# Parametric Study of Ultra-Wideband Dual Elliptically Tapered Antipodal Slot Antenna

Authors: Xianming Qing; Zhi Ning Chen; Michael Yan Wah Chia
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

Parametric study of the impedance and radiation characteristics of a dual elliptically tapered antipodal slot antenna (DETASA) is undertaken in this paper. Usually, the performance of the DETASA is sensitive to the parameters, the effects of major geometry parameters of the radiators and feeding transition of the DETASA on antenna performance are investigated across the frequency range of 1–18 GHz. The information derived from this study provides guidelines for the design and optimization of the DETASAs which are widely used for UWB applications.


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## Body

## 1. Introduction

The cochannel interference and multipath effects of wireless communication systems
can be reduced by using directional antennas [1–4]. Some of the current point-to-multipoint systems
are using horn antennas for this reason, but the horn antennas are too bulky to be
integrated with the rest of the wireless packages and suffer high cost of fabrication.
Tapered slot antennas (notch antenna, Vivaldi antenna) have been widely used in phased
and active arrays for radar systems for many years [5, 6]. They are good candidates for multifunction
communication applications because of their stable directional patterns and
consistent impedance matching over a very broad operating frequency range without any
tuning elements as well as low profile and unobtrusive planar structures. Therefore,
they have been proposed for emerging UWB wireless communications and radar
applications [7–10].The dual elliptically tapered antipodal slot antenna (DETASA)
[11, 12] is a modified
version of the antipodal Vivaldi radiator [13]. It
differs from the conventional antipodal Vivaldi antennas since the inner and outer
edges of the slotline radiator of the DETASA are elliptically tapered. The slotline
radiator is fed by a pair of parallel strip lines which are transformed from a
microstrip line. The variations of antipodal Vivaldi antennas were studied both
analytically and experimentally [14, 15]. However, the reports hardly discuss effects of antenna parameters on
the impedance and radiation characteristics of the DETASA, which is vital for an
engineer to design and optimize the antenna.Therefore, this paper investigates the effects of major geometrical parameters of
the DETASA on the impedance matching, gain, and radiation patterns to provide engineers
with a clear design guideline. First, Section2
shows a design as a reference for the following discussion. The geometry of the
DETASA as well as comparison of simulated and measured results is introduced.
Then, Section 3 demonstrates the effects of the
antenna parameters on the impedance matching. After that, Section
4 discusses the impact of the antenna
parameters on the radiation characteristics including gain, cross-polarization
levels, and radiation patterns in both E-and H-planes. Finally, conclusions
are drawn in the last section.


## 2. Antenna Configuration

Consider a typical DETASA antenna shown in Figure1. It comprises two main parts:
tapered slotline radiator and feeding transition, which are usually printed on a
piece of PCB. The tapered slotline radiator shown in Figure
1(b) is configured by two
conducting arms which are symmetrically on opposite sides of a substrate with
respect to the y-axis. The slot tapers of the conducting arms
follow the outline of a quarter ellipse with major axis
a1 and minor axis b1; the
outer tapers are also elliptically tapered, and take the profile of a quarter ellipse
with major axis a2 and minor axis
b2. Tapering the outer conductor edge provides the
convenience for antenna feeding and additional design degrees of freedom for
optimizing antenna impedance and radiation performance. It is known that the
lower frequency limit of this type of antenna is determined by the cutoff
mechanism of the flare, namely, at the lowest operating frequency, the aperture
(Wr) is λs/2, where
λs is the wavelength of the
slotline [5, 11, 16]. The feeding transition is shown in Figure
1(c) where a 50-Ω microstrip line (strip width,
Ws, and ground width, Wg) is
transformed to a parallel offset strip line (width
Wp, offset S) to feed the tapered
slotline radiator. The strip line is linearly tapered, while the ground plate is
elliptically tapered. The profile of the ground taper takes the outlines of two
quarter ellipses which are determined by major and minor axes
(af1, bf), and
(af2,bf),
respectively.Geometry of a typical DETASA: (a) overall view,
(b) tapered slotline radiator, and (c)
feed line transition.
(a)(b)(c)The E-plane of the DETASA shown in Figure1 is on x-y plane
(θ = 90°), and the H-plane is
on y-z plane (ϕ = 90°). The maximum radiation will be in
y-direction
(θ = 90°,
ϕ = 90°). The parametric studies will be
carried out over a frequency range of 1–18 GHz,
where the parameters a1,
a2, Wg,
bf, S, and
Wp will be considered. When a selected
parameter is investigated, the rest of the parameters are unchanged.
For comparison, the aperture Wr of the
slotline radiator is fixed during the study to fix the lower edge
of the operating frequency range. The parametric study will be
conducted by the aid of using commercial software XFDTD
[17] which is based on FDTD
method.To validate the simulation results, a DETASA prototype was
simulated by using the XFDTD software first; the prototype was then
fabricated and measured. The parameters of the reference design
area1 = 50 mm, b1 = 25 mm, a2 = 20 mm, b2 = 24 mm, Wg = 51 mm, Ws = 1.86 mm, Wp = 1.0 mm, S = 0.5 mm,
af1 = 26 mm,
af2 = 24 mm, bf = 25 mm; substrate = RO4003,
thickness = 0.8128 mm, ϵr =
3.38-j0.002. The simulated and measured results in  terms of return loss, gain, and
radiation patterns are illustrated in Figures
2 and
3. It is found that the agreement
is very good. Therefore, the using of simulated results for
further parametric study is viable.Comparison of simulated and measured return loss and gain of the reference DETASA shown in Figure1; (a) return loss; (b) gain.
(a)(b)Comparison of simulated and measured radiation patterns of the reference DETASA
shown in Figure1; (a) E-plane, (b) H-plane.
(a)(b)


