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CHAPTER 8

COMPENSATED CIRCULARLY POLARISED HEXAGONAL MICROSTRIP ANTENNA

_______________________________________________________________________________

8.1. INTRODUCTION

Matching the polarization in both the transmitter and receiver antennas is important in

terms of decreasing transmission losses. The use of circularly polarized antennas presents

an attractive solution to achieve this polarization match which allows for more flexibility in

the angle between transmitting and receiving antennas, reduces the effect of multipath

reflections, enhances weather penetration and allows for the mobility of both the

transmitter and the receiver. Circular polarization is beneficial because current and future

commercial and 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 RF loss of any array 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

combination of design steps. The first step involves designing an antenna to operate at 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

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the antenna with dual feeds equal in magnitude with 90° physical phase shift. The shape

and the 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, and 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. These grooves can result in

forcing the current to follow extra semi circular patch, which effectively lowers the

resonant frequency of the modified square patch. It was also found that the required size of

the truncated corners for CP operation increases with increasing antenna size reduction.

This behavior gives the proposal design a relaxed manufacturing tolerance for achieving a

compact circularly polarized microstrip antenna [4-6].

In previous chapters we have seen, additional dielectric layer on top of the microstrip patch

may occur as a result of physical conditions 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 main performance characteristics of the structure may be

adversely affected if permittivity and thickness of the dielectric are not chosen properly. It

has also been observed that performance of a microstrip structure can be improved by

operating it around the resonant frequency. Resonant frequency of the microstrip structure

is shifted to a lower value as a result of dielectric shielding or increasing medium

temperature. In such cases, this shift may cause unexpected changes in the behavior of the

antenna structure and, hence, the operation of the supporting electronic circuitry is also

affected. The resonant frequency shift must be compensated without disturbing the original

configuration and degrading its performance.

Based on the information obtained, dielectric of different thickness and material were

loaded on the square-ring microstrip antenna for evaluation. The results show that the

antenna performances such as centre frequency; bandwidth and radiating efficiency are

159

reduced as expected. Furthermore, the axial ratio measurement shows that material with

lower dielectric constant is more preferable if thicker dielectric is to be implemented [7].

However, in order to compensate the shielding effects on the main resonant characteristics

of a microstrip ring structure, air-gap tuning is used and found that in order not to cause a

degrade in the operating performance, air-gap thickness must be adjusted by taking the

structural 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 in the

space-communication applications, which is useful in minimizing the interference caused.

The proposed approach will also be used in the biomedical, geophysical, and millimeter

wave integrated circuit applications providing flexibility in the adjustment of the desired

characteristic without disturbing the original structure and without requiring a new

manufacturing [8].

Therefore in present chapter, focus has been given to achieve CP radiation from hexagonal

microstrip antenna as well as to compensate the dielectric cover effects on its behavior.

Selection of this antenna leads to the advantages of compact structure and, ease of

designing and simple feeding technique. The further details have been described in next

sections.

8.2. DESIGN OF A HEXAGONAL MICROSTRIP ANTENNA

8.2.1. Design Specifications

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)

Coaxial cable dimensions:

Inner radius a : 0.635 mm

Outer radius b : 2.0445 mm

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The elemental side of the patch antenna was chosen to be 26.94 mm and

corresponding to this the resonant frequency is 2.48 GHz.

Figure 8.1 Geometry of hexagonal patch antenna

The Figure 8.1 shows the geometry of hexagonal patch antenna. Hence authors have

designed a microstrip antenna having a ground plane and above which a substrate is

mounted and the substrate a hexagonal patch was build through which the radiation takes

place. In this we have excited antenna through coaxial feeding technique to have proper

radiation.

8.2.2. Results for Antenna without Superstrate

Return loss

For hexagon side length of 26.94 mm we achieve resonant frequency at 2.43 GHz with

return losses -18.52 dB as shown in Figure 8.2.

Figure 8.2 Return loss of the hexagonal microstrip antenna

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1 1.5 2 2.5 3 3.5

dB

(retu

rn

loss

)

Frequency(GHz)

161

VSWR

The VSWR is 2.068 at 2.43 GHz as in Figure 8.3

Figure 8.3 VSWR of the hexagonal patch antenna

Radiation Pattern

Figure 8.4 Radiation pattern of the hexagonal antenna

The radiation pattern shows that the antenna is omni-directional and is linearly polarized

with small levels of cross polarization.

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

dB

(VS

WR

)

Frequency (GHz)

162

Antenna Gain

The gain for the optimized antenna is 5.861 dB as shown in Figure 8.5; the radiation pattern

is omni-directional.

Figure 8.5 Gain of the proposed antenna

Input Impedance

As shown in Figure 8.6 input impedance of the antenna is 46 ohm at 2.43 GHz.

Figure 8.6 Impedance response of the proposed antenna

-25

-20

-15

-10

-5

0

5

10

-200 -100 0 100 200

dB

(gain

)

theta(deg)

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

Imp

ed

en

ce(o

hm

)

Frequency(GHz)

163

Axial Ratio

Axial ratio plot with respect to frequency is shown in Figure 8.7. Axial ratio plot shows the

circular polarized from the electromagnetic waves.

Figure 8.7 Axial rat io plot of the proposed antenna

In axial ratio plot, it is found that axial ratio in dB at resonant frequency (2.43 GHz) is less

than 3 dB which is around 1.245 dB and axial ratio bandwidth is about 1.41 % that shows

circularly polarized from of electromagnetic wave.

