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ELECTROMAGNETICALLY COUPLED MICROSTRIP ANTENNA WITH

DIELECTRIC COVER

_________________________________________

6.1. INTRODUCTION

The present chapter deals with the study of effects of dielectric covers on the performances

of electromagnetically coupled and electromagnetically gap coupled microstrip patch

antennas. A great deal of research has been devoted to increasing the absolute bandwidth of

MPAs. The methods fall into three categories: electromagnetic-coupled patches (EMCP),

use of parasitic elements and log-periodic arrangement of an array of patches. It is possible

to increase the absolute bandwidth of MPAs by simply using thicker substrates also. This

however, introduces several problems. The first is the excitation of surface waves, which

distorts the normal radiation pattern and introduces additional loss; the second is the

excitation of higher-order modes with Z dependence, which introduces further distortions

on the pattern and impedance characteristics. The third is that the application of common

feeding techniques i.e. directs feeding by either a coplanar microstrip line or a

perpendicular coaxial line.

Consider first a coaxial feed, since the probe (extension of inner conductor of the coaxial

line) introduces a series reactance almost proportional to the substrate thickness, the lead

inductance will become significant with respect to the antenna radiation resistance for thick

substrates and will therefore prevent proper matching. Second, a patch which is edge-fed

by a coplanar microstrip line. For a fixed impedance level the line width is almost

proportional to the dielectric thickness. Since the patch dimensions for a fixed resonant

frequency are only weakly dependent on the dielectric thickness (through the fringing field)

the width of the feed line will become non-negligible as the substrate reaches a certain

thickness. As a result the radiation pattern of the antenna will be disturbed partly due to the

covering of the radiating patch edge by the line and partly due to increased radiation from

the feed line. In view of the above problems, electromagnetic coupling (instead of direct

coupling) has been studied using a possible feed technique for electrically thick MPAs. In

121

particular, promising results have been obtained for the stacked dual-patch geometry which

will be discussed in the following sections.

In the early to mid-1980s, parasitically coupling patches in a horizontal manner to the

driven patch were proposed and investigated. The philosophy behind this technique is that

if the resonant frequency of the coupled element or elements is slightly different to that of

the driven patch, then the bandwidth of the entire antenna may be increased. Stacking

patches on top of each other is probably the most common procedure utilized to enhance

the bandwidth of a microstrip antenna. Figure 6.1 shows a schematic diagram of an edge-

fed stacked patch configuration, where an arbitrarily shaped patch is etched on a grounded

substrate and is fed by a microstrip transmission line.

Figure 6.1 Schematic of edge fed arbitrarily shaped stacked microstrip patches

Another patch antenna is mounted on a second laminate (with no ground plane) and is

placed directly above the driven patch. Interestingly, when stacking was first proposed in

the late 1970s to increase the bandwidth of direct contact fed patches, only moderate

improvements were achieved. One possible reason as to why such minor improvements

were observed can be attributed to the relative complexity nature of these printed antennas.

By close observation of Figure 6.1, shows that there are many variables in this

configuration and thus a rigorous full wave analysis to accurately model the performance of

the antenna is required. Importantly, the analysis needs to be not only efficient but also

computationally fast so that trends in the impedance nature can be accurately and rapidly

observed and then later optimized. Such accurate and fast codes are available nowadays. A

122

thorough investigation into how to design broadband direct contact stacked patches was

undertaken, in particular, focusing on what optimum parameters are needed to achieve

good bandwidth characteristics. From this study, direct contact feed-stacked patches with

bandwidths approaching 30% have been achieved. This order of bandwidth can also be

achieved using non-contact, feed-stacked patches, such as aperture-coupled stacked

patches, of which a schematic diagram is shown in Figure 6.2.

.

Figure 6.2 Schematic diagram of aperture-coupled rectangular stacked microstrip patches

Advantages of utilizing direct contact feed-stacked patches over aperture-coupled stacked

patches include ease of fabrication and minimal backward-directed radiations. As

mentioned previously, aperture-coupled patches do have more degrees of freedom than

direct contact fed patches and therefore an aperture-coupled stacked patch is somewhat

easier to design.

