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A Wideband Resonant Cavity Antenna Assembled Using aMicromachined CPW Fed Patch Source and a Two-LayerMetamaterial Superstrate
Citation for published version:Kanjanasit, K & Wang, C 2019, 'A Wideband Resonant Cavity Antenna Assembled Using a MicromachinedCPW Fed Patch Source and a Two-Layer Metamaterial Superstrate', IEEE Transactions on Components,Packaging and Manufacturing Technology, vol. 9, no. 6, pp. 1142-1150.https://doi.org/10.1109/TCPMT.2018.2870479
Digital Object Identifier (DOI):10.1109/TCPMT.2018.2870479
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Abstract—This paper reports the development of a wideband
resonant cavity antenna (RCA). The RCA device was obtained by
using an optimized aperture coupled CPW (coplanar waveguide)
fed wideband patch antenna and a metamaterial based superstrate
design. The wideband behavior is based on the effects of Fabry-
Perot cavity on either side of the resonant frequency of the
metamaterial superstrate and the finite size of the superstrate to
obtained enhanced gain around this frequency. In addition, the
metamaterial superstrate in our work contains two identical layers
of a frequency selective surface (FSS) based on a two dimensional
patch array rather than dissimilar arrays as used in the previous
studies. Two such FSS layers are assembled using a laser
micromachined PMMA rim and a SU8 based bonding method.
The air spacer based metamaterial design offers better
performance by minimizing the dielectric loss between the metallic
patch arrays and also offers flexibility in control of the separation
between the two FSS layers as well as resulting in a light weight
RCA device. The RCA device was designed to operate in the X-
band. The dimensions of the compact RCA device are 45x45x24
mm3. The gain of the device was measured to be 13 dBi with a 3-
dB bandwidth of 46% and a corresponding 1-dBi-ripple
bandwidth as wide as 40%. The measured impedance bandwidth
was 44%.
Index Terms—Resonant cavity antenna, fabrication, assembly,
partially reflective surface (PRS), directive antennas, gain
bandwidth, wideband antenna.
I. INTRODUCTION
HE resonant cavity antenna (RCA) has been studied for
many years due to its ability to provide a narrow beam
response with enhanced directivity over the conventional single
element antenna devices [1-28]. In a typical RCA device the
feed (source) antenna is placed inside a resonator which is
usually a Fabry-Perot (FP) based cavity structure. The bottom
reflector is commonly made of a blank metal surface or a
patterned (defected) surface or an impedance surface which
Manuscript received May 30, 2018;
Komsan Kanjanasit was the Institute of Sensors, Signals and Systems (ISSS), School of Engineering and Physical Sciences, Heriot-Watt University,
Edinburgh EH14 4AS, United Kingdom. He is now with the College of
Computing, Prince of Songkla University, Phuket, 83120, Thailand (email: komsan.k@psu.ac.th). Changhai Wang is with the Institute of Sensors, Signals
and Systems, School of Engineering and Physical Sciences, Heriot-Watt
also acts as the ground plane of the feeding antenna device. The
top reflector (superstrate) can be a single dielectric substrate or
more dielectric layers called an electromagnetic band gap
(EBG) structure. The required high reflectivity is obtained
using a high permittivity dielectric material with a quarter
wavelength thickness [1-4]. The resonant condition in the
dielectric superstrate based RCA devices has been examined
theoretically based on the transmission line model [1][4], the
leaky wave propagation model [2] and the EBG based approach
[3][5-6]. In EBG based superstrates, Weily et al. used a three
layer EBG structure as the superstrate in a RCA device fed by
a slot array [7]. The gain and bandwidth of the RCA device
were 22.7 dBi and 13.2% respectively. Al-Tarifi et al. proposed
a double cavity RCA device consisting of two dielectric
superstrates [8] which has a bandwidth of 17.9% based on the
results of simulation work as compared to 9% of a single cavity
device, but no practical device was constructed to demonstrate
the enhanced bandwidth. The recent work by Hashmi et al. [9]
demonstrated a wideband RCA device using an all dielectric
superstrate consisting of 3 layers of dielectric substrates. A
large 3-dB bandwidth of 22% was achieved with a peak gain of
18.2 dBi. Although the dielectric material based superstrate is
easy to design, the resultant antenna device is bulky. Recently
a new superstrate design made of unprinted dielectric materials
having nonuniform permittivity in the transverse direction, has
been used to realise a wideband RCA with a 3-dB directivity
bandwidth of 52.9% [10]. However the lack of choice in
dielectric slab materials with the required thicknesses also
restricts design flexibility of dielectric material based
superstrates.
