Compensation of dielectric cover effects on cp hexagonal microstrip antenna

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
INTERNATIONAL JOURNAL OF ELECTRONICS AND
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 1, January- February (2013), pp. 43-54
IJECET
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COMPENSATION OF DIELECTRIC COVER EFFECTS ON CP
HEXAGONAL MICROSTRIP ANTENNA

Ravindra Kumar Yadav1, Jugul Kishor1 and Ram. Lal Yadava2
1,2
Department of Electronics and Communication Engineering,
1
I.T.S Engineering College, Greater Noida, Uttar Pradesh, India,
2
Galgotia's college of Engineering and Technology, Greater Noida, Uttar Pradesh, India
ravipusad@gmail.com, er.jugulkishor@gmail.com and rly1972@gmail.com

ABSTRACT

This communication describes the design and analysis of a dielectric layer loaded
circularly polarized (CP) hexagonal patch antenna in the frequency range 2.4000-2.4835
GHz. The obtained results indicate that there are significant changes in the performances of
the antenna. In particular the axial ratio at resonant frequency 2.43 GHz is around 1.245 dB
followed by the axial ratio bandwidth around 1.41 % hence the proposed antenna confirms
the circularly polarized behaviour. Therefore the change in various response parameters due
to such loading is compensated by introducing an air gap between the ground plane and the
substrate of patch antenna. The thickness of the air gap is chosen such that the shifted
responses are brought in the desired range. Due to air gap, the resonant frequency of
dielectric loaded antenna shifted from 2.39 GHz to 2.44 GHz which is within the operating
range of antenna and other performance characteristics of the antenna like input impedance,
VSWR, return loss etc. also get improved, and the impedance bandwidth improved up to
around 1.51 %.

INDEX TERM - Hexagonal Patch Antenna, Circular Polarization, Superstrate loading

I. INTRODUCTION

In any communication system, matching the polarization in both the transmitter and
receiver antennas is very important in terms of decreasing transmission losses. The use of
circularly polarized antennas presents an attractive solution to achieve this polarization match
which allows more flexibility in the angle between transmitting and receiving antennas. It
also reduces the effect of multipath reflections and enhances weather penetration. Circular

43


polarization is beneficial because current and future commercial as well as military
applications require the additional design freedom of not requiring alignment of the electric
field vector at the receiving and transmitting locations. Single feed circularly polarized
antennas are currently receiving much attention, because it allows a reduction in the
complexity, weight and the RF loss of any antenna feed and is desirable in situations where it
is difficult to accommodate dual orthogonal feeds with a power divider network. Circularly
polarized microstrip antennas have the additional advantage of small size, weight, suitability
in conformal mounting and compatibility with microwave and millimeter wave integrated
circuits, and monolithic microwave integrated circuits (MMICS) [1-3].
A single patch antenna can be made to radiate circular polarization if two orthogonal
patch modes are simultaneously excited with equal amplitude and ± 90o out of phase with the
sign determining the sense of rotation. A patch with a single point feed generally radiates
linear polarization, however in order to radiate CP, it is necessary for two orthogonal patch
modes with equal amplitude and in phase quadrature to be introduced. This can be
accomplished by slightly perturbing a patch at appropriate locations with respect to the feed.
Designing a circularly polarized microstrip antenna is challenging; as it requires a
combination of design steps. The first step involves designing an antenna to operate on a
given frequency. However in the second step circular polarization is achieved by either
introducing a perturbation segment to a basic single fed microstrip antenna, or by feeding the
antenna with dual feeds equal in magnitude with 90° physical phase shift. The shape and
dimensions of the perturbation have to be optimized to ensure that the antenna achieves an
axial ratio < 3 dB at the desired design frequency. Various perturbation techniques for
generating CP have been reported in the literatures, which operate on the same principle of
detuning degenerate modes of a symmetrical patch by perturbation segments. A well-known
method of producing a single feed circular polarization operation of the square microstrip
antenna by truncating a pair of patch at two opposite corners has also been presented. It is
also found that this method can also be applied to a modified square microstrip patch with
four semi-circular grooves along the four edges of the patch of equal dimensions to achieve a
CP operation with compact design along with relaxed manufacturing tolerances. The
compactness of the proposed CP design is achieved due to the semicircular grooves at the
patch edges of the square patch. It was also found that the required size of the truncated
corners of CP operation increases with increasing antenna size reduction. This behavior gives
the proposal of designing a relaxed manufacturing tolerance for achieving a compact
circularly polarized microstrip antenna [4-6].
On the other hand an additional dielectric layer on top of the microstrip patch may
occur as a result of physical condition changes such as snow and ice or may be directly
introduced as a radome in the manufacturing stage for the purpose of protection from the
environmental hazards. The performance characteristics of the antenna structure may be
adversely affected if relative permittivity and thickness of the dielectric substrate are not
chosen properly. It has been also observed that the resonant frequency of the microstrip
antennas is shifted to a lower value as a result of dielectric shielding on the antennas. In such
cases, this shift may cause unexpected changes in the behavior of the antenna structure and,
hence the operations of the supporting electronic circuitry are also affected. So the resonant
frequency shift needs to be compensated without disturbing the original configuration and
degrading its performances.
In a study, the dielectric layers of different thickness were loaded on the square-ring
microstrip antenna and found that the antenna performances such as centre frequency;
bandwidth and radiating efficiency are reduced. The axial ratio data show that material with

