ELECTROMAGNETICALLY COUPLED MICROSTRIP ANTENNA WITH...
Transcript of ELECTROMAGNETICALLY COUPLED MICROSTRIP ANTENNA WITH...
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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
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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
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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.
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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
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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.
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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,
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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
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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
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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
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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'
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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
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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'
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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'
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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.
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