In recent years it has been shown that a partially reflecting
surface (PRS) consisting of one or more layers of periodic
metallic or aperture elements on a dielectric substrate known as
a frequency selective surface (FSS), can be used effectively as
the superstrate leading to low-profile RCA devices [11-14]. The
metamaterial based PRS was first proposed and studied by
Trentini [11]. Both the reflection coefficient and the associated
University, Edinburgh EH14 4AS, United Kingdom (e-mail:
c.wang@hw.ac.uk). Part of the work was presented at META’16.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org. Digital Object Identifier
A Wideband Resonant Cavity Antenna
Assembled Using a Micromachined CPW Fed
Patch Source and a Two-Layer Metamaterial
Superstrate
Komsan Kanjanasit and Changhai Wang, Member, IEEE
T
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phase are significant factors in design of a RCA device and have
been considered in the ray theory [8-25]. Wideband capability
of RCA devices based on one FSS layer as the PRS was studied
by Feresidis et al. [11] but the available bandwidth was limited
by the intrinsic characteristics of the magnitude and phase of
the reflection characteristics of the single layer FSS based PRS
designs. Therefore bandwidth enhancement has been a subject
of significant interest using double FSS layer based PRS
structures for RCA devices. Feresidis and Vardaxoglou [22]
proposed a double layer superstrate design consisting of two
FSS layers with different dimensions of patch elements on each
dielectric board. An enhanced bandwidth of 9% was obtained
using a waveguide based feed source. In their recent work, a
waveguide coupled RCA using 3 dissimilar patch arrays printed
on dielectric substrates was demonstrated with a 3 dB
bandwidth of 15% [23]. In another study Konstantinidis et al.
designed an RCA using a slot coupled microstrip antenna and
three double-sided periodic arrays consisting of a patch array
and a cross shape based arrays on each side of a dielectric slab
[24]. A 3dB directivity bandwidth of 10.7% was achieved. Ge
et al. [25] investigated a double layer superstrate for bandwidth
enhancement of a RCA device utilizing two dissimilar dipole
arrays printed on each side of a dielectric board. A monopole
antenna was used as the feed source and a bandwidth of 13.6%
was achieved. In [26], Wang et al. has just reported a wideband
RCA device using a two layer metamaterial superstrate
consisting of a patch array and a perforated metal layer with
periodic square holes, the two FSS layers were printed on each
side of a dielectric board. In their work a wideband microstrip
fed patch antenna with a suspended patch element [29] was used
as the feed source. The measured impedance bandwidth was
about 25% and the gain bandwidth was 28%. In the recent work
by Liu et al. [28], bandwidth enhancement was obtained by
using a metasurface as the PRS and a probe fed patch antenna
as the primary source. The measured 3 dB gain bandwidth is
approximately 20.3%. The metasurface was a layer of square
metal rings with progressive reduction of size from the centre
of the surface, produced on a PCB substrate. In addition dual-
band RCA devices using metallo-dielectric superstrates have
also been reported [30, 31] Lee et al. developed a dual-band
PCA using a two layer metallo-dielectric superstrate which was
based on two dissimilar rectangular patch arrays one on each
side of a dielectric board [30]. Recently Abdelghani et al.
reported a dual band RCA using a double-layered periodic
partially reflective surface with unit cells based the same square
ring design but with different dimensions for each layer of the
two arrays [31].
In this paper, we present the results of design and
implementation of a wideband RCA device using a double layer
metamaterial based superstrate (PRS) consisting of identical
periodic metallic patch elements on thin film liquid crystal
polymer (LCP) substrates [32]. Impedance and gain bandwidth
as large as 44% and 46% have been obtained from a practical
device. The results were achieved using an optimized wideband
patch antenna as the source device with the two layer
metamaterial superstrate. In Section II, we describe design and
modelling of the wideband RCA device. In Section III,
fabrication and assembly of the RCA device are discussed as
well as the results of characterization and measurements. In
Section IV, we summarize the development and performance of
the wideband RCA device.