## 3. Parametric Studies: Impedance  Characteristics

Figure4 shows the return loss of the DETASAs with
varying side intercepts, a1−a2, of the slotline radiator.
It is seen that compared with the reference design with
a1 = 50 mm and
a2 = 20 mm, the larger side
intercepts (a1 = 70 mm, a2 = 20 mm, and a1 = 50 mm,
a2 = 10 mm) degrade impedance matching
characteristic in particular at the lower edge of the bandwidth. The smaller side
intercepts (a1 = 30 mm,
a2 = 20 mm, and
a1 = 50 mm,
a2 = 40 mm) lead to the worse results than the
larger. The reason is that the small-side intercepts bring the
outer edges of the slotline radiator close to the slot edges,
which makes the conducting arms too narrow to maintain the slotline
characteristics, especially at lower frequencies.Return loss for varying the side intercepts,a1−a2,
of the slotline radiator.
(a)(b)Figure5 illustrates the impact of the feeding
transition on the impedance matching. Figure 5(a)
shows that the impedance matching is slightly affected by the length of the transition
over 1–18 GHz. The similar phenomena can be observed when changing
width of the tapered ground plate, Wg, as shown in Figure
5(b). Therefore, it is concluded that the length of
the feeding transition and the width of the tapered ground plate have little impact
on the impedance matching of the DETASA. From impedance matching point of view, a
compact DETASA can be realizable by using a miniaturized feeding transition.Return loss for different feeding transition configuration.
(a)(b)Figure6 shows the return loss of the DETASAs for
different parallel offset strip lines by varying the width,
Wp, and the offset, S. From Figure
6(a), it is found that the effect of the strip width
to impedance matching is limited. Smaller Wp is preferable
for better impedance matching at lower frequencies. Figure
6(b) shows that the offset of the parallel strip
line, S, has a significant impact on the return loss of DETASA;
larger offset degrades the impedance matching a lot below 7 GHz. It suggests
that smaller Wp and S are adequate in
DETASA design for better impedance matching, especially for lower frequencies.
Furthermore, Wp and S can
be optimized for specific DETASA configuration.Return loss for varying parallel strip lines.
(a)(b)