6.2. RECENT DEVELOPMENTS IN ELECTROMAGNETICALLY COUPLED PATCH

ANTENNA

The present research in electromagnetically coupled patch antenna points to the

development of antennas which caters the need of low profile, large bandwidth and

compact communication applications. The antenna designers around the world are

concentrated in the design of compact antennas with efficient radiation characteristics. The

following modules provide a comprehensive survey about the developments in the state of

art electromagnetically coupled patch antenna technology around the world.

123

A wideband electromagnetic-coupled single-layer microstrip patch antenna is studied

experimentally by Mark et al. in [1]. A notable structure in the feeding design is that an

inverted L-shaped strip is connected to the end of the microstrip line and no matching

network is required. The remarkable feature of the antenna is that a small step is introduced

at the end of the feed line. Moreover the noncontact structure facilitates the fabrication of

antenna arrays. W.S.T. Rowe et al. [2] presented a broadband CPW fed stacked patch

antenna for integration with monolithic and optical integrated circuits. A large aperture is

used as resonator within the operating band. Thick slabs of Rogers 5880 duroid and foam

are used as substrates. The high dielectric feed substrate caused an opposite effect on the

coupling strength and also limited the maximum achievable bandwidth of the antenna.

Broadband microstrip patch antennas for MMICs were presented by Rowe et al. [3]. The

stacked antenna consists of a 50Ω microstrip feed line and a patch element fabricated on

alumina substrate which emulates the high dielectric constant materials used in MMICs.

Good efficiency, a broad impedance bandwidth and large front to back ratio eliminates the

need for cavities or other structures to reduce back radiation.

A single layer CPW fed active patch antenna was presented by Kenneth H. Y. Ip. et al. [4].

The group presented a single-layer CPW fed active patch antenna at 2.75 GHz. The patch

antenna acts both as a resonator and a radiator, and Electromagnetic coupling was utilized

for providing the appropriate closed-loop positive feedback. A broadband two-layer shorted

patch antenna with low cross-polarization was presented by Baligar [5]. The antenna has a

bandwidth of 11% centered around 1.975 GHz with a gain of 8.6 dB, and exhibits well than

-13 dB cross-polarization levels in the H-plane. The stacked geometry is found to reduce

radiated cross-polarization levels significantly and offers a larger impedance bandwidth, a

higher gain and radiation efficiency as compared to the co-planar structure as well as the

patch antenna structure.

The technique for the reduction of backward radiation for CPW fed Aperture stacked patch

antennas on small ground planes was presented by W.S.T. Rowe et al. [6]. The proposed

antenna is mounted on a finite sized ground plane that incorporates a reflector element to

reduce backward radiated field. By altering the reflector element parameters, the rear field

pattern can be adjusted to provide field cancellation in arbitrary directions

A Wide-Band Dual-Polarized Stacked Patch Antenna was proposed by Serra et al. [7]. The

dual polarization in the wide band is achieved by stacking two square aluminum patches

124

fed by two microstrip lines through a couple of crossed slots opened in copper ground

plane. However, Lau et al. presented a dual-band stacked folded shorted patch antenna [8]

suitable for the indoor wireless communication systems that are required to cover the

operating bandwidths of three wireless communication systems.

Latter compact vertical patch antenna for dual band WLAN operation was presented by

F.S. Chang et al. [9]. The antenna consists mainly of one driven patch and one shorted

parasitic patch, both of which wind along two concentric circles. The antenna can be quite

practical in applications of ceiling-mount access points. In addition a dual-band antenna

design consists of two patch radiators suspended above a ground plane was presented by

Toh et al. in [10]. The performance in the lower and upper bands can be controlled with

less mutual coupling effect. The antenna also features less beam squinting of the radiation

patterns at bore sight for both operating bands. In May 1990, Jackson designed linear array

of electromagnetically coupled microstrip patches through the sections of transmission line

embedded within the substrate [11]. While in January 2004, De Doncker presented

electromagnetic coupling to transmission lines under complex illumination [12]. The

proposed method relies on the plane wave spectrum representation of the excitation fields

and on the complex equivalent length formalism.