II. ANTENNA DESIGN AND MODELLING
In this section, the configuration and design principle of the
resonant cavity antenna for directivity enhancement and
wideband operation are discussed. The design and modelling
work was carried out using the Ansys-HFSS software tool. An
aperture coupled CPW fed patch antenna with a suspended
patch was designed for wideband operation. The design and
characteristics of a two layer metamaterial are studied for
application as a superstrate for a wideband RCA device. The
metamaterial substrate structure is then incorporated into the
RCA configuration for modelling and optimization of the
resultant antenna device.
A. Configuration of the RCA antenna design
Fig. 1(a) shows a schematic cross-sectional view of the
proposed resonant cavity antenna for high directivity and
wideband operation. The primary source is a wideband CPW
fed patch antenna using a suspended patch design on an FR4
substrate. The patch element is on a thin film liquid crystal
polymer (LCP) substrate. The latter is supported using a laser
micromachined PMMA (polymethyl methacrylate) based
polymer rim. The thickness of the PMMA rim and hence the air
gap between the LCP and the FR4 substrates, is denoted by Hg.
The layout of the aperture for coupling and the coplanar
waveguide for feeding is shown in Fig. 1(b).
Fig. 1. Geometry of the broadband resonance cavity antenna, (a) a cross sectional view, (b) a plane view of the CPW feed structure, the coupling
aperture and the patch element, and (c) the square patch array in each layer of
the PRS.
The PRS is a two-layer metamaterial and is separated from
the ground plane of source antenna by the cavity height, Hc.
Unlike the PRS designs in the previous work, our PRS consists
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of two identical layers of square patch based arrays printed on
the same thin film LCP material as that used in the source
antenna. Fig. 1(c) shows a schematic layout of the patch array
on the LCP film. Two such LCP film substrates are combined
together using a PMMA based rim structure by aligning and
bonding the LCP films to each side of the PMMA rim.
B. Design of a CPW-fed aperture-coupled patch antenna as a
wideband primary source
For wideband operation of a RCA device, it is necessary to use
a wideband antenna as the source element. In our previous work
[29][32] we showed that wideband patch antenna devices can
be obtained by using an aperture coupled design and a
suspended patch [29] or patches in the case of stacked patch
antennas [33]. Using a microstrip feed design, a bandwidth of
~20% was obtained for a single patch based design [29]. In this
work we developed a CPW based feed design to obtain
increased bandwidth using a short end CPW line. In the CPW
fed configuration shown in Fig. 1, a wide impedance bandwidth
can be obtained based on the coupling of the resonances
associated with a double tuned aperture [34] and the suspended
patch. The design and simulation work was carried out in a
similar approach as described in our previous work [29]. Briefly
the aperture was designed in the first step, the total length of the
aperture was one guided wavelength at 10 GHz and the width
was one tenth of the length. Then a short-end CPW was added
to the aperture on the ground plane as the feeding structure. The
CPW line acts as an impedance transformer and was designed
to obtain a double-tuned aperture with wide impedance
bandwidth. By studying the reflection characteristic of the CPW
fed aperture, it was found that the desired length for the CPW
line was one wavelength for wideband impedance operation of
the double tuned aperture. In the last design step, a rectangular
patch was added to the CPW fed aperture with an air gap
between the thin film LCP substrate of the patch and the
aperture. The length of the patch was half wavelength at 10
GHz, the patch resonance was coupled to those of the double
tuned aperture resulting in triple resonances thus broadening the
impedance bandwidth of the antenna design. The air gap
determines the coupling between the patch and the aperture. A
range of air gap values was investigated and it was found that
the air gap of 3 mm provides the best radiation performance.
