design, implementation and comparative study … design, implementation and comparative study...

10
1 Design, Implementation and Comparative Study Slotted Waveguide Antennas João Carlos Ferreira Monteiro Instituto Superior Técnico Avenida Rovisco Pais, 1 1049-001 Lisboa Joã[email protected] Abstract: Nowadays, wireless networks appear as a tool capable of providing the most varied services (Television, Internet, Phone, etc). Such networks are available to all users, whether in study places (Universities, etc), in recreational sites (bars, shopping malls, etc) or even created by the users themselves for their own entertainment. The growing demands for such networks, has led to the development of projects in this area, as well as optimization of existing recourses in order to make this service more competitive. In this paper a comparative study of two antennas made of rectangular waveguides with slots in two orthogonal Planes, operating in 2,45GHz band, is performed. The antennas have the same number of slots, but the position of the slots differs; one contains the slots in the Plane zx, and the other in the Plane zy. They also differ in the offset parameters; the antenna with slots in Plane zx requires the dimensioning of the offset, while the antenna with slots in zy Plane presents no offset. The antennas analysis will be based on characteristic parameters such as -3dB bandwidth, the gain in different polarization Planes and the SWR. The project has included a simulation study using the CST-MWS software and measurements in an anechoic chamber. 1. INTRODUCTION Currently, wireless communication allows you to establish communication between two devices through the propagation of electromagnetic waves (for example radio waves, infrared light, laser, etc.). In the telecommunications industry, its applicability is remarkable in the transmitters and radio receivers, remote controls, computer networks, among others. In this context, the antennas made of slots assume greater importance in the wireless communication system (Wireless). A slot in a waveguide is a metal radiating element. Their behavior is identical to the functioning of an electric dipole. By analogy, an aggregate of slots has the identical behavior of an aggregation of dipoles. It is in this context that emerges the primary objective of this thesis: the analysis of two antennas composed by waveguides with different positioning of the slots in the guide, so that it can be done a comparison between antennas. The antennas consist of a rectangular waveguide with 4 slots made in the zy plane (antenna 1) and in the zx plane (antenna 2). In this chapter it is possible to understand its mode of functioning. In order to compare two types of antennas, it will be presented their mode of operation, dimensioning, positioning, width and length of the slots to insert in the guide. Waveguide The wave guide used as a basic element of these antennas was produced from an anodized aluminum profile, which results in a low cost for these antennas and also in the ease of manufacture, two important characteristics especially in military applications. These guides were designed so to operate in fundamental mode TE10. Figure 1 Rectangular waveguide The measurements of the guides used are shown in Table 1. Table 1 Measurements of the used waveguide Dimensions (mm) Width (a) 37 Height (b) 97 Thickness 1,5 Propagation direction

Upload: ngoduong

Post on 12-May-2018

231 views

Category:

Documents


1 download

TRANSCRIPT

1

Design, Implementation and Comparative Study Slotted

Waveguide Antennas

João Carlos Ferreira Monteiro

Instituto Superior Técnico

Avenida Rovisco Pais, 1 — 1049-001 Lisboa

Joã[email protected]

Abstract: Nowadays, wireless networks appear as a tool capable of providing the most varied services

(Television, Internet, Phone, etc). Such networks are available to all users, whether in study places

(Universities, etc), in recreational sites (bars, shopping malls, etc) or even created by the users themselves

for their own entertainment. The growing demands for such networks, has led to the development of

projects in this area, as well as optimization of existing recourses in order to make this service more

competitive. In this paper a comparative study of two antennas made of rectangular waveguides with slots

in two orthogonal Planes, operating in 2,45GHz band, is performed. The antennas have the same number

of slots, but the position of the slots differs; one contains the slots in the Plane zx, and the other in the

Plane zy. They also differ in the offset parameters; the antenna with slots in Plane zx requires the

dimensioning of the offset, while the antenna with slots in zy Plane presents no offset. The antennas

analysis will be based on characteristic parameters such as -3dB bandwidth, the gain in different

polarization Planes and the SWR. The project has included a simulation study using the CST-MWS

software and measurements in an anechoic chamber.