In May 5, 2004, D. A. White and M. Stowell described the modeling of distributed

electromagnetic coupling effects in analog and mixed-signal integrated circuits [13], and in

December 15,2005 P. Cézanne investigated the electromagnetic coupling between a two-

dimensional grating of resonant gold nano particles and a gold metallic film and observed

multi peaks in the extinction spectra attributed to resonant modes of the hybrid system,

resulting from the coupling between the localized Plasmon of the nano particles with the

underlying surface plasmon mode [14]. However in June 2006, Mcphee presented new

technique for efficient computation of electromagnetic coupling [15]. Higher integration

and smaller layout size, two major trends in today's industry, lead to more prominent

parasitic electromagnetic coupling.

In June 2007, Q. Rao presented an electromagnetically coupling fed broadband low profile

microstrip antenna (MSA) array [16]. Radiation element is an E-shaped MSA that is fed by

an electromagnetically coupled strip and covered by a low loss radome.

125

In September 2007, Nandgaonkar designed high gain two-layer electro-magnetically

coupled patch antenna in the ISM band [17]. In November 19, 2007 L. D. Negro, N.N.

Feng and A. Gopinath explored the potential of one-dimensional and two-dimensional

deterministic a periodic plasmonic arrays for the design of electromagnetic coupling and

plasmon-enhanced, sub-wavelength optical fields on chip-scale devices [18].

Latter, in March 2008, Ikeda enhanced bandwidth of a low profile microstrip antenna

which is electromagnetically coupled with a Folded Inverted L-shaped Probe [19]. In May

2008, W.Ying performed Electromagnetic coupling on airborne structures. He presents a

quantitative approach to analyze this coupling mechanism [20]. In February 2009, Duan

proposed a novel wideband and broad-beam microstrip antenna loaded with gaps and stubs

[21]. The antenna is based on a two-layer stacked electromagnetic coupling microstrip

patch antenna (ECMSA). The impedance bandwidth was found up to 34.6%. In this

impedance bandwidth, the pattern bandwidth was 13%.

R. Q. Lee, K. F. Lee and J. Bobinchak in their studied have found that depending on the

spacing d, the characteristics of the rectangular antenna can be separated into three regions

[22].

The region is associated with bandwidths exceeding 10%;

Region has abnormal radiation patterns, while region c is associated with narrow

beam widths.

The values of ‘d’ separating these regions depend on the dielectric material between the

two-layers. The antenna gains of 9 and 11 dB have been obtained in case of air and Teflon.

As discussed, the dielectric covers on microstrip patch antennas have been found to have

pronounced effects on gain and resonant characteristics. It has been reported that high gain

can be achieved if the thickness of the substrate and multiple superstrate layers are chosen

properly. Recently, experiments with patch antennas covered with dielectric layer showed

that resonant frequency decreases monotonically with superstrate thickness. Since the input

impedance is a function of resonant frequency, it therefore, also changes with cover

thickness. The effect of dielectric cover on a two-layer electromagnetically coupled patch

(EMCP) antenna is of interest because of its potential for high-gain and broadband

applications. The studies of R. Q. Lee, K. F. Lee and J. Bobinchak for the two-layer EMCP

antenna show that, depending on the separation between antenna layers, there exist two

regions of operation [23]. Broadband is possible in a region with spacing less than 0.151,

126

while high-gain is achievable with spacing exceeding 0.3 . However, R. Q. Lee, A. J.

Zaman and K. F. Lee have described, the radiation and impedance characteristics of a two-

layer EMCP antenna covered with dielectric superstrate and found that for the antenna

operating in the high-gain region, the resonant input impedance increases and the 3dB

beam-width decreases with the dielectric thickness [24]. When the antenna is operating in

the broadband region, both the resonant frequency and the bandwidth decrease with the

dielectric thickness. In general, the impedance matching becomes poor with increases in

dielectric thickness.

Due to smaller size, better impedance and bandwidth compared to the square, rectangular

and circular microstrip antenna for given frequency we have choose pentagonal shaped

microstrip antenna. By using this pentagonal geometry we proposed

(i) a electromagnetically coupled antenna and (ii) electromagnetically gaps coupled antenna

feed with a coaxial cable and investigate the effects of accumulation of the water on the

surface of the patch antennas, which is termed as superstrate. Therefore in present chapter

we will discuss effect of dielectric cover (water) on electromagnetically coupled and

electromagnetic gap coupled pentagonal patch antenna.

