1. Antipodal Linearly Tapered Slot Antenna System
Using Substrate Parallel Plate Waveguide
Feeding Structure
Fan Fan He#*1
, Ke Wu *2
, Wei Hong# 3
#
Poly-Grames Research Center, École Polytechnique de Montreal
Montreal, Quebec, H3C 3A7, Canada
1
fanfan.he@polymtl.ca
2
ke.wu@polymtl.ca
*
State Key Laboratory of Millimeter Waves, Southeast University
Nanjing, 210096, P.R. China
3
weihong@emfield.org
Abstract— An antipodal linearly tapered slot antenna (LTSA)
structure fed by substrate parallel-plate waveguide is presented
in this paper. First, propagation constant of the substrate
parallel plate waveguide is analyzed by full-wave software CST.
The simulated results show that this waveguide support TEM
mode. Second, an LTSA element fed by our proposed parallel
plate waveguide is designed, fabricated and measured. A very
wide bandwidth is shown on the basis of the simulated and
measured results.
I. INTRODUCTION
The linearly tapered slot antenna (LTSA) has attracted
much interest in many practical applications due to its
attractive features such as narrow beam width, high element
gain, wide bandwidth and small transverse spacing between
elements in arrays [1]-[3]. To design an LTSA, the most
important part is how to develop the feeding structure which
determines the performances of this antenna. So far, many
feeding structures such as fin line, inverted microstrip line,
coplanar waveguide, microstrip to slot line transition,
balanced microstrip line and substrate integrated waveguide
(SIW) have been proposed and designed [4]-[6].
Generally speaking, some of the above-mentioned feeding
structures such as SIW and microstrip line transition have the
limitation for wide bandwidth LTSA. As is well known, the
parallel plate waveguide as a TEM mode transmission line has
many applications in wide-band components [7]-[8].
Therefore, a wide-band LTSA excited by parallel plate
waveguide is proposed in this work. The parallel plate
waveguide is fabricated on substrate while it has a small size
feature with finite ground planes. The propagation
characteristics of the substrate parallel plate waveguide are
relatively easy to analyze. This structure is found to be a good
candiddate for microwave and millimeter-wave applications.
Details of the LTSA with substrate parallel plate waveguide
feeding mechanism including the prototype, simulated and
measured results are presented in this paper.
(a)
(b)
Fig. 1. (a) 3D schematic of the substrate parallel plate waveguide. (b) Electric
fields over the cross section.
II. SUBSTRATE PARALLEL PLATE WAVEGUIDE
An The substrate parallel plate waveguide is inspired and
developed from the concept of substrate integrated waveguide
with four additional air gaps, as shown in Fig. 1(a). Four air
gaps allow isolating central flat plates from side metallic walls.
Metallic walls can suppress any potential radiation of the
signal on the fringes of the two central flat plates. Moreover,
metallic walls can reduce the size of the central plates. Fig.
2(b) shows the electric fields over the cross section related to
the working mode. Through 2D simulations, the propagation
constant of this structure can be obtained, as shown in Fig. 3.
The results show that this substrate parallel plate waveguide
can support TEM mode.
978-2-87487-013-2 © 2009 EuMA 28-29 September 2009, Rome, Italy
Proceedings of the 2nd European Wireless Technology Conference
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2. Fig. 2. Qualitative dispersion behavior of substrate parallel plate waveguide
mode.
III. DESIGN OF THE PROPOSED LATA
A. Operating Principle of the Proposed LTSA
Fig. 3 depicts the physical 3D configuration of an LTSA
with substrate parallel plate waveguide feeding structure and
microstrip-to-parallel plate waveguide transition, where the
dielectric constant and substrate thickness are denoted by İr
and t, respectively. The transparent and yellow areas identify
the substrate area and metal covers of the substrate. The
parallel plate waveguide-microstrip transition is used for
connecting 50ȍ measurement system, W50 and Wstrip are the
widths at the both ends of the microstrip taper for matching
the antenna impedance to 50ȍ, and Ltaper is the length of the
taper. The parallel plate waveguide section simply transforms
the unbalanced microstrip to the balanced parallel plate
waveguide feeding system for the ALTSA, where D and S are
the diameter and period of the metallic vias used as equivalent
metal wall like SIW, Wh is the distance between tow rows of
metallic vias, Wgap is the air gap width and Lp is the length
of parallel plate waveguide section and. ALTSA has a flare
angle of 2Į and a length of Lant.