1. INTRODUCTION

Currently, wireless communication allows you

to establish communication between two

devices through the propagation of

electromagnetic waves (for example radio

waves, infrared light, laser, etc.). In the

telecommunications industry, its applicability is

remarkable in the transmitters and radio

receivers, remote controls, computer networks,

among others. In this context, the antennas

made of slots assume greater importance in the

wireless communication system (Wireless). A

slot in a waveguide is a metal radiating element.

Their behavior is identical to the functioning of

an electric dipole. By analogy, an aggregate of

slots has the identical behavior of an

aggregation of dipoles. It is in this context that

emerges the primary objective of this thesis: the

analysis of two antennas composed by

waveguides with different positioning of the

slots in the guide, so that it can be done a

comparison between antennas.

The antennas consist of a rectangular waveguide

with 4 slots made in the zy plane (antenna 1)

and in the zx plane (antenna 2). In this chapter it

is possible to understand its mode of

functioning.

In order to compare two types of antennas, it

will be presented their mode of operation,

dimensioning, positioning, width and length of

the slots to insert in the guide.

Waveguide

The wave guide used as a basic element of these

antennas was produced from an anodized

aluminum profile, which results in a low cost

for these antennas and also in the ease of

manufacture, two important characteristics

especially in military applications. These guides

were designed so to operate in fundamental

mode TE10.

Figure 1 – Rectangular waveguide

The measurements of the guides used are shown

in Table 1.

Table 1 – Measurements of the used waveguide

Dimensions (mm)

Width (a) 37

Height (b) 97

Thickness 1,5

Propagation

direction

2

Fundamental Mode

The fundamental mode of propagation, in the

guides with the dimensions mentioned above, is

the TE10 mode. This mode has its cutoff

frequency 1.55 GHz, thus allowing operation at

central frequency of work, 2.45 GHz.

In table 2 we can observe how to calculate the

characteristics of the mode of propagation in the

guide.

Table 2 –Waveguide characteristic parameters

Generic

writing

Rectangular waveguide

[rad ]

[GHz]

[mm]

[rad ]

[mm]

Type and positioning of the slots

The sizing of slots, by other words, the width

and length are the same for both antennas. The

criterion for the design corresponds to ensure

maximum radiation for each slot.

A half-wave resonant dipole or a resonant slot

has a length of 0.475 (1). Elliott and Kurtz

concluded that the length of the slot is given by

0.483 (2).

The same authors, using the curves of Stegen,

determined that the slot width is given by:

However, for the antenna with slots in the plane

zy simulations were performed to optimize the

width of the slot. In the following table you can

view the measurements of the slots

Table 3 – Slots Characteristics

Antenna ZY Plane XZ Plane

L (lenght) 59 59

W (width) 7 4

ZY Plane Slots

Figure 2 –Electric field lines, magnetic field and

electric current distribution (3)

Based on this structure of current lines (Figure

2), the slots were scaled so that they could have

a maximum radiation. The current lines have a

distribution along the guide according to figure

2. This distribution can be expressed by the

following expressions:

These expressions, as well as the distribution of

current lines in Figure 2, are essential to

understand the positioning of the antenna with

slots in the plane zy. The slots are placed in the

zy plane which corresponds to the points of

convergence and divergence of the electric

power lines (zx plane). The arrangement of the

slots along the guide can be seen in Figure 3.

Figure 3 – ZY Plane slots

3

ZX Plane Slots

Figure 4 – ZX Plane slots

Figure 4 shows that the slots are displaced from

the longitudinal axis of the face of the antenna,

this deviation is called offset. The next step is to

calculate this factor. Starting from the initial

formula of Stevenson (2):

where represents the conductance of the

slot and the represents the

conductance of the guide. Elliot, with the help

of curves Stegen, made some adjustments, so

the previous equation has acquired the

following form:

Using the above equation it is possible to

determine the value of the

disregarding the value of

. Then this

value is used, replacing it in the equation, along

with the and other parameters, and so we

can determine the offset that is given by d.

2. SIMULATION AND EXPERIMENTAL

MEASUREMENTS

ZY Plane Slots Antenna

Initially it was used in the simulations, an non

adapted antenna. The results are presented in

Table 4.