6.3. DESIGN AND ANALYSIS OF PROPOSED PATCH ANTENNAS

6.3.1. Design Specifications

Table 6.1 Design parameters of the proposed antennas

Parameters Electromagnetically

coupled antenna

Electromagnetically gap

coupled antenna

(gap size, d =0.4 mm)

Designed frequency (GHz) 2.39 2.41

Substrate1(FR-4) εr1 = 4.4, tanδ = 0.02 εr1 = 4.4, tanδ = 0.02

Size of the pentagon

(l1= l2) 28.52mm 28.52mm

Substrate2(Plexiglas) εr2 = 3.4, tanδ = 0.001 εr2 = 3.4, tanδ = 0.001

Dielectric cover εr = 81, tanδ = 0.0 εr = 81, tanδ = 0.0

Feed location 9.0 mm from the center 9.0 mm from the center

127

6.3.2. Electromagnetically Coupled Patch Antenna

In this configuration it is considered that the inner conductor of the coaxial probe is

connected to the lower patch and is, therefore energy coupled with upper patch only

through the fringing field as shown in the Figure 6.3 (a, b).

Figure 6.3 (a) Schematic diagram of electromagnetically coupled patch antenna

Figure 6.3 (b) Schematic diagram of electromagnetically coupled patch antenna with water

layer

The patches are assumed to be a perfect conductor of zero thickness printed on the

dielectric substrate. The l1 and l2 is the side arm of the pentagonal patch antenna. The

thickness of the lower substrate is h1 and the permittivity is εr1 while h2 and εr2 are the

substrate thickness and permittivity of the upper patch respectively. The proposed antenna

structure has been analysed and obtained results are shown in Figure 6.5 to 6.7. in which t

= 0 represents antenna without any layer while t = 0.1,0.2 and 0.3 mm are thickness of the

water layer poured on patch surface however Table 6.2 summarised the obtained result.

h1, εr1, l1

h2, εr2, l2

Y0

h1, εr1, l1

h2, εr2, l2

Y0

Water layer

128

Figure 6.4 Structure of proposed electromagnetically coupled patch antenna

Figure 6.5 Return loss of proposed electromagnetically coupled patch antenna with/without

water layer

Figure 6.6 SWR of proposed electromagnetically coupled patch antenna with/without water

layer

-28

-24

-20

-16

-12

-8

-4

0

1.8 2 2.2 2.4 2.6 2.8 3

S1

1 (

dB

)

Frequency (GHz)

dB(S11) at t=0 mm

dB(S11) at t=0.1 mm

dB(S11) at t=0.2 mm

dB(S11) at t=0.3 mm

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

2.1 2.2 2.3 2.4 2.5 2.6

SW

R

Frequency (GHz)

VSWR at t=0 mm

VSWR at t=0.1 mm

VSWR at t=0 .2mm

VSWR at t=0 .3mm

129

Figure 6.7 Impedance of proposed electromagnetically coupled patch antenna with/without

water layer

Table 6.2 Simulated parameters of the proposed electromagnetically coupled patch antenna

Types Electromagnetically coupled antenna

Without

dielectric

cover

With dielectric cover

At t=0.1 At t=0.2 At t=0.3

Designed frequency (GHz) 2.39 2.3 2.27 2.26

Return loss(dB) -26.83 -16.84 -15.54 -14.2

Impedance(Ω) 47.15 40.06 37.12 33.9

VSWR 1.095 1.335 1.401 1.484

BW(MHz) 62.3 53.8 48.8 47.2

Figures 6.5, 6.6, 6.7 shows the resonant frequency, return loss, impedance, VSWR

variation of the electromagnetically coupled patch antenna with different water levels. The

variation of the resonant frequency of the antenna can be explained by the variation of the

effective permittivity with accumulation of the water level on the surface of the antenna.

From the Table 6.2, it is observed that the accumulation of the water level on the surface of

the patch antenna reduces the antenna parameters such as return loss, impedance, VSWR

etc.

