Chapter 2

Modified Rectangular Patch Antenna with Truncated Corners

2.1 Introduction of rectangular microstrip antenna

2.2 Design and analysis of rectangular microstrip patch antenna

2.3 Design of modified rectangular microstrip patch antenna with

truncated corners.

2.4 Design of gap coupled truncated rectangular microstrip patch antenna

2.5 Discussion and conclusions.

Chapter 2

Modified Rectangular Patch Antenna with Truncated Corners

2.1 Rectangular Microstrip Antenna

Introduction: Among the common shapes of microstrip patch geometry square and

rectangular shape are most widely investigated due to their simplified mathematical modeling

and associated boundary conditions. Extensive theoretical and measured analysis on these patch

antennas may be seen in available literature on microstrip antennas. Derneyrd [1978] performed

the theoretical investigation of microstrip antenna and reported that the radiation took place

predominantly from the fringing end at the open circuited ends. Samras et al. [2004] theoretical

investigated the changes of input impedance of a rectangular patch antenna with feed position.

The basic rectangular patch antenna having width W and length L designed on a substrate

having substrate thickness (h), relative permittivity (ε r) and patch height (t) is supported by an

infinite ground plane on the back side of the substrate. For a rectangular patch antenna length is

normally 0.333λ < L < 0.5 λ, where λ is the wavelength of free space. And conductor patch

thickness is given by t < < λ /10. From simple formula given below we can calculate length and

width of RMSA. Take width W of the patch smaller or bigger than obtained value from equation.

If w is lesser than gain and band width will decrease and if W is greater, than bandwidth

increases due to the increase in the radiated fields.

0.49 λ

L ≈ 0.49 λd = √ εr

L = resonant length

λd = wavelength in PC board

λ = wavelength in free space

εr = dielectric constant of the PC board material

C W = √ 2/ (εr +1)

2f0 The feed is used to excite the patch either by probe fed or by edge feed, a fringing field is

developed between ground plane and underneath of the patch due to which antenna radiates.

In the next section the performance of the conventional rectangular rmicrostrip patch

antenna has been reported. This antenna is simulated and its performance is analyzed in free

space.

2.2 Design and Analysis of Rectangular Microstrip Patch Antenna

In the first step: a conventional rectangular microstrip patch antenna is simulated and

designed by using IE3D simulator software. The basic requirements for any design of microstrip

patch antenna (in this case rectangular patch antenna) are:

a) Selection of the substrate (Єr): substrate material used in this design is FR4 substrate,

having loss tangent tanδ =0.002 and relative permittivity Єr =4.4 to reduces the

dimensions of the patch usually substrate of high dielectric constant is to be chooses.

b) substrate height (h): antenna height should be kept as small as possible .The standard

height for the available material FR4 is 1.59 mm so for this antenna the height is consider

as h=1.6mm.

c) (f0)- Resonant frequency: it should be choose appropriately for the proposed antenna.

For rectangular patch it is selected as 1.5 GHz which lies in personal communication

system band. 1.5 GHz to 5.2 GHz band of the frequency spectrum is referred as S band.

Many satellites transmit at S band.

d) Selection of feeding method: the antenna is fed through coaxial cable SMA connector of

50Ω. The (Xf &X y) feed location is optimized to excite the patch.

Fig2.1- Meshing in patch antenna geometry

37

Simulation of the antenna is done by IE3D simulator software. This software is based on

methods of moment and divides the prototype geometry in small grids (meshes) as shown in Fig

2.3 the simulation accuracy depends upon the number of grids. There is a compromise between

the desired level of accuracy, the amount of available computing resources and the size of the

mesh. According to the size of the basic elements the accuracy of the solution depends. Solutions

based on coarse meshes are not so accurate than Solutions based on fine meshes. To generate a

precise description of the current each element of the mesh must occupy a region that is small

enough for the current to be adequately interpolated from the normal value. However meshes

with large number of elements require a significant computing memory and power. Therefore it

is desirable to use a mesh that is fine enough to obtain an accurate current solution but not so fine

that it exhausts the available amount of computing power and memory.

2.2.1 Antenna Design Using above equation and design parameter given above, the dimension of the antenna are

calculated so that it can resonate at frequency 1.5 GHz and figure 2.4shows geometry of the

proposed rectangular microstrip patch antenna .The substrate used for proposed antenna design

was FR4 whose thickness is 1.6 mm and dielectric constant of 4.4. The geometrical parameters

for proposed RMSA antenna are, length of rectangular patch L = 47mm and width of rectangular

patch W = 62mm. The matching of input impedance of antenna with 50 ohms impedance of feed

line is achieved by selecting an inset feed point. The antenna is fed from feed point (Xf = 38mm,

Yf = 26mm) through coaxial cable SMA connector of 50Ω .A trial and error mechanism is

followed to analysis the reflection coefficient (S11) minimum value at the resonant frequency.