Tabela 4 – Simulação da antena sem stub

3D H Plane E Plane

S11 (dB) -6,25

VSWR 2.9

Gain (dBi) 10,01 10,0 10,0

SLL (dB) - 4,25 6,28

-3dB

bandwidth

- 10,3 147,7

As you can see in the table, the values of S11 (-

6.26 dB) and VSWR (2.9) reflect the misfit of

the antenna.

Later the antenna was adapted using a stub, first

it was scaled using the Smith Chart and then

optimized by successive simulations. For this

antenna the stub is at 40mm from the top of the

guide and has a depth of 13mm.

We then performed the analysis of the results

obtained by simulation of the antenna already

adapted

Figure 5 – S11 (Simulation)

Through the analysis of figure 5 it can be seen

that the antenna has a value of S11 of -18.25

dB, which results in a VSWR of 1.279. These

values can state that the antenna is adapted.

Figure 6 – Radiation diagram (H Plane)

Figure 7 – Radiation diagram (E Plane)

4

Analyzing figure 6, we can observe that for the

H plane the antenna has a main lobe with

approximately 11dB of gain and a level of

secondary lobes (NLS) of -3.7 dB. So is it

possible to analyze the figure 7, where the gain

has a value of 10dB and an NLS of -3.7.

Experimental measures

Figure 8 – S11 (Experimental)

In figure 8 we can observe that the experimental

S11 value is very close to-15dB. This value is

very close to the minimum acceptable value

which is-15dB. Despite it is slightly above the

desired value is considered acceptable.

Figure 9 – Polarization (E Plane)

Figure 10 – Polarization (E Plane)

Through the analysis of the previous figures we

can conclude that, in both planes of polarization,

the cross-polarization always has a value far

below to the normal polarization. Therefore the

antenna rejects the cross-polarization.

Figure 11 – Gain (E Plane)

Figure 12 – Gain (H Plane)

In the figures 11 and 12, in blue is the

distribution of gain along the azimuth, in red is

the maximum value obtained and in green the

level of secondary lobes. It is worth noting the

high amplitude of secondary lobes, which can

cause interference in the radiation. The

summary of the results obtained in the two

previous figures as well as the other

experimental measurements is present in Table

5.

Table 5 –Experimental measures

H Plane E Plane

S11 (dB) -13,8

VSWR 1.52

Gain (dBi) 10,75 10,61

SLL (dB) 5,18 6,48

-3dB bandwidth 9 146

Simulation and Experimental results

comparison

Figure 13 – Comparison S11

-15

-10

-5

0

2,4 2,45 2,5

S 11 [

dB

]

Frequency [GHz]

S11

S11

-80

-60

-40

-20

0

-180 -120 -60 0 60 120 180

Gai

n [

dB

]

Azimuth [º]

Polarization and Cross polarization

E

Ex

-70

-55

-40

-25

-10

-60 -40 -20 0 20 40 60

Ga

in [

dB

]

Azimuth [º]

Polarization and Cross polarization

H

Hx

-20

-10

0

10

20

-180 -120 -60 0 60 120 180

Ga

in [

dB

]

Azimuth [º]

E Plane Gain

E

Max

SLL

-40 -30 -20 -10

0 10 20

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Ga

in [

dB

]

Azimuth [º]

H Plane Gain

H Max SLL

-25

-20

-15

-10

-5

0

5

2,4 2,45 2,5

S 11 [

dB

]

Frequency [GHz]

Comparison S11

NA

CST

5

The values of S11, experimental and simulated,

don't have the same value, showing a difference

of about 3dB. This may be due to the manual

adjustment of the stub, and to the difficulty in

placing the same to a precise depth of

penetration in the guide.

Figure 14 – Comparison (H Plane Gain)

Figure 15 – Comparison (E Plane)

The figures above show that the distribution of

the gain in H plan or in the E plan is coincident

almost always throughout the graph. It can be

concluded that the antenna, for these two

parameters, presents itself as an ideal antenna.

For a better understanding of the comparison

between simulations and experimental results is

presented in Table 6.