44


lower dielectric constant is more preferable if thicker dielectric is chosen for design [7].
However, in order to compensate the shielding effects on the resonant characteristics of a
microstrip ring structure, air-gap tuning is used and found that in order to avoid degradation
in the operating performances, air-gap thickness must be adjusted by taking the geometrical
parameters of both substrate and dielectric layers into consideration. In addition, it is also
found that there is the possibility of controlling the bandwidth of antennas useful in the
space-communication applications specially to minimize the interference caused. The
proposed approach will also be useful in the biomedical, geophysical, and millimeter wave
integrated circuit applications providing flexibility in the adjustment of the desired
characteristics without altering the original structure and not adding nay new components [8].
Therefore in this paper, an attempt has been made to achieve CP radiation from the hexagonal
microstrip antenna as well as to compensate the dielectric cover effects on the performances
of the antenna. The selection of such antennas leads to the advantages of compact structure
and, ease of designing and a simple feeding technique as well.
II. DESIGN SPECIFICATIONS
Design parameters of proposed hexagonal patch antenna are as follows;
Feeding technique : Coaxial feed
Substrate material : RT Duroid
Relative permittivity of the substrate ( ߝ௥ ሻ: 2.32
Operating frequency range : 2.4-2.4835 GHz
Thickness of dielectric substrate : 1.575 mm
Elemental side : 26.94 mm
Feed location (x, y) : (-4.3 mm, -4.3 mm)
Inner radius a : 0.635 mm
Outer radius b : 2.0445 mm

Fig. 1a. HFSS geometry of hexagonal patch antenna

Fig. 1b. Fabricated Hexagonal patch antenna

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However the Figures 1a & b show the geometry of the hexagonal patch antenna. The
reason behind selecting the hexagonal microstrip antenna that, it has smaller size compared to
the square and circular microstrip antennas, as well as better impedance bandwidth over
rectangular and square microstrip antennas for a given frequency. Therefore, authors have
designed a coaxial fed hexagonal patch antenna and circularly polarized radiation has been
achieved by adjusting the position across the antenna.
Since a circular disc is the limiting case of the polygon with large number of sides, the
resonant frequency for the dominant as well as for the higher order modes can be calculated
from the formula of the circular disc by simply replacing radius a by equivalent radius ܽ௘௤.
ଡ଼′౤౦ .ୡ
f୬୮ ൌ (1)
ଶπୟ౛౧ √ε౨

Where ܺ௡௣ are the zeros of the derivative of the Bessel function Jn(x) of the order n.
′

The equivalent radius ܽ௘௤. is determined by comparing areas of a regular hexagon and a
circular disk of radius ܽ௘௤.
ଷ√ଷ ୗమ
πaୣ୯ ଶ ൌ ଶ
(2)
or
aୣ୯. ൌ 0.9094 S (3)

Thus the resonant frequency of a hexagonal element may be written:
ଡ଼′౤౦ .ୡ ଵ.ଵଡ଼′౤౦ .ୡ
f୰ ൌ ଶπ.ሺ଴.ଽ଴ଽସୗሻ. ൌ ଶπୗ√ε౨
(4)
√ε౨

For the lowest order mode ܶ‫ܯ‬ଵଵ

X ୬୮ ൌ 1.84118
′
(5)

Using above design parameters and design expressions, the proposed antenna has been
designed and performances are examined using HFSS, and the obtained results are described
in the following sections.