The material properties and design parameters are given in
Table I and II respectively. TABLE I
PROPERTIES OF THE DIELECTRIC SUBSTRATES
LCP [35] FR4 [36] PMMA [37]
r 2.9 4.1 2.6
tan 0.0025 0.02 0.008
TABLE II
DESIGN DIMENSIONS FOR WIDEBAND PATCH
ANTENNA FEED SOURCE (MILLIMETERS)
Lp Wp Ls Ws Wf Lf G Hp Wg
10 20 17 1.0 1.4 20 0.2 3 45
The simulation results of the reflection coefficient and
directivity of the wideband patch antenna are shown in Fig. 2.
An impedance bandwidth of about 46% has been achieved. It is
believed that this is the widest bandwidth obtained for a single
patch based antenna device. The directivity is much higher than
that of the conventional printed patch antenna on a PCB board
by using the air gap based design and a low loss LCP thin film
substrate [29].
Fig. 2. Simulation results of reflection coefficient (S11) and the directivity of
the patch antenna source.
C. Design of the metamaterial based superstrate
In this work we have studied design of an effective PRS using
a two-layer metallo-dielectric based metamaterial as shown in
Fig. 1. The metamaterial based PRS consists of two identical
patch arrays each printed on an LCP film substrate and
separated by an air gap. In our previous work [38], we
investigated Fano resonance in such metamaterials with a
narrow bandwidth for high-Q applications. In this paper we
demonstrate that the same metamaterial design but with broad
resonance characteristics can be used to design an effective PRS
for a wideband RCA device. Unlike in [38], in this work
approximately quarter wavelength pitches have been found to
provide suitable transmission and reflection characteristics for
incorporating into a RCA device for broad band operation. The
resonant frequency of the metamaterial PRS was chosen to be
10 GHz matching that of the source antenna as discussed
previously. The resonant frequency of the two-layer material
PRS depends on the pitch of the patch array, the dimensions of
the square patch element and the gap between the two LCP film
substrates. The design work was carried out using a unit cell
based approach thus assuming an infinite array size [13] [16]
[18] [22-27]. The implementation of the unit cell for design and
simulation work is shown in Fig. 3(a). Full wave EM simulation
was performed based on the Floquet’s theorem using the Ansys
HFSS software tool. It has been shown that this method is
accurate and efficient in study of infinite size two dimensional
periodic arrays [39].
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Fig. 3. Unit cell structures for simulation work on (a) PRS design and (b) cavity
model for antenna application based on image theory.
Five such metamaterial designs have been considered for
PRS applications. The pitch of the patch array was 7 mm in all
cases. The values of the air gap between the two arrays are 1, 2,
3, 4 and 5 mm respectively. To maintain the resonant frequency
of the material structures, the dimensions of the square patch
elements were adjusted in the design process. The
corresponding values were found to be 6.95 mm, 6.77 mm, 6.45
mm, 6 mm and 5.3 mm respectively. Fig. 4 shows the
transmission and reflection characteristics of the two-layer
metamaterial designs. The resonant behavior has a strong
dependence on the air gap thickness between the two array
layers. As the air gap increases, the effect of Fano resonance
decreases since the electromagnetic coupling between the
scattered waves becomes weaker [38]. Fig. 4(c) shows the
associated reflection phase change as a function of frequency
corresponding to the magnitude of the reflection coefficient
shown in Fig. 4(b). The reflection phase change contributes to
the total phase of the Fabry-Perot cavity of the RCA device
(Fig. 1(a)) determining the cavity condition and the operation
of the RCA device [11]. The reflection phase behavior of the
metamaterial as the PRS is included in the simulation work
described in Section II D.
The properties of the metamaterial designs for RCA
applications are studied by forming a Fabry-Perot cavity
between the metamaterial superstrate and the ground plane of
the source antenna. The latter is considered to be an infinite
perfect electric conductor (PEC). This approach allows easy
study of the behavior of the metamaterial based PRS designs
for RCA applications. Both the magnitude of the reflection
coefficient and the associated phase change contribute to the
transmission characteristics of the Fabry-Perot cavity. The
transmission study is based on the electromagnetic image
theory [6] [40], the unit cell for this purpose is shown in Fig.