Table 6 – Comparison (Resume)

Simulation results Experimental results

H Plane E Plane H Plane E Plane

S11(dB) -18.25 -13,8

VSWR 1,28 1,52

Gain(dBi) 10,8 10,8 10,75 10,61

SLL (dB) 5,1 7,1 5,18 6,48

-3dB Bandwidth 10,3 147,9 9 146

ZX Plane slots Antenna

Simulations

Like the previous study, for this antenna it was

also made simulations for an antenna without

the stub, in other words, a non adapted antenna.

The results can be viewed in the following table.

Table 7 –Antenna simulations without stub

3D H

Plane

E

Plane

S11 (dB) -6,59

VSWR 2,76

Directivity

(dBi)

12,6

1 12,60 12,60

Gain(dBi) 11,4

9 11,50 11,50

SLL (dB) - -13,80 -18,70

-3 dB

bandwidth - 19,90º 74,10º

As we can see, the parameter values of the S11

and of the VSWR are distant from the intended,

respectively-15dB and 1.5, therefore, the

antenna is not adapted. This fact forced the

design of a stub (dimensioned similarly to the

dimension to the antenna above). The stub

appears as a characteristic length of 31mm to

40mm and is placed at the top of the guide.

After the adjustment of the antenna, new

simulations were conducted. The results of these

simulations are presented below.

Figure 16 – S11 (Simulation)

The figure 16 shows the distribution of the S11

over the frequency, the value of this for the

working frequency (2.45 GHz) is -19.34 dB,

which implies a VSWR of 1.24. The values

obtained confirm the good adaptation of the

antenna.

Figure 17 – Radiation diagram (H Plane)

Figure 18 – Radiation diagram (E Plane)

-60

-40

-20

0

20

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Ga

in [

dB

]

Azimuth [º]

Gain comparison (H plan)

CST

CA

-20

-10

0

10

20

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

Ga

in [

dB

]

Azimuth [º]

Gain comparison (E plan)

CST

CA

6

In Figure 17 it is possible to identify a main

lobe with a range of 12.5 dB, there are also

visible two side lobes prominent in relation to

the others, this results in an S of -13.8 dB.

The SLL has a main lobe with -3dB bandwidth

of 19.9. On the E plane (Figure 18) the antenna

has a maximum gain of 12.5 dB and an SLL of -

18.7 dB. In this plane, the main lobe with -3dB

bandwidth of 74.3.

Experimental measures

Figure 19 – S11 (Experimental)

Looking at Figure 19, it appears that the S11 has

a value of around 20dB. This value is below-

15dB, which fact attests to the good design of

the stub, and the consequent good antenna

adaptation.

Figure 20 – Polarization (E Plane)

Figure 21 – Polarization (H Plane)

According to the figures analysis, the values of

the cross-polarization component distribution

are always lower than the normal polarization.

In H plan, for some azimuths the cross-

component is higher than the normal

polarization. However it may be said that this

antenna rejects the cross-polarization.

Figure 22 – Gain (H Plane)

Figure 23 – Ganho (Planeo E)

The gains of the different planes of radiation are

shown in Figures 22 and 23. At first, we can see

that there is a main lobe, the maximum

amplitude of it is 12.41dB, flanked by two side

lobes. The green line represents the SLL, which

has a value of -14.12dB. In the second Figure

SLL is much smaller than the previous one,

which means that, in this Plan, the secondary

lobes will cause less interference than H Plan

ones.

In the following table, we can see the analyzed

results so far and the remaining parameters in

the analysis.

Table 8 – Experimental results (Resume)

H Plane E Plane

S11 (dB) -19,74

VSWR 1,23

Gain (dBi) 12,41 12,35

SLL (dB) -14,12 -30,20

-3 dB

Bandwidth

20 73

-25

-20

-15

-10 -5

0

2,4 2,45 2,5

S11

[dB

]

Frequency [GHz]

S11 (4 Fendas)

S_11

-70 -60 -50 -40 -30 -20 -10

-180 -120 -60 0 60 120 180

Ga

in [

dB

]

Azimuth [º]

Polarization and Cross polarization

Ex

E

-80 -70 -60 -50 -40 -30 -20 -10

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Ga

in [

dB

]

Azimuth [º]

Polarization and Cross polarization

Hx

H

-40 -30 -20 -10

0 10 20

-90 -60 -30 0 30 60 90

Ga

in [

dB

]

Azimut [º]

H Plane Gain (4 Slots) H

Máximo

NLS

-40 -30 -20 -10

0 10 20

-180 -120 -60 0 60 120 180

Ga

in [

dB

]

Azimuth [º]

E Plane Gain (4 Slots) E

Máximo

NLS

E…..