0
-2
dB(Return loss)

-4
-6
-8
-10
-12
-14
-16
-18
-20
1 1.5 2 2.5 3 3.5
Frequency(GHz)

Fig. 2. Return loss of the hexagonal microstrip antenna
The resonant frequency of the conventional hexagon antenna of side length of 26.94 mm, is
found to be 2.43 GHz with a return loss around -18.52 dB as shown in Figure 2. Whereas the
value of VSWR is 2.068 at 2.43 GHz, and corresponding values of VWSR with frequency is
plotted is Figure 3.

46


60
50
40

VSWR
30
20
10
0
0 1 2 3 4
Frequency (GHz)

Fig. 3. VSWR of the hexagonal patch antenna

Fig. 4. Radiation pattern of the hexagonal antenna
The radiation pattern of the antenna shows that it is omni-directional as well as linearly
polarized with small levels of cross polarization as shown in Figure 4.. The gain for the
optimized antenna is 5.861 dB, and shown in Figure 5, however the input impedance of the
antenna is 46 at 2.43 GHz (Figure 6). Axial ratio with respect to frequency is shown in
Figure 7, and found that axial ratio at the resonant frequency (2.43 GHz) is around 1.245 dB
and axial ratio bandwidth is about 1.41 %.

10
5
0
dB(gain)

-200 -100 -5 0 100 200
-10
-15
-20
-25
theta(deg)

Fig. 5. Gain of the proposed antenna

47


60
50

Impedence(ohm)
40
30
20
10
0
0 1 2 3 4
Frequency(GHz)

Fig. 6. Impedance response of the proposed antenna

10

8
dB(axial ratio)

6

4

2

0
1 1.5 2 2.5 3 3.5
Frequency(GHz)

Fig.7. Axial ratio plot of the proposed antenna

III. HEXAGONAL MICROSTRIP ANTENNA WITH DIELECTRIC COVER
The geometry of a dielectric loaded hexagonal patch antenna is shown in Figure 8, where
Plexiglas, ሺߝ௥ ൌ 3.4) have been used as dielectric covers and the effects on the different
antenna parameters are analyzed and shown in Figures 9-13.

Fig.8. Structure of proposed antenna with dielectric cover

48


0

-5

-10

S11 (dB)
-15

-20
1.5 2 2.5 3
Frequency(GHz)

Fig.9. Return loss of proposed antenna with dielectric cover

60
50
40
VSWR

30
20
10
0
0 1 2 3 4
Frequency(GHz)

Fig.10. VSWR of the proposed antenna with dielectric cover

60

50
Impedence(ohm)

40

30

20

10

0
0 1 2 3 4
Frequency(GHz)

Fig.11. Impedance of proposed antenna with dielectric cover

49


Fig.12. Radiation pattern of proposed antenna with dielectric cover

10
5
0
-200 -100 -5 0 100 200
dB

-10
-15
-20
-25
Theta(degree)

Fig.13. Gain of proposed antenna with dielectric cover

Figures 9-13 show the performance characteristics of the proposed antenna with a dielectric
cover of thickness 0.5 mm. The Figure 9 indicates that the return loss of the antenna is -17.24
dB at 2.39 GHz. However Figure 10 shows that the VSWR is nearly equal to 2. The Figure
11 shows the magnitude of the input impedance of the antenna. The radiation pattern and gain
of the antenna are shown in Figures 12 and 13 respectively.

IV. COMPENSATED HEXAGONAL PATCH ANTENNA

As reported in reference [9], we know that due to dielectric loading, capacitance
of the antenna system increases, which decreases the overall performances of the antenna
such as resonant frequency, impedance bandwidth and radiating efficiency. Therefore, in
order to compensate dielectric loading effect, one should/decrease change the capacitance of
the antenna system. Hence in this work, to achieve the original capacitance of the antenna, an
air gap is created between ground plane and substrate of the antenna. Due to such air gap the
capacitance of the antenna system further decreases causing significant improvements in
overall performances of the antenna system.