0

5

10

15

20

25

30

35

40

45

50

1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6

Z1

1,Ω

Frequency (GHz)

mag(Z11) at t='0mm'

mag(Z11) at t='0.1mm'

mag(Z11) at t='0.2mm'

mag(Z11) at t='0.3mm'

130

6.3.3. Electromagnetically Gap Coupled Antenna

The geometry of the electromagnetically gap coupled patch antenna is shown in Figures 6.8

(a, b) and 6.9. The l1 and l2 is the side arm of the pentagonal patch antenna. The thickness

of the lower substrate is h1 and the permittivity is εr1 while h2 and εr2 are the substrate

thickness and permittivity of the upper patch respectively. The air gap d, between the two

substrates is 0.4 mm.

Figure 6.8 (a) Schematic diagram of electromagnetically gap coupled patch antenna

Figure 6.8 (b) Schematic diagram of electromagnetically gap coupled patch antenna with

water layer

Water layer

131

Figure 6.9 Structure of proposed electromagnetically gap coupled patch antenna

The propose antenna structure has been analysed and obtained results are shown in Figure

6.10 to 6.12. in which t = 0 represents antenna without any layer while t = 0.1, 0.2 and 0.3

mm are thickness of the water layer poured on patch surface however Table 6.3

summarised the obtained results.

Figure 6.10 Return loss of proposed electromagnetically gap coupled patch antenna

-33

-30

-27

-24

-21

-18

-15

-12

-9

-6

-3

0

1.7 1.9 2.1 2.3 2.5 2.7

S1

1(d

B)

Frequency (GHz)

dB(S11) at t='0mm'

dB(S11) at t='0.1 mm'

dB(S11) at t='0.2 mm'

dB(S11) at t='0.3 mm'

132

Figure 6.11 SWR of proposed electromagnetically gap coupled patch antenna with/without

water layer

Figure 6.12 Impedance of proposed electromagnetically gap coupled patch antenna

with/without water layer

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

1.8 2 2.2 2.4 2.6 2.8

VS

WR

Frequency (GHz)

VSWR at t ='0mm'

VSWR at t ='0.1 mm'

VSWR at t ='0.2 mm'

VSWR at t ='0.3mm'

0

10

20

30

40

50

60

1.8 2 2.2 2.4 2.6 2.8

z1

1,Ω

Frequency (GHz)

mag(Z11 at t='0mm'

mag(Z11 at t='0.1mm'

mag(Z11 at t='0.2mm'

mag(Z11 at t='0.3mm'

133

Table 6.3 Simulated parameters of the proposed electromagnetically gap coupled patch

antenna

Types

Electromagnetically gap coupled Antenna

Without

dielectric

cover

With dielectric cover

At t=0.1 At t=0.2 At t=0.3

Designed frequency (GHz) 2.41 2.31 2.23 2.15

Return loss(dB) -31.73 -17.7 -12.67 -10.06

Impedance(Ω) 51.23 39.68 31.89 27.99

VSWR 1.053 1.299 1.605 1.915

BW(MHz) 67.3 60.6 43 0

Figures 6.10, 6.11, 6.12 show the resonant frequency, return loss, impedance, VSWR

variations of the electromagnetically gap coupled patch antenna with different water level

and d =0.4 mm. The variation of the resonant frequency of the antenna can be explained by

the variation of the effective permittivity with air gap spacing as for the cavity. The

effective permittivity of the lower cavity decreases as air gap spacing increases, hence

resonant frequency increases. From the Table 6.3 it is also observed that the accumulation

of the water level on the surface of the patch antenna varies the antenna characteristics such

as return loss, impedance, VSWR etc.

6.4. CONCLUSIONS

Designed proposed antenna operating at 2.39 GHz has been analysed with and without gap

and obtained results reveal that the antenna performance falls down with increasing with

accumulation of water over its surface. However, the gap coupled has been designed at

2.41 GHz and effect of water layers plays similar role in particular for t = 0.3 mm, antenna

structure totally stop functioning as bandwidth is almost zero. As fractal microstrip antenna

provides multiband characteristics, the next chapter is dedicated to describe the effects of

dielectric loading on its operating performances.

134

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