtLl1l2l3l4l5l6l7LTMagnitude
(mm)2160.71313514.33144.10.61.83013Figure 5
Simulated values of gain and values of directivity (dBi).This
figure also shows that above 10.5 GHz the directivity and gain at boresight
increase and it is a reason for this: each slot of the cross-slot is a branch of the square ring. These
branches have theWt=1 mm and this is less than WS=6 mm. This causes high current
distribution flows to the cross-slot rather than square-ring slot and the gain
increases at boresight above the 10.5 GHz. So the combination of the cross-slot and
square ring slot improves the gain and
directivity more than that in [9]. Figure 6 shows the measured maximum gain and gain at boresight.Figure 6
Measured values of the gain.This figure shows that the gain at
boresight is more than 2 dBi and is relatively constant from 8 to12 GHz. The electric
field distribution on the square ring and cross-slot for proposed antenna was
simulated with the IE3D simulation software. Figure7 shows the electric field
distribution on the slots.Figure 7
Simulated electric field distribution on the slots at the frequency of 10 GHz.Figures8 and 9 show the measured H-plane
(xz plane) and E-plane (yz plane) radiation patterns for both
co- and cross-polarizations at f=3.5,5.5,7.5,9.5, and 10.5 GHz. From the results, it is
concluded that the proposed antenna in operating frequencies provides the same
polarization planes and similar radiation patterns.Measured radiation pattern of cross-slot antenna in H-plane (φ=0°). The solid line is copolar and the dash line is cross-polar
component.
(a)(b)(c)(d)(e)Measured radiation pattern of cross-slot antenna in E-plane (φ=90°). The solid line is copolar and the dash line is cross-polar component.
(a)(b)(c)(d)(e)To use the antenna as a linearly
polarized antenna, the radiation pattern in the E-plane is better than that in H-plane. The E- and H-plane patterns start to introduce spurious radiation in high frequency (i.e., from 8.5 GHz) because the
U-shaped stub length is almost equal to
a half wavelength. However, the radiation patterns start to change in high frequencies
and show higher directivities in other directions. In the E-plane, the cross-polar
radiation is at least −10 dB less than the copolar radiation.


## 4. Conclusions

A novel microstrip square-ring slot is merged by cross-slot
and fed with a U-shaped tuning stub. The proposed antenna has been designed, simulated, optimized, and measured for the broadband operation. The novel
antenna promotes gain, impedance bandwidth, and radiation pattern. The proposed antenna provides a good
impedance matching from 3 GHz to 12 GHz. This antenna has a favorable field
gain across the matching band as a desirable feature for UWB applications. The
gain and directivity at boresight are
close to each other and provide high radiation efficiency.



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*Source: 262981-2008-08-17.xml*