3(b). The unit-cell is excited by a plane-wave source at normal
incidence. It was found that the air gap value of 4 mm provides
the best performance in terms of low ripple transmission over a
broad band, although based on the transmission characteristics
shown in Fig. 4(b) the 5 mm air gap design should also provide
wideband transmission. Fig. 5 shows the results of transmission
for different values of the cavity height (Hc) for an air gap (Hr)
value of 4 mm between the two patch arrays. As Hc increases
the broad band transmission shifts to the lower frequency side.
The cavity height also has an effect on the profile of the
transmission band as frequency. The results show unity of
transmission coefficient at 10 GHz as expected from Fig. 4(b)
with zero reflectivity at this frequency. The weak resonance at
each side of the frequency is associated with the defect mode of
the PEC [6]. The weak resonance is due to the broad reflection
characteristic around 10 GHz of the metamaterial as shown in
Fig. 4(b) resulting in wideband transmission for broad band
antenna applications.
(a)
(b)
(c)
Fig. 4. Simulation results for the PRS design based on the unit cell shown in Fig. 3(a), (a) transmission magnitude, (b) reflection magnitude, and (c)
reflection phase.
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Fig. 5. The transmission property (S21) of a Fabry-Perot cavity formed by the
PRS with an air gap of 4 mm between the two LCP substrates and a PEC at
different cavity heights using the unit cell shown in Fig. 3(b).
D. Design of RCA for practical implementation
Synthesis of a wideband RCA device is carried out by
implementing the corresponding 3D structure of the cross-
sectional view as shown in Fig. 1(a) in Ansys HFSS
environment. The source antenna as described in section II B
was incorporated in the RCA design without modification. As
described before, the analysis of the metamaterial based PRS is
initially made based on the infinite-size condition and a plane-
wave excitation. However, the actual PRS structure in a RCA
device has a finite array of patch elements. Therefore the RCA
synthesis work was carried out using finite element based full
wave simulation to take into account of the effect of the finite
size of the PRS. For compactness both of the PRS and the
substrate of the primary source should be small while
maintaining a wideband performance. Fig. 6 shows the results
of broadside directivity as a function of frequency for array
sizes of 5x5, 7x7 and 9x9 in the superstrate respectively. It can
be seen that the bandwidth of directivity is strongly affected by
the size of the superstrate. This effect has already been observed
in other RCA devices [6][14] and has been discussed in detail
in [9]. A larger superstrate results in lateral propagation within
the cavity and phase non-uniformity across the radiation
aperture thus causing degradation in antenna performance.
Hence a smaller superstrate is better for a RCA device, the
leaked wave from the edges of the superstrate also increases the
radiation bandwidth [9]. In our case it was found that the
optimum array size for a wideband RCA device is 5x5. Further
reduction of superstrate size weakens the effect of the Fabry-
Perot resonator and thus reducing the performance of the RCA
device. Comparing the results in Figs. 2 and 6, it can be found
that as the superstrate becomes very large (e.g. 9x9 array) no
enhancement in directivity can be obtained at 10 GHz since
there is no cavity effect at this frequency as the metamaterial
superstrate is at resonance with 100% transmissivity as shown
in Fig. 4(a) and correspondingly zero reflectivity as shown in
Fig. 4(b). In effect there is no Fabry-Perot cavity at 10 GHz
between the superstrate and the ground plane of the patch
antenna when the former is very large since it behaves like an
infinite size superstrate.
Fig. 6. Comparison of broadside directivity of the RCA design for different
arrays in the finite-size PRS. The cavity height (Hc) is 20 mm.
Fig. 7 shows the results of both radiation and reflection
characteristics of the RCA device with the 5x5 array based PRS
design for different cavity heights. It can be seen that the cavity
height has a significant effect on the radiation response of the
cavity. The best RCA performance is obtained at the cavity
height of 20 mm with the lowest ripple response in antenna
directivity. The corresponding impedance bandwidth is 45% as
shown in Fig. 7(b).
(a)
(b)
Fig. 7. Radiation (a) and reflection (b) characteristics of the RCA device.