50


0

dB(Return loss)
-5

-10

-15

-20
1.5 2 2.5 3
Frequency(GHz)

Fig.14. Return loss of compensated hexagonal patch antenna
In particular, we inserted an air gap of 0.1mm between the substrate and the ground plane. As we have
seen that using a 0.5mm thick dielectric cover over the patch causes the shifting of resonant frequency
from 2.43 GHz to 2.39 GHz which is beyond the operating range of antenna (i.e. 2.4-2.4835 GHz) and
hence the performance of antenna get deteriorated. When we create an air gap between the ground
plane and the substrate, the resonant frequency of dielectric loaded antenna shifted back from 2.39
GHz to 2.44 GHz which is within the operating range of the antenna. The obtained compensated
performance characteristic impedance bandwidth, input impedance, VSWR, return loss etc. are shown
Figure 14-19. In particular, return loss with the dielectric cover decreased from -18.52 dB to -17.2407
dB, again improved to around -18 dB.

60
50
40
impedence(ohm)

30
20
10
0
0 1 2 3 4
Frequency (GHz))

Fig.15. Input impedance of compensated hexagonal patch antenna

60
50
40
VSWR

30
20
10
0
0 1 2 3 4
Frequency(GHz)

Fig.16. VSWR of compensated hexagonal patch antenna

51


10
5
0
-200 -100 -5 0 100 200

Gain(dB)
-10
-15
-20
Theta(deg)
-25

Fig.17. Gain of the compensated hexagonal patch antenna

Fig.18. Radiation pattern of compensated hexagonal patch antenna
Similarly input impedance decreased from 53 to 42 , is improved back to 56 in case of
compensated antenna, while VSWR is improved from 2.42 to 2.37, along with the gain improvement
from 5.998 dB to 5.83 dB. The comparison of the obtained results of the proposed antenna are listed
in Table 1

10
9
8
Axial Ratio(dB)

7 AxialRatio_Hexagonal
6 Patch_Without Dielectric cover
5
4 AxialRatio_Hexagonal with
dielectric Cover
3
2 AxialRatioValue_Hexagonal
1 compensated
0
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
Frequency (GHz)

Fig.19. Axial ratio of hexagonal patch antenna

52


Table 1 Comparison of antenna parameters
With
Without dielectric
Antenna Compensated
dielectric loading
parameters values
loading of 0.5
mm
Resonance frequency 2.43 2.39 GHz 2.44 GHz
(GHz) GHz
Return loss (dB) -18.52 -17.2407 -17.931
Impedance ( ) 53 42 56
VSWR 2.068 2.42 2.376
Gain (dB) 5.861 5.998 5.8307
Impedance bandwidth 1.45% 1.30% 1.51%

V. CONCLUSIONS

Thus a dielectric covered hexagonal patch antenna is designed and analyzed with the
help of HFSS. And found that due to dielectric layer the resonant frequency of the antenna
goes beyond the operating range; hence the performance of antenna deteriorates. In addition
various parameters; return loss, input impedance, bandwidth, VSWR, gain also get altered. In
addition basic antenna provides circularly polarized radiation (AR < 3dB) at the frequency
2.2 GHz. However, the dielectric loading deteriorates the circular polarization characteristics
of the antenna and axial ratio values goes beyond 3dB. Therefore, the main focus has been
given to compensate these changes by introducing an air gap between the ground plane and
substrate of the hexagonal patch antennas. The thickness of the air gap is chosen such that the
shifted responses are brought in the desired range. It is also found the proposed compensation
technique does not play an effective role to get back the same circular polarization radiation.
That is the compensation of superstrate loading effects on the CP antenna can be chosen for
further research.

ACKNOWLEDGMENT

The authors express their appreciation to Dr. B. K. Kanaujia, Professor, Department
of Electronics and Communication, Ambedkar Institute of Technology, New Delhi for allows
us to use HFSS simulation software and experimentations.

REFERENCES

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resonator," IEEE Tran. Microwave Theory and Techniques, Vol. 30, pp. 825-828, 1982.

53


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