Fig. 3. Physical 3D configuration and parameters of the proposed LTSA.
Fig. 4. Photograph of the proposed LTSA.
In Fig. 3, the substrate parallel plate waveguide is designed
to support TEM mode free from cut-off frequency. The TLSA
is designed by gradually flaring the metallic covers on
opposite sides of the substrate by an angle of 2Į. In order to
match the high impedance of LTSA with the low impedance
parallel plate waveguide, the flaring metal covers are
overlapped each other. The metallic covers gradually stretch
out on the opposite side of the substrate by angle 2Į. Linear
tapers from the overlapped metallic covers to tapered slot
antenna can change the vertical field polarization of the
parallel plate waveguide to the horizontal field polarization of
the LTSA. A good impedance matching can be obtained by
adjusting length Lant and angle 2Į. If a good matching
performance cannot be obtained, we may match the LTSA
with air by adjusting width W1, consequently the entire LTSA
matching can be adjusted to its optimal performance. The gain
of an LTSA is mainly determined by length Lant [2].
In this work, the proposed LTSA is designed only for
demonstration purpose. The parameters of the antenna are
listed in Table. I. The antenna is designed and fabricated on
Rogers Duroid 5880 substrate with İr=2.2 and t=0.254mm.
Fig. 4 shows the photograph of the fabricated LTSA sample.
TABLE II
DIMENSIONS OF THE DESIGNED LTSA
D(mm) 0.3 Ltaper (mm) 20
S (mm) 0.6 Lt (mm) 40
Lp (mm) 10 Wh (mm) 4.8
Wp (mm) 4 Į(degree) 19.8
W50(mm) 0.75 W1(mm) 5
Wgap (mm) 0.2 WA (mm) 5
B. Simulated and Measured Results
The LTSA using substrate parallel plate waveguide as
feeding structure is simulated by the full-wave CAD software
(CST in our case).
Simulated and measured return losses are shown in Fig. 4,
which show a very good agreement. Measured return loss S11
is lower than about -10dB from 12 GHz to 50GHz. Actually
simulated return loss is lower than -10dB from 12.5 GHz to 80
GHz. The highest measured frequency is limited by the
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3. measurement system. The results show that the proposed
antenna has a very wide bandwidth.
The antenna gain is measured from 10 to 40 GHz because
some measurement equipment above 40 GHz is not available
during this work. Fig. 5 shows simulated and measured
antenna gain versus frequency. Both simulated and measured
results indicate that the antenna gain slowly increases from 6
to 12 dBi as the working frequency is swept from 10 to 40
GHz. This trend with regards to these curves is coincident
with the Zucker’s standard curves for TSA [2].
Fig. 4. Simulated and measured return losses of the proposed LTSA cell.
Fig. 5. Simulated and measured gain versus frequency
Fig. 6 shows the measured radiation patterns of the LTSA
with substrate integrated parallel plate antenna feeding
structure at 18GHz and 30 GHz respectively. The sidelobe is
below than -10 Db at both frequencies. The E-plane radiation
pattern has a 3dB beamwidth of 50 degree and 28 degree at
18GHz and 30 GHz, respectively. The H-plane radiation
pattern has a 3dB beamwidth of 78 degree and 57 degree at
18GHz and 30 GHz, respectively. At these two frequencies,
antenna gain is 8dBi and 9.5dBi shown in Fig. 5.
(a)
(b)
Fig. 6. Measured normalized radiation patterns for the proposed LTSA.(a)
Radiation patterns at 18GHz. (b) Radiation patterns at 30GHz.
IV.CONCLUSIONS
The antipodal linearly tapered slot antenna using the
substrate parallel plate waveguide feeding structure has been
designed, fabricated and tested. The proposed substrate
parallel plate waveguide supporting TEM mode, can be used
in not only wideband antenna but also other wideband
components such as wideband divider, coupler, etc. For the
purpose of demonstration, wideband LTSA using this feeding
structure are measured. The results indicate that the LTSA
using the proposed feeding structure can yield wideband
characteristics.
ACKNOWLEDGMENT
The authors are grateful to Guohua Zai for his help during
the test and to Liang Han and Jing Zhang for revision.
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