Max

SLL

E…..

Max

SLL

7

Simulation and experimental measures

comparison

Figure 24 – Comparison S11

In Figure 24 we can see the correlation between

experimental value and the value obtained by

simulation. For this parameter the objectives

have been met.

Figure 25 – Comparison (H Plane)

Figure 26 – Comparison (E Plane)

For the H plane (Figure 25) experimental and

simulated values are nearly coincident.

In the E plane (Figure 26) there are small

variations, particularly in the area of the side

lobes, however, the remaining values of the

graph are almost coincident.

The following table presents a summary of

comparisons made between experimentally

values and values obtained by the simulations

done.

Table 9 – Comparison (Resume)

Simulation Real

H Plane E Plane H

Plane

E

Plane

S11 (dB) -19,34 -19,74

VSWR 1,24 1,23

Gain (dBi) 12,51 12,51 12,41 12,35

SLL (dB) -13,80

-18,70

-14,12 -30,20

-3 dB

Bandwidth

19,90º 74,10º 20º 73º

3. ANTENNAS COMPARISON

Presented the two antennas, which are an

integral part of this study, it is time to compare

them taking into account the parameters used to

analyze them individually.

Results obtained by simulations

The first parameter to be analyzed will be the

standing wave ratio (S11).

Figure 27 – Antennas Comparison (Simulation -

S11)

From the viewpoint of results obtained using

simulations, the antennas have very close values

of S11, this values are approximately equal to -

20dB. However, the zx plane slots antenna has a

better adaptation, due to theirlower S11.

Figure 28 – Antennas Comparison (H Plane

gain-simulation)

-25

-20

-15

-10

-5

0

2,4 2,45 2,5

S11

[d

B]

Frequency [GHz]

Comparison S11 (4 Slots)

S_11 (NA)

S_11 (CST)

-50 -40 -30 -20 -10

0 10 20

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Ga

in [

dB

]

Azimuth [º]

CST e CA gain comparison Ganho CA

Ganho CST

-40

-30

-20

-10

0

10

20

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

Ga

in [

dB

]

Azimuth[º]

CST e CA gain comparison Ganho CA

Ganho CST

-30

-20

-10

0

2,4 2,45 2,5

S 11 [

dB

]

Frequency [GHz]

Antennas S11 (simulation)

-40

-20

0

20

-90 -60 -30 0 30 60 90

Ga

in [

dB

]

Azimut [º]

Antennas Gain (H plane)

8

Figure 29 – Antennas E Plane Gain (Simulation)

In Figure 28 we can observe two main lobes,

one in the red curve and another in the blue

curve, but the lobe that belongs to the red curve

has a greater bandwidth than the blue curve.

This characterizes the antenna with slots in the

plane zy as more directive. However the

antenna with slots in zx plane has a higher gain,

of approximately 2dB.

In the plane E, Figure 29, the roles are inverted,

the antenna with slots in the plane zx is more

directive in the result of a narrower main lobe,

the antenna with slots in zy plane is nearly

isotropic, due to a larger width of the lobe main.

In the following table can have a perception of

all parameters examined in this comparison.

Table 10 – Comparison (Simulations)

ZX Plane slots

antenna

ZY Plane slots

antenna

Planeo

H

Planeo

E

Planeo

H

Planeo

E

S11(dB) -19,34 -18,25

VSWR 1,24 1,28

Gain (dBi) 12,51 12,51 10,8 10,8

SLL (dB) -13,80 -18,70 -5,1 -7,1

-3dB

Bandwith 19,90 74,10 10,3 147,9

Experimental measures

Similar to the comparison between values

obtained using simulations, a comparison was

made based on experimental results obtained for

both antennas

Figure 30 – Antennas Comparison

(Experimental – S11)

Contrary to what happened with the

measurements obtained by simulation, the

experimental measurements of the two antennas

have different values. This difference has a

value of about 5 dB. This is due to the fact that

adapting the antenna with slots in zy plane was

not expected. The manual adjustment of the

stub, and the difficulty in setting it may be the

reason for this bad adaptation.