38

Figure2.2 - rectangular microstrip patch antenna

39

40

40

2.2.2 Results and Discussion

Fig. 2.5 represents the return loss for the proposed designed antenna .An excellent

reflection coefficient approx -31dB has been achieved at the resonant frequency 1.49 GHz

corresponds to dominant TE11 mode of excitation. The simulated result has a bandwidth of 2.68

% across a range of freq 1.47 GHz to 1.51 GHz, below the -10 dB RL at central frequency of 1.49 GHz. The change in simulated value of VSWR Vs frequency is shown in fig 4.4. VSWR

presented by antenna across a bandwidth area is less than 2:1 value which is good for matching

between feeding circuit and antenna. The simulate value of VSWR at resonant freq is 1.06.

Figure 2.3 - Variation of return loss Vs frequency

41

41

To match an antenna the impedance locus needs to be shifted as near as possible to the centre of

the smith chart (matching point). As shown in fig2.7 the impedance matching point is very close

to the centre of the smith chart. Figure 2.8 depict the simulated graph of input impedance of

design antenna with freq.At resonance freq 1.49 GHz the simulated input impedance of antenna

is 51+ j 2.77ohms which is in good agreement with the 50 ohms impedance of feeding network.

Figure 2.4 - Variation of VSWR Vs. frequency

Figure 2.5 - Impedance Loci

42

Figure 2.6 - Graph of input impedance of antenna vs. frequency

43

44

44

The other radiation characteristics of antenna such as directivity, gain and efficiency are shown

in fig 2.9, 2.10.and 2.11respectively. At resonance freq the total field directivity is 6.4 dBi as the

typical value of directivity for microstrip antenna should be 5-8 dbi and the maximum gain of

about 1.16 dBi at resonance frequency is obtained. at resonance freq antenna and radiation

efficiencies of antenna is about 30% as shown in figure 2.11.the E-plane and H- plane elevation

pattern and azimuth pattern of antenna at resonance freq 1.49ghz are shown in fig 2.12 and 2.13

respectively which indicates that the radiation intensity is maximum normal to the patch.

Figure 2.7 - change in simulated value of directivity of antenna Vs frequency

45

45

Figure 2.8 - change in simulated value of gain of antenna Vs. frequency

Figure 2.9 - Variation of efficiency of antenna vs. frequency

46

Figure 2.10 - display of Elevation pattern of antenna

47

Figure 2.11 - display of Azimuth pattern of antenna

48

49

49

All the results presented above for a rectangular patch antenna suggests that the designed antenna

present in this form is unsuitable for modern communication systems. Therefore we have to

modify this antenna to improve its overall performances as reported in next section.

2.3 Design of Simple Truncated Rectangular Microstrip Patch Antenna

A Rectangular patch antenna discuss in previous section is modified to achieve dual

frequency/dual band performance. In these when two or more resonance frequencies of a

microstrip antenna are close to each other, one gets broadband characteristics. The dual band

antenna can be used in various applications like cellular systems, WLAN, radar, and radio

frequency identification systems because of their advantages like low profile, light weight and

reduced cost. Generally single layer dual band microstrip antenna are possible by utilizing the

multi resonance characteristics of a single patch antenna by loading the patch with stub, using

shorting post, introducing notches, corner chopped, and by loading slots.The detailed inspection

of dual frequency microstrip antenna is available in open literature of antenna. Wong and Chen

[1998] presented bow tie patch dual- frequency antenna by loading a pair of narrow slots. Gao et

al. [2002] reported a rectangular microstrip patch antenna with a shorting pin and achieved large

bandwidth and good reduction in antenna size. A.A. Heidari et al.[2009] presented a circularly

polarized stub loaded microstrip patch antenna for G PS application. Wenquan et. al[2011]

design a broadband circularly polarized microstrip antenna with a truncated corner patch using a

single chip-resistor loading which gives effective axial ratio and wide bandwidth. The methods

discuss above for obtaining dual frequency have their own merits and demerits.

In this section a single layer rectangular microstrip antenna is modified by chopped the

opposite corners of the patch antenna as this modification gives good impedance matching and

better gain. It also provides a dual frequency behavior out of which one is similar to the resonant

frequency of a conventional patch while the other is originating due to modification in geometry.

This modification gives better performance as compare to simple rectangular patch .In further

chapters dimensions of a rectangular patch are modified and various broadband techniques are

50

50

applied such that the resonance frequencies of the two orthogonal modes are close to each other

to obtain broad bandwidth.