## 4. Parametric Studies: Radiation  Characteristics

In this section, we will address the impact of the geometry parameters of the
DETASA on its radiation characteristics: gain, cross-polarization, radiation patterns
including main beam, side lobe, and back lobe levels. Note that the gain addressed in
this paper is the realized gain which includes the mismatching loss of the
antenna.Figure7 shows the impact of varying lengths of
slot taper and outer taper of the slotline radiator on gain and cross-polarization
levels. Figure 7(a) shows that the increase in the
length of slot taper, a1, for the fixed outer edge
(a2 = 20 mm)
results in higher gain and cross-polarization levels in particular at higher frequencies. The
gain drops significantly when a1 is reduced to
30 mm because the narrow conducting arms cause the slotline radiator to not
operate well. From Figure 7(b), we can find that the
outer taper primarily affects the cross-polarization levels. Larger a2 offers lower cross-polarization level. Again, the narrower conducting arm
(a1 = 50 mm, a2 = 40 mm) reduces the gain especially at the frequencies higher than
about 15 GHz.Gain and cross-polarization levels for varying the length of slot taper,a1, and outer taper, a2, of the slotline radiator.
(a)(b)As shown in Figure8, it is found that the gain of
the DETASA is almost unaffected by the feeding transition. The length of the tapered
ground, bf, has a great effect on the cross-polarization
levels of the antenna as the operating frequency is higher than 5 GHz as
shown in Figure 8(a) whereas the width of the tapered ground,
Wg, hardly affects the cross-polarization
level as can be seen in Figure 8(b).Gain and cross polarization with different ground profiles of feeding transition.
(a)(b)Figure9 shows the gain and cross-polarization
levels of the DETASAs for changing parallel offset strip lines. It is clear that
width of the strip lines, Wp, and offset
of the strip lines, S, have little effect on the gain. Also,
Wp does not affect the cross-polarization level
below 12.5 GHz but the increasing Wp lowers
cross-polarization levels at higher frequencies. Figure
9(b) shows that the increasing S results in lower
cross-polarization levels at higher frequencies.Gain and cross-polarization levels for parallel strip lines.
(a)(b)In general, all cases suffer higher cross-polarization levels at higher frequencies.
The reason is the inherent asymmetrical features of antipodal structures, namely, two
conducting arms of the DETASA are positioned at opposite sides of the substrate,
which causes the fields in slotline to be skew as shown in Figure10. The skewness of the field in the slotline is
more serious in the starting area which corresponds to radiation at higher
frequencies. In the area close to the aperture, the separation of the conducting arms
become larger so that the skewness of field is brought down and lower
cross-polarization is observed at lower frequencies. Higher cross polarization at
higher frequencies is the drawback of the microstrip-line-fed DETASA, the effective
way to reduce cross polarization of such antenna has been
reported yet.Cross-section view of the DETASA: (a) cross section of the DETASA next to the feeding
transition, (b) cross section of the DETASA close to the aperture.
(a)(b)The characteristics of radiation patterns in E- and H-planes are investigated in
terms of the beamwidth, sidelobe, and backlobe levels. Figure11 compares the radiation patterns of the DETASAs at
9 GHz for varying length of slot taper, a1.
Figures 11(a) and 11(b),
respectively, show the patterns in the E- and H-planes. The E-plane patterns are
asymmetrical because of the instinct of the antipodal structure. The length of the
slot taper is found to have a slight effect on the beamwidth in the E-planes but a
significant impact on the sidelobe and backlobe levels. Figure
11(b) demonstrates that the length of the slot taper
has the largest effect on the beamwidth, sidelobe, and backlobe levels in the
H-planes. The longer the length a1 is, the narrower the main beam
is. The angular locations of the peaks and nulls of sidelobes as well as the
shape and levels of backlobes are changed as well. It should be noted that with
a1 = 30 mm, the sidelobes, and backlobes increase
a lot in both E- and H-planes.Radiation patterns of DETASAs for varying length of slot taper,a1, in (a) E-plane and
(b) H-plane.
(a)(b)Figure12 discusses the effects of varying length
of the outer edge, a2, on the radiation characteristics
of the DETASAs. Figure 12(a) shows the radiation patterns in the E-plane.
The outer taper mainly affects the sidelobe and backlobe of the patterns. The smaller
a2 leads to lower sidelobe levels. The influences of the
outer taper on H-plane patterns are shown in Figure
12(b). Again, the outer taper has a great effect on
the sidelobe and backlobe levels of the patterns. The angular
location and the level of sidelobe as well as backlobe change significantly for
varying
a2. Therefore,  the length of outer
edge, a2, can be optimized for desired sidelobe
and backlobe performance in both the E- and H-planes.Radiation patterns of DETASAs for varying length of outer taper,a2,
in (a) E-plane, (b) H-plane.
(a)(b)For further understanding the radiation characteristics of the DETASA, the current
distribution on the conducting arms is calculated by using IE3D
[18] which is based on moment method. Figure
13 shows the currents of two DETASAs with different
slotline radiator configurations. For the DETASA which has large side
intercept, a1−a2 (shown in Figure
13(a)), the currents along the edges of the slot
taper are large in quantity so they dominate the radiation of the antenna.
The currents along the outer edge are oppositely directed and small in quantity so
that they contribute less to the radiation. However, when conducting arms becomes
very narrow, that is, the side intercept is very small (shown in Figure
13(b)), the currents along the inner and outer edges
of the slot taper are similarly directed and almost equal in quantity. The radiating
structure does not behave as a Vivaldi radiator but more like a V-shaped dipole.
This is the reason of those DETASAs, which have small
side intercepts, demonstrate lower gain, higher sidelobe and backlobe levels.Current distribution for DETASAs with different profile of slotline radiator: (a)a1 = 50 mm, a2 = 20 mm, (b) a1 = 30 mm,
and a2 = 20 mm.
(a)(b)Figure14 illustrates the radiation patterns
of the DETASAs in the E- and H-planes for changing length of tapered ground,
bf. Refer to Figure
14(a), the length of the tapered
ground has main effect on sidelobe and backlobe of the E-plane patterns.
The pattern becomes more symmetrical when tapered ground is longer. In H-planes,
no significant impact is observed but the low backlobe is achieved for a specified
ground length, bf = 25 mm.Radiation patterns of DETASAs for changing length of tapered ground,bf,
in (a) E-plane, (b) H-plan.
(a)(b)Figure15 presents the E-plane and H-plane
patterns of the DETASA with the width of the tapered ground, Wg, varying from 11 mm to 71 mm. Again, the width of the
tapered ground affects the sidelobe and backlobe levels in the E-planes and the
backlobe levels in the H-planes. Conclusively, a small width
Wg is conducive to the
symmetry of E-plane patterns.Radiation patterns of DETASAs for changing width of tapered ground,Wg, in (a) E-plane, (b) H-plane.
(a)(b)It is concluded that the effect of the feeding transition to the radiation
patterns is limited to the sidelobe and backlobe. As shown in Figure13, the currents on the tapered ground plate are
small and mainly along the tapered edges. For those feeding transitions which are
longer in length, for example, bf = 60 mm or smaller in
width, for example, Wg = 11 mm, the antenna structure tends to be
more symmetrical with respect to the y-axis; furthermore, the
direction of the currents on the ground plate tends to y-direction
so that they cause less distortion to the radiation, the antenna patterns becomes
more symmetrical in E-plane.


