Compensation of dielectric cover effects on cp hexagonal microstrip antenna

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN

0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME

43

COMPENSATION OF DIELECTRIC COVER EFFECTS ON CP

HEXAGONAL MICROSTRIP ANTENNA

Ravindra Kumar Yadav

1, Jugul Kishor

1 and Ram. Lal Yadava

2

1,2

Department of Electronics and Communication Engineering, 1I.T.S Engineering College, Greater Noida, Uttar Pradesh, India,

2Galgotia's college of Engineering and Technology, Greater Noida, Uttar Pradesh, India

[email protected], [email protected] and [email protected]

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

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44

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



45

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 ( : 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



46

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 .

f ′ .

π√ε (1)

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 .

πa √

(2)

or

a. 0.9094 S (3)

Thus the resonant frequency of a hexagonal element may be written:

f$ ′ .

π.%&.'&'(.√ε

).) ′ .

π√ε (4)

For the lowest order mode *+))

X′ 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.

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.

-20-18-16-14-12-10-8-6-4-20

1 1.5 2 2.5 3 3.5

dB

(Ret

urn

loss

)

Frequency(GHz)



47

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 Ω 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 %.

Fig. 5. Gain of the proposed antenna

0

10

20

30

40

50

60

0 1 2 3 4V

SW

R

Frequency (GHz)

-25

-20

-15

-10

-5

0

5

10

-200 -100 0 100 200

dB

(ga

in)

theta(deg)



48

Fig. 6. Impedance response of the proposed antenna

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, % 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

0

10

20

30

40

50

60

0 1 2 3 4Im

ped

ence

(oh

m)

Frequency(GHz)

0

2

4

6

8

10

1 1.5 2 2.5 3 3.5

dB

(axia

l ra

tio

)

Frequency(GHz)



49

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

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

Fig.11. Impedance of proposed antenna with dielectric cover

-20

-15

-10

-5

0

1.5 2 2.5 3

S1

1 (d

B)

Frequency(GHz)

0

10

20

30

40

50

60

0 1 2 3 4

VS

WR

Frequency(GHz)

0

10

20

30

40

50

60

0 1 2 3 4

Imp

eden

ce(o

hm

)

Frequency(GHz)



50

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

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.

-25

-20

-15

-10

-5

0

5

10

-200 -100 0 100 200

dB

Theta(degree)



51

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.

Fig.15. Input impedance of compensated hexagonal patch antenna

Fig.16. VSWR of compensated hexagonal patch antenna

-20

-15

-10

-5

0

1.5 2 2.5 3d

B(R

etu

rn loss

)

Frequency(GHz)

0

10

20

30

40

50

60

0 1 2 3 4

imp

eden

ce(o

hm

)

Frequency (GHz))

0

10

20

30

40

50

60

0 1 2 3 4

VS

WR

Frequency(GHz)



52

Fig.17. Gain of the compensated hexagonal patch antenna

Fig.18. Radiation pattern of compensated hexagonal patch antenna

Similarly input impedance decreased from 53 Ω to 42 Ω, is improved back to 56 Ω 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

Fig.19. Axial ratio of hexagonal patch antenna

-25

-20

-15

-10

-5

0

5

10

-200 -100 0 100 200

Gain

(dB

)

Theta(deg)

0

1

2

3

4

5

6

7

8

9

10

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Axia

l R

ati

o(d

B)

Frequency (GHz)

AxialRatio_Hexagonal

Patch_Without Dielectric cover

AxialRatio_Hexagonal with

dielectric Cover

AxialRatioValue_Hexagonal

compensated



53

Table 1 Comparison of antenna parameters

Antenna

parameters

Without

dielectric

loading

With

dielectric

loading

of 0.5

mm

Compensated

values

Resonance frequency

(GHz)

2.43

GHz

2.39 GHz 2.44 GHz

Return loss (dB) -18.52 -17.2407 -17.931

Impedance (Ω) 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.

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54

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Published by IAEME.

Compensation of dielectric cover effects on cp hexagonal microstrip antenna

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