2.3.1 Antenna Design Figure -2.14 shows the truncated RMSA, the design parameters for the proposed TRMSA design

structure are length of patch (L) is 47mm, width of the patch (W) is 62 mm and two notches of 5

mm are introduces at two corners of rectangular microstrip patch .The dielectric constant (Єr) of

the substrate is 4.4 mm and the thickness of the dielectric substrate is 1.6 mm. The patch is

printed on inexpensive glass epoxy FR4 substrate. The 50-ohm coaxial cable with SMA

connector is used for feeding. The proposed patch antenna gives dual resonance frequency f1 =

1.16 GHz with impedance Bandwidth equal to 1.72 %and f2= 1.5 GHz with impedance Bandwidth equal to 2.66% over a range of frequency in between 1 GHz to 2 GHz, at

appropriate feed point location( Xf=41mm,Yf= 60mm). A trial and error mechanism is followed

to analysis the reflection coefficient (S11) minimum value at the resonant frequency. The

simulation of this design antenna is done by IE3D simulator software. For a good impedance

matching across a wide range of frequency, notches are also introduced at the two corners of the

rectangular patch antenna as shown.

Fig2.12- truncated rectangular microstrip patch antenna

51

2.3.2 Results and Discussion

The proposed antenna resonating at two frequencies corresponding to different modes of

excitation. Fig 2.15 represents the simulated return loss of the proposed design antenna. The first

resonance freq is 1.16 GHz is due to notches and second resonance freq is 1.5 GHz which is

similar to rectangular patch antennas studied in previous section. the simulated variation of

VSWR presented by antenna at both the resonant frequency are display in fig 2.16 which

indicates that VSWR bears values lower than 2:1 at both the frequencies this result confirms

good matching of this antenna with the feed network. Figure 2.17 indicates the simulated graph

of input impedance of design antenna with freq , the simulated values of input impedance of

antenna at two resonance freq are 35.05 +j0.419 ohms and (50.87+ j1.572) the real parts of input

impedances are in fair agreement with 50ohms impedance of feed line.

Fig.2.13- Variation of return loss Vs frequency

52

Fig 2.14- Variation of VSWR Vs. frequency

53

Fig 2.15- Graph of input impedance of antenna vs. frequency

54

Fig. 2.16 Impedance Loci

55

The other radiation characteristics of antenna such as directivity, gain and efficiency are shown

in fig 2.19, 2.20 and fig 2.21 respectively. At resonance freq the total field directivity at 1.16

GHz is 6.19 dBi and at second resonant frequency of 1.5 GHz is 6.39 dBi and maximum gain of

about -1.45 dBi and 1.20 dBi at both resonance frequencies respectively. The directivity is

somewhat unaffected over the frequency range and gain is marginally better in compare with that

of previous case of rectangular antenna. The simulated elevation pattern and azimuth pattern of

at both resonating freq are given in fig 2.22 and 2.23 respectively which indicates that the

radiation intensity is maximum normal to the patch.

Fig 2.17 change in simulated value of directivity of antenna Vs frequency

56

Fig 2.18 change in simulated value of gain of antenna Vs. frequency

57

Fig 2.19 Variation of efficiency of antenna vs. frequency

58

Fig 2.20a- display of Elevation pattern of antenna

59

Fig 2.20b- display of Elevation pattern of antenna

60

Fig 2.21a- display of Azimuth pattern of antenna

61

Fig 2.21b- display of Azimuth pattern of antenna

62

63

63

The modified antenna discussed above radiates at two frequency with good broad side radiation

properties .however the impedance bandwidth of this antenna at both the freq are still narrow (of

order of 1.72 % and 2.66 % corresponding to freq 1.16 and 1.5 GHz respectively) hence antenna

in its present form is still unsuitable for communication systems and further improvement is

required.

2.4 Design of gap coupled truncated rectangular microstrip antenna

In following section an attempt is made to further improve the impedance Bandwidth of

TRMSA (truncated rectangular microstrip antenna) as discussed in above section. For this

purpose one horizontal slot parallel to non-radiating edge and two vertical slots parallel to

radiating edge are applied in the radiating patch forming an H shape slot as shown in

figure2.24.there fore the patch is divided in to six independent patches and gap coupled with one

or more independent patches.

2.4.1 Antenna Design – The optimized design parameter for the proposed antenna are, L1 = 47mm length of rectangular

patch, W = 62 mm width of rectangular patch, notches n=5mm and slot s=1mm as shown in

fig1.The dielectric constant (Єr) of the substrate is 4.4 mm and the thickness of the dielectric

substrate is 1.6 mm. The patch is printed on inexpensive glass epoxy FR4 substrate. The 50-ohm

coaxial cable with SMA connector is used for feeding. The proposed patch antenna gives wide

bandwidth having resonance frequency f0 = 2.2 GHz with impedance Bandwidth equal to 9.0%,

over the range of frequency 2.1 GHz to 2.32 GHz, at appropriate feed point location Xf=38 and

Yf=60. Simulation of the designed antenna is done by IE3D simulator software. A trial and error

mechanism is followed to analysis the reflection coefficient (S11) minimum value at the resonant

frequency. For a good impedance matching over a wide range of frequency, notches are

introduced on the two corners of the rectangular patch antenna as shown.

