A Wideband Resonant Cavity Antenna Assembled Using a

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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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1

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: [email protected]). 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:

[email protected]). 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




2

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




3

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].




4

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.




5

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




6

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




7

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




8

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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