Fig. 8 illustrates the radiation patterns in both E- and H-plane
at 10 GHz for three different superstrate designs. Beam-
splitting is observed and is found to be associated with the 9x9
array based superstrate. Beam splitting is an undesirable
behavior for RCA devices and can occur in designs with an
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infinite [41-42] or a finite superstrate [43-44]. The total
radiation aperture for the optimal superstrate with 5x5 array of
patch elements is approximately 1.2x1.2, comparable to the
aperture size of 1.5x1.5 for the RCA device with an all
dielectric based superstrate [9] but in our case the superstrate
structure is substantially thinner (4 mm as compared to 14 mm).
(a)
(b)
Fig. 8. Radiation characteristics (a) H-plane and (b) E-plane for finite-size FSS
designs in PRS and the source (patch) antenna at 10 GHz.
III. FABRICATION AND ASSEMBLY
A. Fabrication and assembly of the primary patch antenna
The wideband patch antenna was produced on an FR4
substrate. The thickness of the substrate was 1.6 mm. A similar
fabrication approach was used as in our previous work [33].
However a laser micromachined PMMA rim was used in this
work to obtain a suspended patch instead of the SU8 based
polymer rim that was produced by a surface micromachining
based method. The new approach is efficient and better for
producing millimeter thick polymer structures than the previous
method. The CPW feed line and the coupling aperture was
produced on a copper clad FR4 board using a microfabrication
approach. A layer of photoresist (AZ9260) was deposited on the
FR4 board by spin coating. After drying and baking the
photoresist layer, it was exposed to UV light on a mask aligner
using a film photomask with the design patterns for fabrication.
After development of the photoresist layer, the exposed copper
layer was etched in a FeCl3 based solution. The photoresist was
then then removed to obtain the aperture and CPW structure as
well as the surrounding ground plane. The patch was fabricated
on a thin film (100 m) copper clad LCP substrate using a
similar process. The PMMA rim for supporting the patch on the
LCP substrate was produced using a CO2 laser based
micromachining system [45]. The inner dimensions of the rim
were 36 x 36 mm2 and the width of the PMMA track was 2 mm.
A PMMA plate of 3 mm of thickness was used in this work to
match the design requirement for wideband operation of the
resultant patch based source antenna. In the assembly process,
the PMMA rim was aligned to the aperture on the FR4 substrate
and attached to it using the SU8 based bonding method [33].
The patch on the LCP film substrate was then aligned to the
coupling aperture and the LCP substrate was bonded to the
PMMA rim using the same method as for bonding of the
PMMA rim to the FR4 substrate. Finally a SMA connector was
mounted to the FR4 board by soldering to complete the
construction of a wideband patch antenna as the primary source
for a wideband RCA device.
B. Fabrication and assembly of the PRS structure
Based on the results of design and simulation work as
described in section II, the PRS design with a 5x5 array of
square patch elements in each FSS layer was chosen for
construction of a RCA device. As shown in Fig. 7, the best
performance for a RCA device is expected from incorporation
of this PRS design. The 5x5 array of square patch elements was
fabricated on a copper clad LCP film substrate. The patch array
was the fabricated on the LCP substrate using the same
microfabrication method as for fabrication of the primary
source antenna. Two arrays were fabricated each on an LCP
film as necessary for construction of the double layer
metamaterial based PRS. Two 2 mm thick laser machined
PMMA rims were prepared and stacked together to a total rim
thickness of 4 mm as required for optimal performance for the
RCA device. The two LCP layers were then aligned and
attached to each side of the stacked PMMA rim to complete the
construction of the PRS structure.
Fig. 9. A photograph of the assembled RCA device showing a CPW-fed
aperture coupled patch antenna and the PRS superstrate. The cavity height is 20
mm.
C. Assembly of RCA device
The primary source antenna and the PRS as described in the
above sections were assembled to complete the construction of
the wideband RCA device. A screw based method, similar to
that used in [44], was used to mount the PRS on the FR4
substrate of the source antenna. Alignment was made using 4
pre-drilled matching holes on the FR4 substrate and the PRS
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structure. The holes were located on each corner of the square
FR4 board and the PRS assembly. Fig. 9 shows an optical
image of the assembled wideband RCA device. The cavity
height can be controlled by adjusting the position of the screws
securing the PRS structure.