Figure 31 – H Plane Experimental Gain

comparison

Figure 32 – E Plane experimental Gain

As experimental and simulated values in

relation to gain, practically match for both

antennas, the analysis to be made to the figures

31 and 32 matches with the analysis already

made to the figures 28 and 29. The main

conclusions of this analysis are the higher

directivity, in the H plane, of the antenna with

slots in the plane zy. However the antenna with

slots in zx plane is more directive in the plane E,

-20

0

20

-180 -120 -60 0 60 120 180

Ga

in [

dB

]

Azimut [º]

E Plane Gain(Simulation)

-20

-15

-10

-5

0

2,4 2,45 2,5

S 11 [

dB

]

Frequency [GHz]

Antennas S11(experimental)

-30

-20

-10

0

10

20

-90 -60 -30 0 30 60 90

Ga

in [

dB

]

Azimut [º]

H Plane Experimental Gain

-40

-30

-20

-10

0

10

20

-180 -90 0 90 180

Ga

in [

dB

]

Azimut [º]

E plane experimental gain

9

where its competitor plan is practically

isotropic. Table 11 present all values obtained in

the experimental values comparisons.

Table 11 – Comparison (Experimental

measures)

ZX Plane slots

antenna

ZY Plane slots

antenna

H Plane E

Plane H Plane E Plane

S11(dB) -19,74 -13,8

VSWR 1,23 1,52

Gain (dBi) 12,41 12,35 10,75 10,65

SLL (dB) -14,12 -30,20 -5,18 -6,48

-3dB

Bandwidth 20 73 9 146

4. CONCLUSIONS

The development of this work had as the initial

objective, the design, construction and analysis

of two individual antennas. This analysis was

divided into two main parts, results obtained by

simulation comparisons and results obtained

experimentally comparisons.

Subsequently, the objective was focused on a

comparison between antennas, while keeping in

mind all the results obtained in the individual

analysis.

Then, it is possible to see the 3D radiation

diagrams the antennas under study.

Figure 33 – 3D radiation diagram (zx Plane

antenna - E Plane)

Figure 34 –3D radiation diagram (zy Plane

antenna - E Plane)

Through observation of the previous graphics,

Figure 33 and Figure 34, there are differences in

the main lobe, the Figure 6.1 show a higher

bandwidth main lobe than the Figure 6.2.

This allows us to say that the zy plane slots

antenna is more directive, in the plane H, than

the zx plane slots antenna It is still possible to

see that the level of side lobes is greater in the

zy plane slots antenna.

Figure 35 – 3D radiation diagram (zx Plane

antenna - H Plane)

Figure 36 – 3D radiation diagram (zy Plane

antenna - H Plane)

10

Analyzing the figures 35 and 36, corresponding

to the 3D radiation diagrams of the antennas

under study, according to a different

perspective, we can conclude that the antennas

have different behaviors. Comparing the main

lobes, like the study for the figures 6.1 and 6.2,

is possible to verify that the antenna with slots

in the plane zx has an inferior bandwidth when

compared with the bandwidth of the antenna

with slots in the plane zy. For the antenna with

slots in zy plane, we can say that it is almost

isotropic in this plane of radiation. This

situation can be solved by placing a metal net in

the rear of the antenna.

Generally, it was found that the antenna with

slots in the zx plane is presented, accounting for

all parameters in the analysis, as the most viable

option. The parameters which are highlighted

and that gave advantage to the antenna with

slots in the plane zx were the gain, the level of

side lobes and the coefficient of stationary

wave.

Although both antennas are easy to build, the

antenna with slots in the plane zy presents

greater ease of construction because they do not

need an offset dimension. However, there are

features common to both antennas, such as low

cost and robustness. This last feature makes

them an option to be taken into account to

perform in military communications.

REFERENCES 1. Kraus, John D. Antennas. s.l. : McGraw-Hill

Book Company, 1950.

2. Wade, Paul. Microwave Antenna Book.

2003.

3. Faro, M. de Abreu. Propagação e Radiação

de ondas Electromagnéticas. s.l. : Técnica

AIST, 1984.