8.3. DESIGNED PATCH ANTENNA WITH DIELECTRIC LOADING

The geometry of hexagonal patch antenna having dielectric cover is shown in Figure. 8.8.

In reality, the microstrip antenna attached to electronic devices will be protected by a

dielectric cover (superstrate) that acts as a shield against hazardous environmental effects.

Figure 8.8 Structure of proposed antenna with dielectric cover

0

2

4

6

8

10

1 1.5 2 2.5 3 3.5

dB

(axia

l ra

tio)

Frequency(GHz)

164

These shielding materials will decrease the overall performances of the antenna operating

characteristics such as resonant frequency, reflection coefficient, and impedance bandwidth

and radiating efficiency. We have used the dielectric cover of various thicknesses and

analyze the effects of dielectric cover on the different antenna parameters. We have used

Plexiglas ( ) as a dielectric cover.

8.3.1. Results with Dielectric Loading

The frequency response of the microstrip antenna covered with dielectric varies as a

function of the dielectric cover thickness.

Return loss

As we know that with the increase in the thickness of the dielectric the resonance frequency

shifts towards lower value and the return losses increases. Thus the height of the dielectric

should be such that the return losses are good but at the same time operates in the designed

range. Figure 8.9 shows the return loss with dielectric loading with the thickness of 0.5

mm.

Figure 8.9 Return loss of proposed antenna with dielectric cover

VSWR

As shown in the Figure 8.10 VSWR is nearly equal to 2, and with superstrate the VSWR

decreases.

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1.5 2 2.5 3

S11 d

B

Frequency(GHz)

165

Figure 8.10 VSWR of the proposed antenna with dielectric cover

Input Impedance

Figure 8.11 Impedance of proposed antenna with dielectric cover

The Figure 8.11 shows the magnitude of the input impedance of antenna. The input

impedance also decreases with increase in the height of the dielectric.

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

dB

(vsw

r)

Frequency(GHz)

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

Imp

ed

en

ce(o

hm

)

Frequency(GHz)

166

Radiation Pattern

Figure 8.12 Radiation pattern of proposed antenna with dielectric cover

Gain

Gain of antenna increases with the dielectric thickness. It increases from 5.86 dB to 5.99

dB when dielectric cover is used over patch antenna.

Figure 8.13 Gain of proposed antenna with dielectric cover

-25

-20

-15

-10

-5

0

5

10

-200 -150 -100 -50 0 50 100 150 200

dB

Theta(degree)

167

8.4. COMPENSATED HEXAGONAL PATCH ANTENNA

To compensate dielectric loading effect, an air gap is created between ground planes using

RT-Duroid substrate. For a 0.5 mm thick dielectric cover of Plexiglas, an air gap of 0.1mm

is created between substrate and the ground plane in order to compensate the dielectric

loading effect. As we 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 performance of antenna get

deteriorated. When we create air gap between the ground plane and substrate, resonant

frequency of dielectric loaded antenna shifted from 2.39 GHz to 2.44 GHz which is within

operating range of antenna and other performance characteristics of antenna like impedance

bandwidth, input impedance, VSWR, return loss etc. also get improved.

8.4.1. Results for Compensated Patch Antenna

A compensated performance characteristic of proposed antenna is shown Figure 8.14-

8.18.

Return Loss

Return loss of antenna with the dielectric cover decrease from -18.52 dB to -17.2407 dB

which again improved to around -18 dB while compensating using air gap.

Figure 8.14 Return loss of compensated hexagonal patch antenna

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1.5 2 2.5 3

dB

(retu

rn

loss

)

freq(Ghz)

168

Input Impedance

Input impedance of antenna decrease from 47 Ω to 42 Ω when dielectric cover is used and

compensated value of impedance is improved from 42 Ω to 45 Ω.

Figure 8.15 Input impedance of compensated hexagonal patch antenna

VSWR

VSWR of antenna while compensating is improved from 2.42 to 2.37.

Figure 8.16 VSWR of compensated hexagonal patch antenna

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

imp

ed

en

ce(o

hm

)

Frequency (GHz))

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

dB

Frequency(GHz)

169

Gain

Gain of the antenna changes from 5.998 dB to 5.83 dB while dielectric loaded antenna is

compensated.

Figure 8.17 Gain of the compensated hexagonal patch antenna

Radiation Pattern

Figure 8.18 Radiat ion pattern of compensated hexagonal patch antenna

-25

-20

-15

-10

-5

0

5

10

-200 -100 0 100 200

dB

Theta(deg)

170

Table 8.1 Comparison of antenna parameters

Antenna

parameters

Without

dielectric loading

With

dielectric

loading for

gap 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) 47 42 45

VSWR 2.068 2.42 2.376

Gain (dB) 5.861 5.998 5.8307

Impedance bandwidth 1.45% 1.30% 1.51%

8.5. CONCLUSIONS

The hexagonal patch antenna shows different variations when covered with dielectric

materials. A basic hexagonal patch antenna is designed with the help of HFSS. It is found

that performance characteristics of antenna changes with increasing thickness of dielectric

layer. We found that beyond particular thickness of the dielectric layer the resonant

frequency of antenna goes beyond the operating range; hence the performance of antenna

deteriorates. Various characteristics like resonance frequency, return loss, input impedance,

bandwidth, VSWR, gain get shifted. The change in various responses is compensated by

the introduction of an air gap between ground plane and substrate. The thickness of the air

gap is chosen such that the shifted responses are brought in the desired range.

171

REFERENCES

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