(a)
(b)
Fig. 10. The measured and simulation results of reflection (a) and radiation
(b) characteristics of the broad band feed antenna
IV. MEASUREMENTS
Microwave measurements for antenna characterization were
made in an anechoic chamber using a vector network analyzer
(HP 8510). The gain measurements were made using a far field
approach based on the gain transfer method [46]. Two standard
gain horn antennas were used, one was used as the transmitter
and the other one as the reference antenna. The latter has an
average gain of 20 dBi over the operation band between 8.2
GHz and 12.5 GHz. Reflection and radiation measurements
were carried out for both the wideband patch based source
antenna and the resultant RCA device after it was assembled
with the metamaterial superstrate.
Fig. 10 shows the results of reflection and gain measurements
for the source (feed) antenna. Good impedance matching
through coupling of multiple resonances was obtained with an
impedance bandwidth of 43%. The 3dB-gain bandwidth is 40%
covering the X-band with a peak gain of 8 dBi. To our
knowledge the results are the largest bandwidth values obtained
for a patch antenna device. It provides the necessary feed source
for a wideband RCA device. The differences between the
results of simulation and measurement in Fig. 10(a) are
probably due to the fact that the SMT connector was not
included in the simulation work and the accuracy of the
dimensions of the fabricated structures. More detailed analysis
of the results may be made using the FSV (Feature Selective
Validation) method [47]. The large difference between the
results of simulation and measurement below 8.2GHz in Fig.
10(b) is the effect of the cutoff frequency of the gain horn
antenna used in the measurement work.
(a)
(b)
Fig. 11. (a) Results of reflection from measurement and simulation, and (b) results of gain from measurement and simulation as well as the calculated
directivity of the RCA device.
Fig. 11 presents the results of both measurement and
simulation for the RCA antenna device as shown in Fig. 9. The
cavity height was 20 mm. The measured impedance bandwidth
is 44% between 8 GHz and 12.4 GHz from the results shown in
Fig. 11(a). The discrepancy between the results of simulation
and measurement is due to the same factors as discussed for the
results in Fig. 10(a). The measured 3dB gain bandwidth is 46%
from the results shown in Fig. 11(b). The device has excellent
flat-ripple response with a 1-dB-ripple gain bandwidth as wide
as 40% over the frequency band between 7.6 and 11.6 GHz. The
calculated directivity is obtained using the results of gain from
measurement and radiation efficiency from simulation. To
investigate the spatial characteristics of the radiation, angle
dependent measurements were carried out at selected
frequencies and the results are shown in Fig. 12. In general the
measured results are in good agreement with the predicted
performance, except the E-plane results at 10 and 12 GHz
which may be due to the fact that the SMA connector of the
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source antenna was on the E-plane and was not included in the
model of the simulation work. The results of E-plane and H-
plane measurements show the same value of 40 for the 3dB
bandwidth at the frequencies of both 8 GHz and 10 GHz. It is
slightly narrower at 12 GHz with a value of 30. Therefore the
results illustrate the desirable radiation behavior of the RCA
design for broad band applications maintaining similar
radiation patterns over the band of operation.
Fig. 12. E- and H-plane far-field radiation patterns of the RCA device at three
different frequencies of 8 GHz, 10 GHz and 12 GHz respectively.
V. CONCLUSION
A resonance cavity antenna with the characteristics of the
broad radiation bandwidth has been proposed and demonstrated
successfully. A new PRS design based on two identical arrays
of square patch elements was used as the superstrate in the RCA
device. Wideband performance has been achieved in
conjunction with an optimized wideband patch antenna as the
primary source. Thin film LCP substrates were used in
construction of the RCA device enabling high performance and
light weight devices. The associated fabrication and assembly
methods were developed to produce the RCA device. The
impedance and 3 dB radiation bandwidth of the device were
measured to be 44% and 46% respectively representing
considerable improvement over the bandwidth of the similar
devices reported in the previous work. The peak gain of the
device was about 13 dBi. The RCA device is also very compact
with dimensions of only 45x45x24 mm3.
ACKNOWLEDGMENT
The authors would like to thank Rogers Corp. for providing
the LCP film substrate material used in this work.
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