64

64

Fig2.22-Gap coupled truncated rectangular microstrip patch

antenna.

65

65

2.4.2 Results and Discussion The simulated results for the return loss and parametric study for the proposed design are also

studied. Fig. 2.25 represents the return loss for optimized proposed design. The simulated result

has a bandwidth of 9.0 % across a range of freq 2.1 GHz to 2.32 GHz, below the -10 dB RL at

central frequency at 2.22 GHz. The change in simulated value of VSWR Vs frequency is shown

in fig 2.26 VSWR presented by antenna across a bandwidth area is less than 2:1 value which is

good for matching between feeding circuit and antenna. The simulate value of VSWR at

resonant freq is 1.58. Figure 4.5 depict the simulated graph of input impedance of design antenna

with freq .At resonance freq 2.22 GHz the simulated input impedance of antenna is 58.3+ j 25

ohms which is in good agreement with the 50 ohms impedance of feeding network.

Fig.2.23- Variation of return loss Vs frequency

Fig 2.24- Variation of VSWR Vs. frequency

66

Fig 2.25- Graph of input impedance of antenna vs. frequency

67

Fig. 2.26 Impedance Loci

68

69

69

2.4.3 Parametric study and effect of gap slots between parasitic patches.

By changing one parameter of geometry a parametric studies are presented while

remaining parameters of geometry are fixed w.r.t reference design. Fig 2.29 represent the

simulated graph of return loss for proposed design w.r.t freq for various values of spacing ‘S’

between rectangular parasitic patches. From simulated results it is observed that if the gap

spacing ‘S’ decreases from the optimum value, there is decrease in Bandwidth with Slightly

change of frequency range near to lower side of freq. it is observed too that by increasing gap of

slot bandwidth is decreased approximately by 40%. The optimal performance is obtained for S =

1 mm as shown in figure 2.24

Fig. 2.27 Effects of variation of gap between parasitic patches.

70

70

The other radiation characteristics of antenna such as directivity, gain and efficiency are shown

in fig 2.30, 2.31and fig 2.32 respectively. At resonance freq the total field directivity is 7.2 dbi

and maximum gain of about 3.35dBi at resonance frequency. It is observed that simulated

variation of gain, efficiency and directivity of antenna increases significantly as compare to that

of simple rectangular and truncated antenna. The simulated elevation pattern and azimuth pattern

of antenna are shown in fig 2.33 and 2.34 respectively which indicates that the antenna is

strongly radiating normal to the patch.

Fig 2.28 - Directivity VS. Frequency

Fig 2.29 gain vs. frequency

71

Fig 2.30 -efficiency vs. frequency

72

Fig 2.31- display of Elevation pattern of antenna

73

Fig 2.32- display of Azimuth pattern of antenna

74

The modified antenna discussed above radiates at resonance frequency of 2.2ghz with good

broadside radiation properties having impedance bandwidth of 9.0% .The reported results

suggest that the overall performance of this gap coupled truncated rectangular microstrip antenna

a is improved considerably. In the next chapter same antenna is further modified to achieve

further improvement in impedance bandwidth and other characteristics.

2.5 Discussion and Conclusion

In this chapter, the investigation of a simple rectangular MSA and modified rectangular

MSA with Truncated Corners are discussed. In first part, the characteristics of simple rectangular

patch antenna is simulated and studied on single layer FR-4 substrate material. The useful

bandwidth for the simple rectangular antenna is 2.68% which is too low for application in

communication system. Further the antenna is modified by introducing two notches at the

corners of rectangular patch which gives the dual frequency operation and better performance as

compare to simple rectangular microstrip patch antenna. The impedance Band width of this

antenna at two resonant frequencies is 1.72% and 2.66% which is also not suitable for

application. In next section a gap coupled arrangement is used with the same parameters of

truncated rectangular microstrip patch antenna by introducing horizontal and vertical slots such

that the antenna is divided in to small patches and gap coupled with each other.

By such arrangement the improved impedance bandwidth of 9.0% at resonance frequency 2.2 GHz is achieved and also there is a good improvement in gain, directivity and efficiency of

the patch antenna. Simulated results verifying the application of such method for single layer

antenna.

In further chapters an efforts are made to increase the impedance bandwidth with same

parameters by using different methods of bandwidth enhancement techniques.