## 5. Conclusions

This paper has investigated the effects of major geometry parameters on the
impedance and radiation performance of the DETASA. The investigation was conducted
to explore the general behavioral trends of the DETASA rather than to design a specific
antenna. The parametric study has been done over 1–20 GHz band and
yielded a wealth of information which will benefit antenna engineers for their design
and optimization of the DETASA.From the study, we can conclude the following points, which can be used as a
guideline for the design of the DETASA.(1)
The side intercepts,a1−a2, of the
slotline radiator and separation of the parallel offset strip lines,
S, have major effects on the impedance matching characteristic.
Large-side intercept and small separation result in good impedance matching
especially at lower frequencies. The profile of the tapered ground plate has a
slight effect on impedance matching.(2)
The length of the slot taper,a1, mainly
controls the gain of DETASA. Usually, longer slot taper offers higher gain but
results in higher cross-polarization levels. The outer edge shows little impact
on the gain. The feeding transition has shown very little effect on antenna
gain but somewhat on the cross-polarization levels(3)
The length of
the slot taper,a1, has shown a significant effect on main beam of the H-plane patterns, while
the main beam of the E-plane patterns is nearly unaffected. The longer the slot taper
is, the narrower the H-plane beamwidth is. The length of the outer taper,
a2, has little effect on the main beam but affects the
sidelobe and backlobe levels. Therefore, it can be optimized to suppress sidelobe and
backlobe levels.(4)
The size of the feeding
transition primarily affects the sidelobe levels of E-plane patterns. Longer or
smaller feeding transitions offer more symmetrical radiation patterns in E-plane.



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*Source: 267197-2008-02-03.xml*